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The X Rendering Extension Version 0.0.15 2000-11-19 Keith Packard keithp@xfree86.org 1. Introduction The X Rendering Extension (Render) introduces digital image composition as the foundation of a new rendering model within the X Window System. Rendering geometric figures is accomplished by client-side tesselation into either triangles or trapezoids. Text is drawn by loading glyphs into the server and rendering sets of them. 2. Acknowledgments This extension was the work of many people, in particular: + Thomas Porter and Tom Duff for their formal description of image compositing. + Rob Pike and Russ Cox who designed the Plan 9 window system from which the compositing model was lifted. + Juliusz Chroboczek and Raph Levien whose proposal for client-side glyph management eliminated font handling from the X server. + Jon Leech, Brad Grantham and Allen Akin for patiently explaining how OpenGL works. 3. Rendering Model Render provides a single rendering operation which can be used in a variety of ways to generate images: dest = (source IN mask) OP dest Where 'IN' is the Porter/Duff operator of that name and 'OP' is any of the list of compositing operators described below, among which can be found all of the Porter/Duff binary operators. To use this operator several additional values are required: + The destination rectangle. This is a subset of the destination within which the rendering is performed. + The source location. This identifies the coordinate in the source aligned with the upper left corner of the destination rectangle. + The mask location. This identifies the coordinate in the mask aligned with the upper left corner of the destination rectangle. + A clip list. This limits the rendering to the intersection of the destination rectangle with this clip list. + The OP to use + Whether the source should be repeated to cover the destination rectangle or whether rendering should be clipped by the source + Whether the mask should be repeated to cover the destination rectangle or whether rendering should be clipped by the mask + Whether the mask has a single alpha value for all four channels or whether each mask channel should affect the associated source/dest channels. These parameters are variously attached to the operands or included in each rendering request. 4. Data types The core protocol rendering system uses a pixel model and applies color only in the final generation of the video signal. A compositing model operates on colors, not pixel values so a new datatype is needed to interpret data as color instead of just bits. The "PictFormat" object holds information needed to translate pixel values into red, green, blue and alpha channels. The server has a list of picture formats corresponding to the various visuals on the screen. There are two classes of formats, Indexed and Direct. Indexed PictFormats hold a list of pixel values and RGBA values while Direct PictFormats hold bit masks for each of R, G, B and A. The "Picture" object contains a Drawable, a PictFormat and some rendering state. More than one Picture can refer to the same Drawable. 5. Errors Errors are sent using core X error reports. PictFormat A value for a PICTFORMAT argument does not name a defined PICTFORMAT. Picture A value for a PICTURE argument does not name a defined PICTURE. PictOp A value for a PICTOP argument does not name a defined PICTOP. GlyphSet A value for a GLYPHSET argument does not name a defined GLYPHSET. Glyph A value for a GLYPH argument does not name a defined GLYPH in the glyphset. 6. Protocol Types PICTURE 32-bit value (top three bits guaranteed to be zero) PICTFORMAT 32-bit value (top three bits guaranteed to be zero) PICTTYPE { Indexed, Direct } PICTOP { Clear, Src, Dst, Over, OverReverse, In, InReverse, Out, OutReverse, Atop, AtopReverse, Xor, Add, Saturate } COLOR [ red, green, blue, alpha: CARD16 ] CHANNELMASK [ shift, mask: CARD16 ] DIRECTFORMAT [ red, green, blue, alpha: CHANNELMASK ] INDEXVALUE [ pixel: Pixel; red, green, blue, alpha: CARD16 ] PICTFORMINFO [ id: PICTFORMAT type: PICTTYPE depth: CARD8 direct: DIRECTFORMAT colormap: COLORMAP or None ] PICTVISUAL [ visual: VISUALID or None format: PICTFORMAT ] PICTDEPTH [ depth: CARD8 visuals: LISTofPICTVISUAL ] PICTSCREEN LISTofPICTDEPTH DITHERINFO [ name: ATOM format: PICTFORMAT width: CARD16 height: CARD16 ] FIXED 32-bit value (top 24 are integer portion, bottom 8 are fraction) POINTFIX [ x, y: FIXED ] POLYEDGE { Sharp, Smooth } POLYMODE { Precise, Imprecise } COLORPOINT [ point: POINTFIX color: COLOR ] SPANFIX [ left, right, y: FIXED ] COLORSPANFIX [ left, right, y: