==================================== DOCUMENTATION OF THE XCF FILE FORMAT ==================================== This document describes the native image file format of GIMP. License ------- Copyright Henning Makholm , 2006-07-11 This is free documentation; you can modify and/or redistribute it according to the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the license, or (at your option) any later version. Table of contents ----------------- Documentation of the XCF file format License Table of contents Audience Scope Status Version history 1. Basic concepts XCF file Basic data types Canvas Color Pixel data: Tiles Pixel data: Levels of detail hierarchy Channels Layers Layer masks Properties Parasites Selections Floating selection Tattoos 2. General properties 3. The Image structure Header Image properties 4. The Channel structure Channel properties 5. The Layer structure Layer properties 6. The Hierarchy structure Levels 7. Tile data organization Uncompressed tile data RLE compressed tile data 8. Miscellaneous The name XCF Audience -------- Audience of this document are developers of GIMP and other software that reads and writes XCF files. Scope ----- The XCF format is designed to store the whole state of GIMP that is specific to one image (i.e., not the cut buffer, tool options, key bindings, etc.) and is not undo data. This makes the full collection of data stored in an XCF file rather heterogeneous and tied to the internals of GIMP. Use of the XCF format by third-party software is recommended only as a way to get data into and out of GIMP for which it would be impossible or inconvenient to use a more standard interchange format. Authors of third-party XCF-creating software in particular should take care to write files that are as indistinguishable as possible from ones saved by GIMP. The GIMP developers take care to make each version of GIMP able to read XCF files produced by older GIMP versions, but they make no special efforts to allow reading of XCF files created by other software. Interchanging image data with other applications is not goal of the XCF format. For this use case GIMP opens and exports common images formats, like JPEG, PNG and PSD. TODO: Role of the ORA format in this context? For the stated reasons and clarification GIMP _saves_ XCF files, but _exports_ to other image formats. Beware that CinePaint's native file format is called XCF, too. While it is derived from the format described here, both formats differ in many details and are _not_ mutually compatible. This document does not describe the CinePaint XCF format. For more information on that see http://www.cinepaint.org/more/docs/xcf.html Status ------ This specification is an unofficial condensation and extrapolation of the XCF-writing and -reading code in version 2.8.10 of GIMP. As of this writing, it has not been approved or proofread by any GIMP developer, though it has been written with the intention of contributing it to the GIMP project for use as official documentation. Some of the normative statements made below are enforced by the XCF code in GIMP; others are just the author's informed guess about "best practices" that would be likely to maximize interoperability with future versions of GIMP. Version history --------------- This section lists the changes between file format versions in bigger terms. Details are denoted in the text. Version 0: Since GIMP 0.99.16, released on 15.12.1997. The initial file format. Everything that is not listed in the following versions is part of this. Version 1: Since GIMP 0.99.16, released on 15.12.1997. Adds color maps. Chapter 3 "The image structure" describes the PROP_COLOR_MAP property. Version 2: Since GIMP 1.3.10, released on 07.11.2002. Adds layer modes "Soft light", "Grain extract", "Grain merge" and painting mode "Color Erase". In chapter 5 "The layer structure" the description of the property PROP_MODE contains the new layer modes. Improves path handling in GIMP 1.3.21, released on 5.10.2003. Chapter 1 "Basic concepts" describes the path handling in general and chapter 2 "General concepts" introduces the PROP_VECTORS property. Version 3: Since GIMP 2.7.1, released on 29.06.2010. Adds layer groups. The chapter 5 "The layer structure" describes the new properties PROP_GROUP_ITEM, PROP_GROUP_ITEM_FLAGS and PROP_ITEM_PATH. 1. BASIC CONCEPTS ================= It is recommended that a software developer who wants to take full advantage of the XCF format be deeply familiar with GIMP at least as a user. The following high-level overview is meant to help those non-users who just need to extract pixel data from an XCF file get up to speed. XCF file -------- An XCF file is a sequence of bytes. In general an XCF file describes a stack of layers and channels on a canvas. It contains a series of data structures, the order of which is in general not significant. The exception to this is that the main image structure must come at the very beginning of the file, and that the tile data blocks for each drawable must follow each other directly. References _between_ structures in the XCF file take the form of 32-bit "pointers" that count the number of bytes between the beginning of the XCF file and the beginning of the target structure. Note that therefore the maximum address of a layer, channel, hierarchy or tile set is 2^32 - 1, i.e. at 4 GB. Everything after will be lost. Currently this doesn't play a role yet. Each structure is designed to be written and read sequentially; many contain items of variable length and the concept of an offset _within_ a data structure is not often relevant. Basic data types ---------------- A WORD is a 32-bit integer stored as 4 bytes in big-endian order, i.e. with the most significant byte first. The word is not necessarily aligned to an offset within the XCF file that is a multiple of 4. Depending on the context the word can be unsigned or (2's complement) signed. UINT32 denotes unsigned words and INT32 denotes signed