extensions/REND/REND_screen_coordinates.txt

XXX - Not complete yet!!!

Name

    REND_screen_coordinates

Name Strings

    GL_REND_screen_coordinates

Version

    $Date: 1998/06/15 20:37:15 $ $Revision: 1.1 $

Number

    155

Dependencies

    OpenGL 1.1 is required
    EXT_fog_coord affects the definition of this extension.
    EXT_cull_vertex affects the definition of this extension.

Overview

    This extension allows the specification of screen coordinate vertex
    data. Screen coordinate vertices completely bypass transformation,
    texture generation, lighting and frustum clipping. It also allow for
    fewer floating point computations to the performed by OpenGL.

    If we get screen coordinate inputs then in order to perspectively
    correct data (eg texture), the input data currently has to be
    specified in one of the following manners

	1. Specify all the data normally
	   eg.
		  glTexture2T(s, t);
	   and the coordinates as
		  glVertex4T(x*w, y*w, z*w, w);
	or
	2. Divide each data by w
	   eg.
		  glTexture4T(s/w, t/w, r/w, q/w);
	   and the coordinates as
		  glVertex3T(x, y, z);

    Most hardware already performs some form of correction of the
    coordinate data with respect to the w term prior to interpolation.
    This is normally in the form of a multiplication of the terms by the
    inverse w. It would be much more efficient to simply specify screen
    coordinates as shown in the following example
	   glTexture2T(s, t, r, q);
    and the coordinates as
	   glVertex4T(x, y, z, w);
    and allow the hardware to bring the interpolated terms into a linear
    screen space.

    Additionally if the application derives screen coordinates it is
    also highly likely that the 1/w term may already be computed. So it
    would be advantageous to be able to specify 1/w directly instead of
    w in the input screen coordinates.

    For hardware that linearly interpolates data, the hardware
    interpolates the following data:
	s/w, t/w, r/w, q/w, x, y, z
    If the input w represents the original 1/w, then the hardware can
    avoid the division and instead interpolate:
	s*w, t*w, r*w, q*w, x, y, z


Issues

    * Should screen coordinates have been done as a hint instead of
      an enable bit?

    RESOLVED: Since this extension specifies changes the semantics of
    OpenGL, we need to make this an enable bit

    * Should the texture matrix be applied to the input screen
      coordinates?

    This spec assumes that the texture matrix is applied

    * Should the raster position valid be set to invalid when screen
      coordinates are enabled.

    RESOLVED: Raster position can still be defined. But the raster
    position calls should act similar to the vertex specification calls.
    So the raster position calls will be specifying screen space
    coordinates. The raster position will always be valid. The

    raster eye z will be defined to be 0 always.

    * Should we allow evaluators in screen space?

    This spec is written assuming the evaluators will act as if it is
    disabled

    * Should we ignore selection if screen coordinates is enabled, or
      select everything?

    * Should we ignore feedback in screen coordinates is enabled, or
      return everything?

New Procedures and Functions

    None.

New Tokens

    Accepted by the <cap> parameter of Enable, Disable and IsEnabled:

    SCREEN_COORDINATES_REND	0x8490
    INVERTED_SCREEN_W_REND	0x8491

Additions to Chapter 2 of the 1.2 Specification (OpenGL Operation)

    Section 2.6 Begin/End Paradigm

    <p 13, amend paragraph 1>
	These associated colors are either based on the current color or
	produced by lighting, depending on whether or not lighting is
	enabled and whether or not screen coordinates are enabled.
	Texture coordinates are similarly associated with each vertex.
	Fig 2.2 summarizes the association of auxiliary data with a
	transformed vertex to produce a processed vertex.

    <p 13, amend paragraph 2> ...
	Vertices and normals are transformed, colors may be affected or
	replaced by lighting and texture coordinates are transformed and
	possibly affected by a texture coordinate generation function.
	However, if screen coordinates has been enabled, then neither
	the vertices nor the normal undergo any transformation, the
	current colors are not modifies by lighting, the texture
	coordinates are transformed but are not affected by texture
	coordinate generation functions.

    <p 13, amend figure 2.2>
	If screen coordinate vertex data is enabled, then the
	coordinates are not transformed.

    <p 13, amend figure 2.3>
	If screen coordinate vertex data is enabled, then clipping is
	bypassed.

    Section 2.7

    <p 20, add after paragraph 1>
	If SCREEN_COORDINATES is enabled then the x, y, z and w values
	represent screen coordinate values. Additionally if
	INVERTED_SCREEN_W is enabled then the w coodinate is treated as
	the actual 1/w coordinate.

    Section 2.10

    <p 28, modify paragraph 1>
	Vertices, normals and texture coordinates are transformed before
	their coordinates are used to produce an image in the
	framebuffer. However if screen coordinates are enabled then only
	the texture coordinate may undergo a transformation.