FIXED left_color: COLOR right_color: COLOR QUAD [ p1, p2, p3, p4: POINTFIX ] TRIANGLE [ p1, p2, p3: POINTFIX ] TRAP [ top, bottom: SPANFIX ] COLORTRIANGLE [ p1, p2, p3: COLORPOINT ] COLORTRAP [ top, bottom: COLORSPANFIX ] GLYPHSET 32-bit value (top three bits guaranteed to be zero) GLYPH 32-bit value GLYPHINFO [ width, height: CARD16 x, y: INT16 off-x, off-y: INT16 ] PICTGLYPH [ info: GLYPHINFO x, y: INT16 ] GLYPHABLE GLYPHSET or FONTABLE GLYPHELT8 [ dx, dy: INT16 glyphs: LISTofCARD8 ] GLYPHITEM8 GLYPHELT8 or GLYPHABLE GLYPHELT16 [ dx, dy: INT16 glyphs: LISTofCARD16 ] GLYPHITEM16 GLYPHELT16 or GLYPHABLE GLYPHELT32 [ dx, dy: INT16 glyphs: LISTofCARD32 ] GLYPHITEM32 GLYPHELT32 or GLYPHABLE 7. Standard PictFormats The server must support a Direct PictFormat with 8 bits each of red, green, blue and alpha as well as a Direct PictFormat with 8 bits of red, green and blue and 0 bits of alpha. The server must also support Direct PictFormats with 1, 4 and 8 bits of alpha and 0 bits of r, g and b. Pixel component values lie in the close range [0,1]. These values are encoded in a varying number of bits. Values are encoded in a straight forward manner. For a component encoded in m bits, a binary encoding b is equal to a component value of b/(2^m-1). A Direct PictFormat with zero bits of alpha component is declared to have alpha == 1 everywhere. A Direct PictFormat with zero bits of red, green and blue is declared to have red, green, blue == 0 everywhere. If any of red, green or blue components are of zero size, all are of zero size. Direct PictFormats never have colormaps and are therefore screen independent. Indexed PictFormats never have alpha channels and the direct component is all zeros. Indexed PictFormats always have a colormap in which the specified colors are allocated read-only and are therefore screen dependent. 8. Compositing Operators For each pixel, the four channels of the image are computed with: C = Ca * Fa + Cb * Fb where C, Ca, Cb are the values of the respective channels and Fa and Fb come from the following table: PictOp Fa Fb ------------------------------------------ Clear 0 0 Src 1 0 Dst 0 1 Over 1 1-Aa OverReverse 1-Ab 1 In Ab 0 InReverse 0 Aa Out 1-Ab 0 OutReverse 0 1-Aa Atop Ab 1-Aa AtopReverse 1-Ab Aa Xor 1-Ab 1-Aa Add 1 1 Saturate min(1,(1-Ab)/Aa) 1 --- Saturate matches GL with FUNC_ADD, SRC_ALPHA_SATURATE, ONE, except that it uses premultiplied alphas while GL uses non-premultiplied alphas. Remember the idea is to apply (src In mask) Saturate Dst so that computing (src In mask) effectively applies alpha values of 'mask' to src; the server could 'short circuit' that computation by only multiplying the alpha channel and then applying the regular GL SRC_ALPHA_SATURATE operator. --- The result of any compositing operator is always limited to the range [0,1] for each component. Components whose value would be greater than 1 are set to 1. When the mask contains separate alpha values for each channel, the alpha value resulting from the combination of that value with the source alpha channel is used in the final image composition. 9. Polygon Rasterization All polygons must be convex. Rendering of concave polygons is unspecified except that the result must obey the clipping rules. Each polygon request fills the region closed by the specified path. The path is automatically closed if the last point does not coincide with the first point. A point is infinitely small and the path is an infinitely thin line. A pixel is inside if the center point of the pixel is inside and the center point is not on the boundary. If the center point is on the boundary, the pixel is inside if and only if the polygon interior is immediately to its right (x increasing direction). Pixels with centers along a horizontal edge are a special case and are inside if and only if the polygon interior is immediately below (y increasing direction). A polygon contains a pixel if the pixel is inside the polygon. Polygons are rasterized by implicit generating an alpha mask and using that in the general compositing operator along with a supplied source image: tmp = Rasterize (polygon) Composite (op, dst, src, tmp) When rasterized with Sharp edges, the mask is generated by setting pixels inside the polygon to 1 and pixels outside the mask to 0. When rasterized with Smooth edges, the mask is generated by creating a square around each pixel coordinate and computing the amount of that square covered by the polygon. Yes, this ignores sampling theory but it provides a precise definition which is close to the right answer. This value is truncated to the alpha width in the fallback format before