words in this document. A FLOAT is stored as a 32-bit IEEE 754 single-precision floating-point number in big-endian order. A STRING is stored as follows: uint32 n+1 Number of bytes that follow, including the zero byte byte[n] ... String data in Unicode, encoded using UTF-8 byte 0 Zero marks the end of the string. Exception: the empty string is stored simply as an uint32 with the value 0. Canvas ------ A canvas is an abstract rectangular viewport for the layers and channels. The image header stores the canvas' dimensions. Color ----- RGB: Three intensity values for red, green, and blue additive color components, each on a scale from 0 to 255. The exact color space is not specified. GIMP displays image data directly on PC display hardware without any software correction, so in most cases the intensity values should be considered nonlinear samples that map to physical light intensities using a power function with an exponent ("gamma") of about 2.5. (This is how PC hardware commonly treat bit values in the video buffer, which incidentally also has the property of making each 1/255th step about equally perceptible to the human eye when the monitor is correctly adjusted). Beware, however, that GIMP's compositing algorithms (as described in the document compositing.txt) implicitly treat the intensities as _linear_ samples. The XCF file format currently has no support for storing the intended gamma of the samples. TODO: Are the statements about color space, gamma and layer modes still valid? Grayscale: One intensity value on a scale from 0 (black) to 255 (white). Gamma considerations as for RGB. Indexed: An 8-bit index into a color map that is shared between all layers. The color map maps each index to an RGB triple which is interpreted as in the RGB model. Pixel data: Tiles ----------------- Basically pixels are organized in a grid of "tiles", each with a width and height of up to 64 pixels. The only tiles that have a width less than 64 are those in the rightmost column, and the only tiles that have a height less than 64 are those in the bottommost row. Thus, a layer measuring 200 x 150 pixels will be divided into 12 tiles: +-----------------+-----------------+------------------+-----------------+ | Tile 0: 64 x 64 | Tile 1: 64 x 64 | Tile 2: 64 x 64 | Tile 3: 8 x 64 | +-----------------+-----------------+------------------+-----------------+ | Tile 4: 64 x 64 | Tile 5: 64 x 64 | Tile 6: 64 x 64 | Tile 7: 8 x 64 | +-----------------+-----------------+------------------+-----------------+ | Tile 8: 64 x 22 | Tile 9: 64 x 22 | Tile 10: 64 x 22 | Tile 11: 8 x 22 | +-----------------+-----------------+------------------+-----------------+ As can be seen from this example, the tiles appear in the XCF file in row-major, top-to-bottom, left-to-right order. The dimensions of the individual tiles are not stored explicitly in the XCF file, but must be computed by the reader. The tiles that are pointed to by a single level structure must be contiguous in the XCF file, because GIMP's XCF reader uses the difference between two subsequent tile pointers to judge the amount of memory it needs to allocate for internal data structures. Pixel data: Levels of detail hierarchy -------------------------------------- The tiles themselves are organized in levels of detail. These levels build a hierarchy. Only the first level structure is used by GIMP's XCF reader, except that the reader checks that a terminating zero for the level-pointer list can be found. GIMP's XCF writer creates a series of dummy level structures (with NULL-pointers to the tiles), each declaring a height and width half of the previous one (rounded down), until the height and with are both less than 64. Thus, for a layer of 200 x 150 pixels, this series of levels will be saved: A level of 200 x 150 pixels with 12 tiles: the actually used one A level of 100 x 75 pixels with no tiles A level of 50 x 37 pixels with no tiles Third-party XCF writers should probably mimic this entire structure; robust XCF readers should have no reason to even read past the pointer to the first level structure. TODO: The XCF file holds (for unclear historical reasons) a level-of-detail hierarchy, but we only use the lowest hierarchy level of it and other XCF consumers are told to do the same. This looks like a mipmap. Would using it to save an image pyramid or the thumbnail for the File dialogs get us some benefits? Channel ------- A channel is a named object that contains a single byte of information for each pixel in the canvas area. Channels have a variety of use as intermediate objects during editing; they are not meant to be rendered directly when the final image is displayed or exported to layer-less formats. A major use of channels is as a store for saved selections. A channel can be edited as if it was a grayscale layer with the same dimensions as the canvas. When it is shown in the GIMP editor UI together with other layers, it is used as if it was the _inverse_ alpha channel of a layer with the same color information in all pixels; this color can be stored in the XCF file as a property of the channel. This "mask" representation is generally thought of as an UI feature rather than an intrinsic semantics of a channel. Though the channel data structure in the XCF file contains a height and width field, these must always be the same as the canvas width and height. TODO: does this apply to any channel or only to selections? Layer ----- A layer is a named rectangular area of pixels which has a definite position with respect to the canvas. It may extend beyond the canvas or (more commonly) only cover some of it. Each pixel of the layer has a color which is specified in one of three ways as described in the "Color" section. All layers in an image must use the same color model. Exception: if the "floating selection" (see below) belongs to a channel or layer mask, it will be represented as grayscale pixels with alpha independently of the image's