path: root/renderproto.txt

			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)