application of the compositing operator. When rasterized in Precise mode, the pixelization will match this specification exactly. When rasterized in Imprecise mode, the pixelization may deviate from this specification by up to 1/2 pixel along any edge subject to the following constraints: + Abutting edges must match precisely. When specifying two polygons abutting along a common edge, if that edge is specified with the same coordinates in each polygon then the sum of alpha values for pixels inside the union of the two polygons must be precisely one. + Translationally invarient. The pixelization of the polygon must be the same when either the polygon or the target drawable are translated by any whole number of pixels in any direction. + Sharp edges are honored. When the polygon is rasterized with Sharp edges, the implicit alpha mask will contain only 1 or 0 for each pixel. + Order independent. Two identical polygons specified with vertices in different orders must generate identical results. Polygons can also be specified with colors for each vertex. These color values are interpolated along the edges and across each scanline. When rasterized in Precise mode, the interpolated colors are exact. When rasterized in Imprecise mode, the color of each pixel may optionally be interpolated from a triangle containing the pixel which is formed from any three polygon vertices. Any interpolated color value can err up to 1 lsb in each channel. 10. Glyph Rendering Glyphs are small alpha masks which can be stored in the X server and rendered by referring to them by name. A set of glyphs can be rendered in a single request. Glyphs are positioned by subtracting the x, y elements of the GLYPHINFO from the requested rendering position. The next glyph rendering position is set to the current rendering position plus the off-x and off-y elements. Glyphs are stored in GlyphSets and are named within the GlyphSet with client-specified 32-bit numbers. Glyphs can be stored in any PictFormat supported by the server. All glyphs in a GlyphSet are stored in the same format. 11. Dithering Each screen supports a list of dithers. There are several standard dithers with defined pixelization, the server is free to offer others as well. The width and height of the dither are a hint about the size of the matrix used if the dither is ordered. An unordered dither will have zero in these fields. The standard dithers are: "Standard2x2" "Standard4x4" "Standard128x128" --- Need a notation for specifying pixelization of dithers. --- 12. Extension Initialization The client must negotiate the version of the extension before executing extension requests. Behavior of the server is undefined otherwise. QueryVersion client-major-version: CARD32 client-minor-version: CARD32 -> major-version: CARD32 minor-version: CARD32 The client sends the highest supported version to the server and the server sends the highest version it supports, but no higher than the requested version. Major versions changes can introduce incompatibilities in existing functionality, minor version changes introduce only backward compatible changes. It is the clients responsibility to ensure that the server supports a version which is compatible with its expectations. QueryPictFormats -> fallback: PICTFORMAT formats: LISTofPICTFORMINFO screens: LISTofPICTSCREEN The server responds with a list of supported PictFormats and a list of which PictFormat goes with each visual on each screen. Every PictFormat must match a supported depth, but not every PictFormat need have a matching visual. The fallback format is used as an intermediate representation in cases where there is no ideal choice. QueryPictIndexValues format: PICTFORMAT -> values: LISTofINDEXVALUE Errors: PictFormat, Match Returns the mapping from pixel values to RGBA values for the specified Indexed PictFormat. If 'format' does not refer to an Indexed PictFormat a Match error is generated. QueryDithers drawable: DRAWABLE -> dithers: LISTofDITHERINFO Returns all of the supported dithers on the screen specified by drawable. 