overall color model. Each pixel of a layer also has an alpha component which specifies the opacity of the pixel on a linear scale from 0 (denoting an alpha of 0.0, or completely transparent) to 255 (denoting an alpha of 1.0, or completely opaque). The color values do not use "premultiplied alpha" storage. The color information for pixels with alpha 0 _may_ be meaningful; GIMP preserves it when parts of a layer are erased and provides (obscure) ways of recovering it in its user interface. The bottommost layer _only_ in an image may not contain alpha information; in this case all pixels in the layer have an alpha value of 255. (Even if the bottommost layer does not cover the entire canvas, it is the only layer that can be without an explicit alpha channel). In images that use the indexed color model, GIMP does not support partial transparency and interprets alpha values from 0 to 127 as fully transparent and values from 128 to 255 as fully opaque. This behavior _may_ change in future versions of GIMP. TODO: has already changed? Layers have certain other properties such as a visibility flag, a global opacity (which is multiplied with individual pixel alphas) a layer group flag and various editing state flags. Layer mask ---------- The layer mask can be attached to a layer. Actually it is represented as a channel structure in the XCF file. It is referred to from its parent layer and not listed in the master list of channels. Its dimensions and placement coincide with those of its parent layer. Unless disabled by the PROP_APPLY_MASK property, the layer mask functions as an extra alpha channel for the layer, in that for each pixel the layer's alpha byte and the layer mask byte are multiplied to find the extent to which the layer blankets the background. Thus a layer mask can make parts of the layer more transparent, but never more opaque. Properties ---------- Properties are an extension mechanism to attribute the image, channels and layers. Some are attributes for general use, such as PROP_END, others are specific to the image, a channel or a layer. Technically properties are implemented as variable-length series of variable-length PROPERTY records which have the following general format uint32 type Numerical type identifier uint32 plength Payload length in bytes (but BEWARE! see below) byte[n] ... Payload - interpretation depends on the type The authoritative source for property type numbers is the file app/xcf/xcf-private.h in the GIMP sources. Only GIMP itself should define new property types. The number of properties in a property list is not stored explicitly; the last property in the list is identified by having type 0; it must have length 0. XCF readers must skip and ignore property records of unrecognized type, and the length word is there to support such skipping. However, GIMP's own XCF reader will _ignore_ the length word of most properties that it _does_ recognize, and instead reads the amount of payload it knows this property to have. This means that a property record is not itself extensible: one cannot piggyback extra data onto an existing property record by increasing its length. Also, some historical versions of GIMP actually stored the wrong length for some properties, so there are XCF files with misleading property length information in circulation. For maximal compatibility, an XCF reader should endeavor to know the native lengths of as many properties as possible and fall back to the length word only for truly unknown property types. There is not supposed to be more than one instance of each property in a property list, but some versions of GIMP will erroneously emit duplicate properties. An XCF reader that meets a duplicated property should let the content of the later instance take precedence, except for properties that contain lists of subitems, in which the lists should generally be concatenated. An XCF writer should never deliberately duplicate properties within a single property list. Parasites --------- Parasites provide a second level of extensibility. A parasite is analogous to a property, but is identified by a string rather than a number. This makes a larger namespace available for parasites. GIMP plug-ins can access the parasites of an image component through the API and can define their own parasite names which will be ignored by other plug-ins. A list of known parasites and their data formats can be found in the file devel-doc/parasites.txt of the GIMP source tree. The PROP_PARASITE property stores the parasites of the image, layers and channels and the PROP_VECTORS property those of the paths. The number of parasites there is not directly encoded; the list ends when the total length of the parasite data read equals the property payload length. GIMP's XCF reader checks that the combined size of all parasites in the property precisely equals the length word, so it is safe for a reader to use the length word to skip the property without parsing the individual parasites. The parasite content may be binary, but often a textual encoding is chosen in order to spare the writing and reading code of having to deal with byte ordering. There can only be one parasite with a given name attached to each element of the image. Some versions of GIMP will erroneously write some parasites twice in the same property list; XCF readers must be prepared to gracefully ignore all but the last instance of a parasite name in each property list. TODO: How shall parasite readers handle lists in duplicate parasites? Selection --------- If the current selection in the editor is nonempty, then GIMP stores it as a channel in the XCF file. Pixels with a value of 255 belong to the selection; pixels with a value of 0 don't, and pixels with intermediate values are partially selected. Floating selection ------------------ A floating selection is a selection, that is attached to a particular layer, channel or layer mask. Technically it is handled as a layer with alpha. If a floating selection exists, it must always be the first layer in the layer list, but it is not rendered at that position in the layer stack. Instead it is logically attached to another layer, or a channel or layer mask, and the content of the floating selection is combined with ("anchored to") that drawable before it is used to render the visible image. The floating selection must not have a layer mask of its own, but if an ordinary (not floating) selection also exists, it will be used as a layer mask for the floating selection. If a floating selection exists, it must also be the active layer. Because the floating selection is modal and ephemeral, users rarely save XCF files containing a floating selection. It may be acceptable for third-party XCF consumers to ignore the floating selection or explicitly refuse to process it. Tattoos ------- A tattoo is a unique and permanent identifier attached to a drawable or path that can be used to uniquely identify it within an image even between sessions. The tattoo of the image, a layer or channel is stored in the PROP_TATTOO property, a tattoo for a path in the PROP_VECTORS property. The PROP_TATTOO property of the entire image stores a "high-water mark" for the entire image; it is greater than OR EQUAL TO any tattoo for an element of the image. It allows efficient generation of new unused tattoo values and also prevents old tattoo numbers from being reused within a single image, lest plug-ins that use the tattoos for bookkeeping get confused. An XCF file must either provide tattoo values for all its elements or for none of them. GIMP will invent fresh tattoos when it reads in tattoo-less elements, but it does not attempt to keep them different from ones specified explicitly in the file. TODO: can this cause confusion and hard-to-find errors? If so, fix. Text ---- GIMP stores text in plain layers with parasites for the text and formattting and PROP_TEXT_LAYER_FLAGS for flags. Vector paths ------------ GIMP stores vector paths as properties of the image. If all paths are continuous sequences of Bezier strokes, then GIMP uses the PROP_PATHS property, otherwise PROP_VECTORS. PROP_PATHS is for old files from GIMP up to version 1.2. 2. GENERAL PROPERTIES ===================== This chapter describes the formats of the defined property records that can appear in more than one context in an XCF file. PROP_END uint32 0 Type identification uint32 0 PROP_END has no payload. The PROP_END pseudo-property marks the end of any property list. PROP_LINKED (editing state) uint32 9 Type identification uint32 4 Four bytes of payload uint32 linked 1 if the layer is linked; 0 if not PROP_LINKED controls the behavior of Transform tools with a layer, channel or path. If a Transform tool is used to transform one of them all other linked elements will be transformed the same way. It appears in the property list for layers, channels and paths. PROP_LOCK_CONTENT (since version 3, editing state) uint32 28 Type identification uint32 4 Four bytes of payload uint32 locked 1 if the content is locked; 0 if not PROP_LOCK_CONTENT specifies whether the layer, channel or path is locked, i.e. cannot be edited. PROP_OPACITY (essential) uint32 6 Type identification uint32 4 Four bytes of payload uint32 opacity Opacity on a scale from 0 (fully transparent) to 255 (fully opaque) PROP_OPACITY records the overall opacity setting for the layer or channel. It appears in the property list of layers and channels. Note that though GIMP's user interface displays the opacity as a percentage, it is actually stored on a 0-255 scale. Also note that this opacity value is stored as a 32-bit quantity even though it has been scaled to fit exactly in a single byte. When reading old XCF files that lack this property, full opacity should be assumed. PROP_PARASITES uint32 21 Type identification uint32 plength Total length of the following payload data in bytes ,----------------- Repeat for each parasite: | string name Name of the parasite | uint32 flags Flags of the parasite | uint32 pplength Length of the payload data in bytes | byte[n] ... Parasite-specific payload `-- PROP_PARASITES stores parasites. It can contain multiple parasite records. See "Basic concepts" and the file parasites.txt for more information about parasites. This property can appear in any property list. PROP_TATTOO (internal GIMP state) uint32 20 Type identification uint32 4 Four bytes of payload uint32 tattoo Nonzero unsigned integer identifier PROP_TATTOO is an unique identifier for the denoted image, channel or layer. It appears in the property list of layers, channels, and the image. PROP_VISIBLE (essential) uint32 8 Type identification uint32 4 Four bytes of payload uint32 visible 1 if the layer/channel is visible; 0 if not PROP_VISIBLE specifies the visibility of a layer or channel. It appears in the property list for layers and channels. For the visibility of a path see the PROP_VECTORS property. When reading old XCF files that lack this property, assume that layers are visible and channels are not. 3. THE IMAGE STRUCTURE ====================== Header ------ The image structure always starts at offset 0 in the XCF file. byte[9] "gimp xcf " File type identification byte[4] version XCF version "file": version 0 "v001": version 1 "v002": version 2 "v003": version 3 byte 0 Zero marks the end of the version tag. uint32 width Width of canvas uint32 height Height of canvas uint32 base_type Color mode of the image; one of 0: RGB color 1: Grayscale 2: Indexed color (see enum GimpImageBaseType in libgimpbase/gimpbaseenums.h) property-list Image properties ,----------------- Repeat once for each layer, topmost layer first: | uint32 lptr Pointer to the layer structure `-- uint32 0 Zero marks the end of the array of layer pointers. ,------------------ Repeat once for each channel, in no particular order: | uint32 cptr Pointer to the channel structure `-- uint32 0 Zero marks the end of the array of channel