13. Extension Requests CreatePicture pid: PICTURE drawable: DRAWABLE format: PICTFORMAT value-mask: BITMASK value-list: LISTofVALUE Errors: Alloc, Drawable, IDChoice, Match, Pixmap, Picture, PictFormat, Value This request creates a Picture object associated with the specified drawable and assigns the identifier pid to it. Pixel data in the image are interpreted according to 'format'. It is a Match error to specify a format with a different depth than the drawable. If the drawable is a Window then the Red, Green and Blue masks must match those in the visual for the window else a Match error is generated. The value-mask and value-list specify attributes of the picture that are to be explicitly initialized. The possible values are: repeat: BOOL alpha-map: PICTURE or None alpha-x-origin: INT16 alpha-y-origin: INT16 clip-x-origin: INT16 clip-y-origin: INT16 clip-mask: PIXMAP or None graphics-exposures: BOOL subwindow-mode: { ClipByChildren, IncludeInferiors } poly-edge: POLYEDGE poly-mode: POLYMODE dither: ATOM or None component-alpha: BOOL The repeat value controls whether the image is replicated when used as the source or mask in a rendering operation. When True, the contents are tiled over the destination instead of clipping to the geometry of the drawable. The alpha channel of alpha-map is used in place of any alpha channel contained within the drawable for all rendering operations. The alpha-mask origin is interpreted relative to the origin of drawable. Rendering is additionally clipped by the geometry of alpha-map. Exposures to the window do not affect the contents of alpha-map. Alpha-map must refer to a picture containing a Pixmap, not a Window (or a Match error results). The clip-mask restricts reads and writes to drawable. Only pixels where the clip-mask has bits set to 1 are read or written. Pixels are not accessed outside the area covered by the clip-mask or where the clip-mask has bits set to 0. The clip-mask affects all graphics requests, including sources. The clip-mask origin is interpreted relative to the origin of drawable. If a pixmap is specified as the clip-mask, it must have depth 1 and have the same root as the drawable (or a Match error results). If clip-mask is None, then pixels are always drawn, regardless of the clip origin. The clip-mask can also be set with the SetPictureClipRectangles request. For ClipByChildren, both source and destination windows are additionally clipped by all viewable InputOutput children. For IncludeInferiors , neither source nor destination window is clipped by inferiors. This will result in including subwindow contents in the source and drawing through subwindow boundaries of the destination. The use of IncludeInferiors with a source or destination window of one depth with mapped inferiors of differing depth is not illegal, but the semantics are undefined by this extension. The graphics-exposures flag controls GraphicsExposure event generation for Composite and Transform requests (and any similar requests defined by additional extensions). Poly-edge and poly-mode control the rasterization of polygons as described above. Dither selects which of the available dither patterns should be used. If dither is None, no dithering will be done. Component-alpha indicates whether each image component is intended as a separate alpha value when the picture is used as a mask operand. The default component values are Component Default ------------------------------- op Over repeat False clip-x-origin 0 clip-y-origin 0 clip-mask None graphics-exposures True subwindow-mode ClipByChildren poly-edge Smooth poly-mode Precise dither None component-alpha False ChangePicture pid: PICTURE value-mask: BITMASK value-list: LISTofVALUE Errors: Picture, Alloc, Pixmap, PictOp, Value The value-mask and value-list specify which attributes are to be changed. The values and restrictions are the same as for CreatePicture. SetPictureClipRectangles picture: PICTURE clip-x-origin: INT16 clip-y-origin: INT16 rectangles: LISTofRECTANGLE Errors: Alloc, Picture This request changes clip-mask in picture to the specified list of rectangles and sets the clip origin. Input and output will be clipped to remain contained within the rectangles. The clip origin is interpreted relative to the origin of the drawable associated with picture. The rectangle coordinates are interpreted relative to the clip origin. Note that the list of rectangles can be empty, which effectively disables output. This is the opposite of passing None as the clip-mask in CreatePicture and ChangePicture. Note that output is clipped to the union of all of the rectangles and that no particular ordering among the rectangles is required. FreePicture pid: PICTURE Errors: Picture This request deletes the association between the resource ID and the picture and destroys the picture. Composite op: PICTOP src: PICTURE mask: PICTURE or None dst: PICTURE src-x, src-y: INT16 mask-x, mask-y: INT16 dst-x, dst-y: INT16 width, height: CARD16 This request combines the specified rectangle of src and mask with the specified rectangle of dst using op as the compositing operator. The coordinates are relative their respective drawable's origin. Rendering is clipped to the geometry of