pointers. The last 4 characters of the initial 13-character identification string are a version indicator. The version will be higher than 3 if the correct reconstruction of pixel data from the file requires that the reader understands features not described in this specification. On the other hand, optional extra information that can be safely ignored will not cause the version to increase. GIMP's XCF writer dynamically selects the lowest version that will allow the image to be represented. Third-party XCF writers should do likewise. Version numbers from v100 upwards have been used by CinePaint, which originated as a 16-bit fork of GIMP, see "Scope". Image properties ---------------- The following properties are found only in the property list of the image structure. Additionally the list can also contain the properties PROP_END, PROP_PARASITES and PROP_TATTOO, defined in chapter 2. PROP_COLORMAP (essential) uint32 1 Type identification uint32 3*n+4 Payload length in bytes uint32 n Number of colors in the color map (should be <256) ,------------ Repeat n times: | byte r Red component of a color map color | byte g Green component of a color map color | byte b Blue component of a color map color `-- PROP_COLORMAP stores the color map. It appears in all indexed images. The property will be ignored if it is encountered in an RGB or grayscale image. The current GIMP will not write a color map with RGB or grayscale images, but some older ones occasionally did, and readers should be prepared to gracefully ignore it in those cases. Note that in contrast to the palette data model of, for example, the PNG format, an XCF color map does not contain alpha components, and there is no color map entry for "transparent"; the alpha channel of layers that have one is always represented separately. The structure here is that of since XCF version 1. Comments in the GIMP source code indicate that XCF version 0 could not store indexed images in a sane way; contemporary GIMP versions will complain and reinterpret the pixel data as a grayscale image if they meet a version-0 indexed image. Beware that the payload length of the PROP_COLORMAP in particular cannot be trusted: some historic releases of GIMP erroneously wrote n+4 instead of 3*n+4 into the length word (but still actually followed it by 3*n+4 bytes of payload). PROP_COMPRESSION (essential) uint32 17 Type identification uint32 1 One byte of payload byte comp Compression indicator; one of 0: No compression 1: RLE encoding 2: (Never used, but reserved for zlib compression) 3: (Never used, but reserved for some fractal compression) PROP_COMPRESSION defines the encoding of pixels in tile data blocks in the entire XCF file. See chapter 7 for details. Note that unlike most other properties whose payload is always a small integer, PROP_COMPRESSION does _not_ pad the value to a full 32-bit integer. Contemporary GIMP versions always write files with comp=1. It is unknown to the author of this document whether versions that wrote completely uncompressed (comp=0) files ever existed. TODO: Why do we use zlib compression to compress only the whole file (.xcf.gz) if it is a built-in feature? PROP_GUIDES (editing state) uint32 18 Type identification uint32 5*n Five bytes of payload per guide ,--------------- Repeat n times: | int32 coord Guide coordinate | byte o Guide orientation; one of | 1: The guide is horizontal, and coord is a y coordinate | 2: The guide is vertical, and coord is an x coordinate (see enum XcfOrientationType in /app/xcf/xcf-private.h) `-- PROP_GUIDES stores the horizontal or vertical positions of guides. It appears if any guides have been defined. Some old XCF files define guides with negative coordinates; those should be ignored by readers. PROP_PATHS uint32 23 Type identification uint32 plength Total length of the following payload in bytes uint32 aindex Index of the active path uint32 n Number of paths that follow path_1 path_2 ... path_n PROP_PATHS stores the paths. Each path has one of three formats Format 1: Format 2: Format 3: string string string name Name of the path uint32 uint32 uint32 linked 1 if the path is linked; 0 if not byte byte byte state 4 if closed; 2 otherwise (for GIMP 1.2 compatibility) uint32 uint32 uint32 closed 1 if path is closed; 0 otherwise uint32 uint32 uint32 np Number of points uint32=1 uint32=2 uint32=3 version Version indicator uint32 uint32 dummy Ignored; always set to 1 uint32 tattoo 0 if none, or see PROP_TATTOO ,---------- ,---------- ,------------------ Repeat for np points: | int32 | int32 | int32 type Type of point; one of | | | 0: Anchor | | | 1: Bezier control point | | | (for GIMP 1.2 compatibility) | int32 | float | float x X coordinate | int32 | float | float y Y coordinate `-- `-- `-- This format is used to save path data if all paths in the image are continuous sequences of Bezier strokes. Otherwise GIMP stores the paths in PROP_VECTORS. Note: the attribute 'linked' was formerly erroneously called 'locked' (but meant 'linked' anyway). A closed path is a path which has the last and the first point connected, for instance a triangle. GIMP's XCF reader _does not_ check that the total size of all path specifications in the property precisely equals the plength word. Note that this is different to PROP_VECTORS. TODO: Clarify: PROP_PATHS cannot represent parasites for paths, but the XCF writer does not check whether all paths are parasite-less when choosing which property to use, so path parasites may be lost upon saving). Is this by design or a bug? There may be paths that declare a length of 0 points; these should be ignored. PROP_RESOLUTION (not editing state, but not _really_ essential either) uint32 19 Type identification uint32 8 Eight bytes of payload float hres Horizontal resolution in pixels per inch (ppi) float vres Vertical resolution in pixels per inch (ppi) PROP_RESOLUTION gives the intended physical size of the image's pixels. Note that for many images, such as graphics created for the web, the creator does not really have an intended resolution in mind but intends the image to be shown at whatever the natural resolution of the viewer's monitor is. Similarly, photographs commonly do not have a well-defined target size and are intended to be scaled to fit the available space instead. Therefore readers should not interpret the information in this property too rigidly; GIMP writes it to XCF files unconditionally, even if the user has not explicitly chosen a resolution. PROP_SAMPLE_POINTS uint32 17 Type identification uint32 plength Total length of the following payload in bytes ,---------------- Repeat for each sample point: | uint32 x X coordinate | uint32 y Y coordinate `-- PROP_UNIT (editing state) uint32 22 Type identification uint32 4 Four bytes of payload uint32 uid Unit identifier; one of 1: Inches (25.4 mm) 2: Millimeters (1 mm) 3: Points (127/360 mm) 4: Picas (127/30 mm) PROP_UNIT specifies the units used to specify resolution in the Scale Image and Print Size dialogs. Note that this is used only in the user interface; the PROP_RESOLUTION property is always stored in ppi. To specify non-standard units use PROP_USER_UNIT. PROP_USER_UNIT (editing state) uint32 24 Type identification uint32 plength Total length of the following payload in bytes float factor 1 inch divided by the length of the unit uint32 digits Number of decimal digits used with the unit string id An identifier for the unit string symbol Short symbol for the unit string abbrev Abbreviation for the unit string sname Unit name in singular form string pname Unit name in plural form PROP_USER_UNIT allows the use of units that are not on the standard list. It is an alternative to PROP_UNIT. TODO: How is this related to the unitrc file? PROP_VECTORS uint32 25 Type identification uint32 plength Total length of the following payload in bytes uint32 1 Version tag; so far always 1 uint32 aindex Index of the active path uint32 n Number of paths that follow ,---------------------- Repeat n times: | string name Name of the path | uint32 tattoo Tattoo of the path (see PROP_TATTOO), or 0 | uint32 visible 1 if path is visible, 0 if not | uint32 linked 1 if path is linked, 0 if not | uint32 m Number of parasites for the path | uint32 k Number of strokes in the first path | ,-------------------- Repeat m times: | | parasite ... In same format as in PROP_PARASITES. | `-- | ,-------------------- Repeat k times: | | uint32 1 The stroke is a Bezier stroke | | uint32 closed 1 if path is closed; 0 otherwise | | uint32 nf Number of floats given for each point; | | must be >= 2 and <= 6. | | uint32 np Number of control points for this stroke | | ,------------------ Repeat np times: | | | uint32 type Type of the first point; one of | | | 0: Anchor | | | 1: Bezier control point | | | float x X coordinate | | | float y Y coordinate | | | float pressure Only if nf >= 3; otherwise defaults to 1.0 | | | float xtilt Only if nf >= 4; otherwise defaults to 0.5 | | | float ytilt Only if nf >= 5; otherwise defaults to 0.5 | | | float wheel Only if nf == 6; otherwise defaults to 0.5 | | `-- | `-- `-- PROP_VECTORS stores the paths. It appears if all paths are continuous sequences of Bezier strokes; otherwise PROP_PATHS is used. GIMP's XCF reader checks that the total size of all path specifications in the property precisely equals the plength word, so it is safe for a reader to use the plength word to skip the property without parsing the individual parasites. (Note that this is _not_ the case for PROP_PATHS). 4. THE CHANNEL STRUCTURE ======================== Channel structures are pointed to from layer structures (in case of layer masks) or from the master image structure (for all other channels). uint32 width Width of the channel uint32 height Height of the channel string name Name of the channel property-list Channel properties uint32 hptr Pointer to the hierarchy structure with the pixels The width and height of the channel must be the same as those of its parent structure (the layer in the case of layer masks; the canvas for all other channels). Channel properties ------------------ The following properties are found only in the property list of channel structures. Additionally the list can also contain the properties PROP_END, PROP_LINKED, PROP_LOCK_CONTENT, PROP_OPACITY, PROP_PARASITES, PROP_TATTOO and PROP_VISIBLE, defined in chapter 2. PROP_ACTIVE_CHANNEL (editing state) uint32 3 Type identification uint32 0 PROP_ACTIVE_CHANNEL has no payload The presence of PROP_ACTIVE_CHANNEL indicates that the channel is the currently active channel. It appears in the property list of the currently active channel. Only zero or one channel must have this property at any time. PROP_COLOR uint32 16 Type identification uint32 3 Three bytes of payload byte r Red component of color byte g Green component of color byte b Blue component of color PROP_COLOR gives the color of the screen that is used to represent the channel when it is visible in the UI. (The alpha of the screen is given as the channel's PROP_OPACITY). TODO: What exactly does "screen" mean here? PROP_SELECTION (editing state?) uint32 4 Type identification uint32 0 PROP_SELECTION has no payload PROP_SELECTION appears in the property list of the channel structure that represents the selection mask. PROP_SHOW_MASKED (editing state) uint32 14 Type identification uint32 4 Four bytes of payload uint32 masked 1 if the channel is shown as a mask, 0 if not PROP_SHOW_MASKED specifies whether a channel is shown as a mask. 5. THE LAYER STRUCTURE ====================== Layer structures are pointed to from a list of layer pointers in the master image structure. uint32 width Width of the layer uint32 height Height of the layer uint32 type Color mode of the layer: one of 0: RGB color without alpha 1: RGB color with alpha 2: Grayscale without alpha 3: Grayscale with alpha 4: Indexed without alpha 5: Indexed with alpha (see enum GimpImageType in