the dst drawable and then to the dst clip-list, the src clip-list and the mask clip-list. If the specified rectangle extends beyond src, then if src has the repeat attribute set, the src picture will be tiled to fill the specified rectangle, otherwise rendering is clipped to the src geometry. If the specified rectangle extends beyond mask, then if mask has the repeat attribute set, the mask picture will be tiled to fill the specified rectangle, otherwise rendering is clipped to the mask geometry. If src, mask and dst are not in the same format, and one of their formats can hold all without loss of precision, they are converted to that format. Alternatively, the server will convert each operand to the fallback format. If mask is None, it is replaced by a constant alpha value of 1. When dst has clip-notify set, a NoExpose event is sent if the rendering operation was not clipped by either src or mask, otherwise a sequence of GraphicsExpose events are sent covering areas in dst where rendering was clipped by src or mask. Scale color-scale: CARD32 alpha-scale: CARD32 src: PICTURE dst: PICTURE src-x, src-y: INT16 dst-x, dst-y: INT16 width, height: CARD16 This request replaces the specified rectangle in dst with the specified rectangle of src with the components multiplied in the following fashion: dst-red = src-red * color-scale / 65536 dst-green = src-green * color-scale / 65536 dst-blue = src-blue * color-scale / 65536 dst-alpha = src-alpha * alpha-scale / 65536 The coordinates are relative their respective drawable's origin. Rendering is clipped to the geometry of the dst drawable and then to the dst clip-list, the src clip-list and the mask clip-list. If the specified rectangle extends beyond src, then if src has the repeat attribute set, the src picture will be tiled to fill the specified rectangle, otherwise rendering is clipped to the src geometry. FillRectangles op: PICTOP dst: PICTURE color: COLOR rects: LISTofRECTANGLE This request combines color with the destination drawable in the area specified by rects. Each rectangle is combined separately; overlapping areas will be rendered multiple times. The effect is equivalent to compositing with a repeating source picture filled with the specified color. Trapezoids op: PICTOP src: PICTURE src-x, src-y: INT16 dst: PICTURE traps: LISTofTRAP This request rasterizes the list of trapezoids. For each span, the left coordinate must be less than or equal to the right coordinate. The y coordinate of the top span must be less than or equal to the y coordinate of the bottom span. Results are undefined otherwise. Triangles op: PICTOP src: PICTURE src-x, src-y: INT16 dst: PICTURE traps: LISTofTRIANGLE This request rasterizes the list of triangles in the order they occur in the list. TriStrip op: PICTOP src: PICTURE src-x, src-y: INT16 dst: PICTURE points: LISTofPOINTFIX Triangles are formed by initially using the first three points and then by eliminating the first point and appending the next point in the list. If fewer than three points are provided, this request does nothing. TriFan op: PICTOP src: PICTURE src-x, src-y: INT16 dst: PICTURE points: LISTofPOINTFIX Triangles are formed by initially using the first three points and then by eliminating the second point and appending the next point int the list. If fewer than three points are provided, this request does nothing. ??? Should I bother with these two compressed triangle representations? ??? ColorTrapezoids op: PICTOP dst: PICTURE triangles: LISTofCOLORTRAP The geometry of the trapezoids must meet the same requirements as for the Trapezoids request. The trapezoids are filled in the order they occur in the list. ColorTriangles op: PICTOP dst: PICTURE triangles: LISTofCOLORTRIANGLE The colored triangles are rasterized in the order they occur in the list. ??? Should I included compressed triangle representations here? ??? Transform op: PICTOP src: PICTURE dst: PICTURE src-quad: QUAD dst-quad: QUAD filter: { Nearest, ... } Errors: Picture, Value This request combines the specified quadrilateral of src with the specified quadrilateral of dst using op as the compositing operator. The coordinates are relative their respective drawable's origin. Rendering is clipped to the geometry of the dst drawable and then to the dst clip-list and the src clip-list. If the specified rectangle extends beyond src, then if src has the repeat attribute set, the src picture will be tiled to fill the specified rectangle, otherwise rendering is clipped to the src geometry. If the specified rectangle extends beyond mask, then if mask has the repeat attribute set, the mask picture will be tiled to