libgimpbase/gimpbaseenums.h) string name Name of the layer property-list Layer properties uint32 hptr Pointer to the hierarchy structure with the pixels uint32 mptr Pointer to the layer mask (a channel structure), or 0 The color mode of a layer must match that of the entire image. All layers except the bottommost one _must_ have an alpha channel. The bottom layer _can_ have an alpha channel. TODO: Check whether the redundant color mode storage potentially causes errors. Wouldn't a alpha bit/flag be sufficient? Exception: If the layer is a floating selection and is attached to a channel or layer mask, then its color mode must be 3 (grayscale with alpha). Layer properties ---------------- The following properties are found only in the property list of layer structures. Additionally the list can also contain the properties PROP_END, PROP_LINKED, PROP_LOCK_CONTENT, PROP_OPACITY, PROP_PARASITES, PROP_TATTOO and PROP_VISIBLE, defined in chapter 2. PROP_ACTIVE_LAYER (editing state) uint32 2 Type identification uint32 0 PROP_ACTIVE_LAYER has no payload The presence of PROP_ACTIVE_LAYER indicates that the channel is the currently active layer. Only zero or one layer must have this property at any time. PROP_APPLY_MASK (essential) uint32 11 Type identification uint32 4 Four bytes of payload uint32 apply 1 if the layer mask should be applied, 0 if not PROP_APPLY_MASK specifies whether the layer mask shall be applied to the layer. If the property does not appear for a layer which has a layer mask, it defaults to true. Robust readers should force this to false if the layer has no layer mask. Writers should never save this as true unless the layer has a layer mask. PROP_EDIT_MASK (editing state) uint32 12 Type identification uint32 4 Four bytes of payload uint32 editing 1 if the layer mask is currently being edited, 0 if not PROP_EDIT_MASK specifies whether the layer mask is currently being edited. If the property does not appear for a layer which has a layer mask, it defaults to false. Robust readers should force this to false if the layer has no layer mask. Writers should never save this as true unless the layer has a layer mask. PROP_FLOATING_SELECTION (essential) uint32 5 Type identification uint32 4 Four bytes of payload uint32 ptr Pointer to the layer or channel the floating selection is attached to PROP_FLOATING_SELECTION indicates that the layer is the floating selection and specifies the pointer to the layer, channel and layer mask it is attached to. It appears in the property list for the layer that is the floating selection. Only zero or one layer must have this property at any time. PROP_GROUP_ITEM (since version 3) uint32 29 Type identification uint32 0 PROP_GROUP_ITEM has no payload PROP_GROUP_ITEM indicates that the layer is a layer group. It appears in the property list if the layer is a layer group. PROP_ITEM_PATH (since version 3) uint32 30 Type identification uint32 plength Total length of the following payload in bytes item-path List of pointers, represented as uint32 values TODO: The code reads that it is a list of pointers, represented as uint32 integers and somehow in the context of layers. What this is for and what do the property values mean? PROP_GROUP_ITEM_FLAGS (since version 3) uint32 31 Type identification uint32 4 Four bytes of payload uint32 flags Flags for the layer, or'ed together from the following set: 0x00000001 Layer group is expanded. (see enum XcfGroupItemFlagsType in app/xcf/xcf-private.h) PROP_GROUP_ITEM_FLAGS specifies flags for the layer group. It appears in the property list if the layer is a layer group. PROP_LOCK_ALPHA (editing state) (called PROP_PRESERVE_TRANSPARENCY in GIMP before 2.3) uint32 10 Type identification uint32 4 Four bytes of payload uint32 lock_alpha 1 if alpha is locked; 0 if not PROP_LOCK_ALPHA prevents all drawing tools in GIMP from increasing the alpha of any pixel in the layer. Decreasing the alpha is possible. PROP_MODE (essential) uint32 7 Type identification uint32 4 Four bytes of payload unit32 mode Layer mode; one of 0: Normal 1: Dissolve (random dithering to discrete alpha) 2: (Behind: not selectable in the GIMP UI. TODO: why not?) 3: Multiply 4: Screen 5: Overlay 6: Difference 7: Addition 8: Subtract 9: Darken Only 10: Lighten Only 11: Hue (H of HSV) 12: Saturation (S of HSV) 13: Color (H and S of HSL) 14: Value (V of HSV) 15: Divide 16: Dodge 17: Burn 18: Hard Light 19: Soft Light (since version 2) 20: Grain Extract (since version 2) 21: Grain Merge (since version 2) PROP_MODE specifies the layer mode. When reading old XCF files that lack this property, assume mode==0. The effects of the various layer modes are defined in the document compositing.txt. Beware that GIMP ignores all other layer modes than Normal and Dissolve for the bottommost visible layer of the image. If a mode>=3 has been specified for this layer it will interpreted as mode==0 (Normal) for display and flattening purposes. This effect happens for one layer only: even if the bottommost visible layer covers only some (or none) of the canvas, it will be the only layer to have its mode forced to Normal. TODO: The layer modes 'Overlay' and 'Soft light' are identical. If this information is still valid - is this state subject to change in GEGL? Should we continue providing two different names for the same thing? PROP_OFFSETS (essential) uint32 15 Type identification uint32 8 Eight bytes of payload int32 xoffset Horizontal offset int32 yoffset Vertical offset PROP_OFFSETS gives the coordinates of the upper left corner of the layer relative to the upper left corner of the canvas. The coordinates can be negative; this corresponds to a layer that extends to the left of or above the canvas boundary. When reading old XCF files that lack this property, assume (0,0). PROP_SHOW_MASK (editing state) uint32 13 Type identification uint32 4 Four bytes of payload uint32 visible 1 if the layer mask is visible, 0 if not PROP_SHOW_MASK specifies whether the layer mask is visible. If the property does not appear for a layer which has a layer mask, it defaults to false. Robust readers should force this to false if the layer has no layer mask. Writers should never save this as true unless the layer has a layer mask. PROP_TEXT_LAYER_FLAGS uint32 26 Type identification uint32 4 Four bytes of payload uint32 flags Flags, or'ed together from the following set: 0x00000001 Do _not_ change the layer name if the text content is changed 0x00000002 The pixel data has been painted to or otherwise modified since the text was rendered. (see the anonymous enum in app/text/gimptextlayer-xcf.c) PROP_TEXT_LAYER_FLAGS specifies the text layer behavior by flags. It appears in property lists for text layers. The actual text (and other parameters such as font and color) is a parasite rather than a property. 