fill the specified rectangle, otherwise rendering is clipped to the mask geometry. The effect of this request is: tmp_image = affine-transform (src, src-quad * dst-quad) tmp_mask = render (dst-quad) Composite (op, dst, tmp_image, tmp_mask) That is, the entire transformed source image is masked by an image of the destination quadrilateral and rendered using the Composite operator. If the specified quadrilateral extends beyond src, then if src has the repeat attribute set, the src picture will be tiled to fill the specified rectangle, otherwise rendering is clipped to the src geometry. It is a Value error to specify a self intersecting quadrilateral for either src-quad or dst-quad. If src and dst are not in the same format, and one of their formats can hold both without loss of precision, they are converted to that format. Alternatively, the server will convert each operand to the fallback format. The compositing operator from the src picture is used to merge the images together. If filter is Nearest, then the nearest (converted) pixel values to each destination pixel is used without averaging. When dst has clip-notify set, a NoExpose event is sent if the rendering operation was not clipped by src, otherwise a sequence of GraphicsExpose events are sent covering areas in dst where rendering was clipped by src. ??? What (small) set of filters should be included ??? --- Need to describe in more detail the semantics here Looks like the geometric extension needs to be tied to the compositing extension (sigh). --- CreateGlyphSet gsid: GLYPHSET format: PICTFORMAT Errors: Alloc, IDChoice, PictFormat, Match This request creates a container for glyphs. The glyphset and all contained glyphs are destroyed when gsid and any other names for the glyphset are freed. Format must be a Direct format, when it contains RGB values, the glyphs are composited using component-alpha True, otherwise they are composited using component-alpha False. ReferenceGlyphSet gsid: GLYPHSET existing: GLYPHSET Errors: Alloc, IDChoice, GlyphSet This request creates an additional name for the existing glyphset. The glyphset will not be freed until all references to it are destroyed. FreeGlyphSet glyphset: GLYPHSET Errors: GlyphSet This request frees the name for the glyphset. When all names have been freed, the glyphset and all contained glyphs are freed. AddGlyphs glyphset: GLYPHSET glyphids: LISTofCARD32 glyphs: LISTofGLYPHINFO data: LISTofBYTE Errors: GlyphSet, Alloc This request adds glyphs to glyphset. The image for the glyphs are stored with each glyph in a separate Z-format image padded to a 32-bit boundary. Existing glyphs with the same names are replaced. AddGlyphsFromPicture glyphset: GLYPHSET src: PICTURE glyphs: LISTofPICTGLYPH Errors: GlyphSet, Alloc This request adds glyphs to glyphset by copying them from src from the locations included in glyphs. Existing glyphs with the same names are replaced. Src may be in a different PictFormat than glyphset, in which case the images are converted to the glyphset format. FreeGlyphs glyphset: GLYPHSET glyphs: LISTofGLYPH Errors: GlyphSet, Match This request removes glyphs from glyphset. Each glyph must exist in glyphset (else a Match error results). CompositeGlyphs8 CompositeGlyphs16 CompositeGlyphs32 op: PICTOP src: PICTURE dst: PICTURE mask-format: PICTFORMAT or None glyphset: GLYPHABLE src-x, src-y: INT16 dst-x, dst-y: INT16 glyphcmds: LISTofGLYPHITEM8 CompositeGlyphs8 glyphcmds: LISTofGLYPHITEM16 CompositeGlyphs16 glyphcmds: LISTofGLYPHITEM32 CompositeGlyphs32 Errors: Picture, PictOp, PictFormat, GlyphSet, Glyph The dst-x and dst-y coordinates are relative to the drawable's origin and specify the baseline starting position (the initial glyph origin). Each glyph item is processed in turn. A glyphset item causes the glyhpset to be used for subsequent glyphs. Switching among glyphsets does not affect the next glyph origin. A glyph element delta-x and delta-y specify additional changes in the position along the x and y axes before the string is drawn; the deltas are always added to the glyph origin. All contained GLYPHSETs are always transmitted most significant byte first. If a GlyphSet error is generated for an item, the previous items may have been drawn. When mask-format is not None, glyphs are rendered in the following way with the effective mask computed in mask-format: tmp = temporary alpha picture Combine (Zero, tmp, tmp, None) for each glyph Combine (Add, tmp, glyph, None) Combine (op, dst, source, tmp) When mask-format is None, glyphs are rendered in the order specified directly to the destination: for each glyph Combine (op, dst, source, glyph)