6. THE HIERARCHY STRUCTURE ========================== A hierarchy contains data for a rectangular array of pixels. It appears in a context: each layer and channel has a pointer to its hierarchy. uint32 width Width of the pixel array uint32 height Height of the pixel array uint32 bpp Number of bytes per pixel given 3: RGB color without alpha 4: RGB color with alpha 1: Grayscale without alpha 2: Grayscale with alpha 1: Indexed without alpha 2: Indexed with alpha uint32 lptr Pointer to the "level" structure ,--------------- Repeat zero or more times | uint32 dlevel Pointer to an unused level structure (dummy level) `-- uint32 0 Zero marks the end of the list of level pointers. The width, height and bpp values are for consistency checking; their correct values can always be inferred from the context, and are checked when GIMP reads the XCF file. Levels ------ The level structure is laid out as follows: uint32 width Width of the pixel array uint32 height Height of the pixel array ,-------------- Repeat for each of the ceil(width/64)*ceil(height/64) tiles | uint32 tptr Pointer to tile data `-- uint32 0 Zero marks the end of the array of tile pointers. The width and height must be the same as the ones recorded in the hierarchy structure (except for the aforementioned dummy levels). Ceil(x) is the smallest integer not smaller than x. 7. TILE DATA ORGANIZATION ========================= The format of the data blocks pointed to by the tile pointers in the level structure of hierarchy differs according to the value of the PROP_COMPRESSION property of the main image structure. Current GIMP versions always use RLE compression, but readers should nevertheless be prepared to meet the older uncompressed format. Both formats assume the width, height and byte depth of the tile are known from the context (namely, they are stored explicitly in the hierarchy structure for regular tiles). Both encodings store a linear sequence of width*height pixels, extracted from the tile in row-major, top-to-bottom, left-to-right order (the same as the reading direction of multi-line English text). In color modes with alpha information, the alpha value is the last of the 2 or 4 bytes for each pixel. In RGB color modes, the 3 (first) bytes for each pixel is the red intensity, the green intensity, and the blue intensity, in that order. TODO: How is this in master, when we work with higher bit depths? Uncompressed tile data ---------------------- In the uncompressed format the file first contains all the bytes for the first pixel, then all the bytes for the second pixel, and so on. RLE compressed tile data ------------------------ In the Run-Length Encoded format, each tile consists of a run-length encoded stream of the first byte of each pixel, then a stream of the second byte of each pixel, and so forth. In each of the streams, multiple occurrences of the same byte value are represented in compressed form. The representation of a stream is a series of operations; the first byte of each operation determines the format and meaning of the operation (opcode): byte n For 0 <= n <= 126: a short run of identical bytes byte v Repeat this value n+1 times or byte 127 A long run of identical bytes byte p byte q byte v Repeat this value p*256 + q times or byte 128 A long run of different bytes byte p byte q byte[p*256+q] data Copy these verbatim to the output stream or byte n For 129 <= n <= 255: a short run of different bytes byte[256-n] data Copy these verbatim to the output stream The end of the stream for "the first byte of all pixels" (and the following similar streams) must occur at the end of one of these operations; it is not permitted to have one operation span the boundary between streams. The RLE encoding can cause degenerated encodings in which the original data stream may double in size (or grow to arbitrarily large sizes if (128,0,0) operations are inserted). Such encodings must be avoided, as GIMP's XCF reader expects that the size of an encoded tile is never more than 24 KB, which is only 1.5 times the unencoded size of a 64x64 RGBA tile. A simple way for an XCF creator to avoid overflow is a) never using opcode 0 (but instead opcode 255) b) using opcodes 127 and 128 only for lengths larger than 127 c) never emitting two "different bytes" opcodes next to each other in the encoding of a single stream. TODO: If each tile has a maximum of 64 pixels (resulting in a maximum of 64 bytes for each color in this tile), do values>64 and long runs apply at all? 8. MISCELLANEOUS ================ The name XCF ------------ The name XCF honors GIMP's origin at the eXperimental Computing Facility of the University of California at Berkeley. TODO: Integrate this document into the API doc. TODO: Some properties are denoted with "essential", "editing state", "not editing state, but not really essential either". What did the original author Henning Makholm mean? TODO: What will happen with the format after the GEGL port? AFAIK the ORA format will play a big role in the GEGL context (correct me if I'm wrong). Will XCF be dropped then or will ORA then be yet another import/export format like PSD etc.?