    Section 2.10.4

    <p 36, modify paragraph 1>
	Texture coordinates associated with a vertex may either be taken
	from the current texture coordinates or generated according to a
	function dependant on vertex coordinates. However if screen
	coordinate data is enabled then the texture coordinate
	associated with the vertex always comes from the current
	texture.

    Section 2.12

    <p 41, modify paragraph 2>
	The coordinates are treated as if they were specified in a
	Vertex command. The x, y, z, and w coordinates are transformed
	by the current model-view and perspective matrices if screen
	coordinates is disabled. These coordinates, along with the
	current values, are used to generate a color and texture
	coordinates just as is done for a vertex. The color and textures
	coordinates so produced replace the color and texture
	coordinates stored in the current raster position's associated
	data. The distance from the origin of the eye coordinate system
	to the vertex as transformed by only the current model-view
	matrix replaces the current raster distance if screen
	coordinates is disabled. If screen coordinates are enabled then
	the current raster distance is always 0. This distance can be
	approximted (see section 3.10).

    <p 42, Modify figure 2.7>
	If screen coordinate vertex data is enabled then the associated
	data always gets the texture coordinates from the current
	texture coordinate and the color from the current color
	(lighting and texgen are as if they are disabled).

    <p 43, Modify figure 2.9>
	If screen coordinate vertex data is enabled then lighting and
	color clipping is bypassed.

    Section 2.13

    <p 43, Modify paragraph 1>
	Next lighting, if enabled and screen coordinate data is not
	enabled, produces either a color index or primary and secondary
	colors. ...


    Section 2.13.8

    <p 54, modify paragraph 1>
	After lighting, clamping or masking, and possibly flat shading,
	colors are clipped if screen coordinate data is not enabled. ...

Additions to Chapter 3 of the 1.2 Specification (Rasterization)

    Section 3.4.1

    <p 66, modify equation 3.2>
	If inverted screen w disabled or screen coordinate data is
	disabled

		(1-t) fa/wa + t fb/wb
	    f =-----------------------
		(1-t) Aa/wa + t Ab/wb

	otherwise

		(1-t) fa.wa + t fb.wb
	    f =-----------------------
		(1-t) Aa.wa + t Ab.wb

     Section 3.5

     <p 71, modify equation 3.4>
	If inverted screen w disabled or screen coordinate data is
	disabled
		a.fa/wa + b.fb/wb + c.fc/wc
	    f =-----------------------------
		a.Aa/wa + b.Ab/wb + c.Ac/wc

	otherwise
		a.fa.wa + b.fb.wb + c.fc.wc
	    f =-----------------------------
		a.Aa.wa + b.Ab.wb + c.Ac.wc

     Section 3.10

     <p 139, modify paragraph 2>
	This factor f is computed according to one of three equations:

	    f = exp(-d.z),		    (3.24)
			  2
	    f = exp(-(d.z) ), or	    (3.25)

		e - z
	    f = ------			    (3.26)
		e - s
	(z is the eye coordinate distance from the eye, (0,0,0,1) in

	eye coordinates, to the fragment center if screen coordinates is
	disabled, z is 0 otherwise). ....

Additions to Chapter 4 of the 1.2 Specification (Per-Fragment Operations and the Framebuffer)

Additions to Chapter 5 of the 1.2 Specification (Special Functions)

    Section 5.1

    <p 166, append at the end of paragraph 1>
	Evaluators act as if they are disabled if screen coordinate data
	is enabled.

    Section 5.2

    <p 172, append at the end paragraph 1>
	If screen coordinate data is enabled then the primitives are
	always selected.

    Section 5.3

    <p 175, append to the end of paragraph 1>
	If screen coordinate data is enabled then every primitive is
	always added to the feedback buffer if it is not an image
	rectangle based primitive (bitmap or DrawPixels or CopyPixels).

Additions to Chapter 6 of the 1.2 Specification (State and State Requests)

    Section 6.2.1

    <p 199, add to the end of Table 6.7>
    Get Value		Type Get Command   Initial   Description Sec. Attribute
					   Value
    ---------		---- -----------   --------  ----------- ---- ---------
    SCREEN_COORDINATES	B    IsEnabled	   False     Input coord -    enable
						     system
    INVERTED_SCREEN_W	B    IsEnabled	   False     Screen coor 2.7  enable
						     w semantics

Additions to the GLX Specification

    None

GLX Protocol

    TBD

Dependencies on EXT_fog_coord

     If EXT_fog_coord is supported then the section 3.10 is
     further modified to read:

	This factor f is computed according to one of three equations:

	    f = exp(-d.c),		    (3.24)
			  2
	    f = exp(-(d.c) ), or	    (3.25)

		e - c
	    f = ------			    (3.26)
		e - s
	If the fog source (as defined below) is FRAGMENT_DEPTH, then c
	is the eye coordinate distance from the eye (0,0,0,1) in eye
	coordinates, to the fragment center if screen coordinates is
	disabled. If screen coordinates is enabled then it is always 0.
	If the fog source is FOG_COORDINATE, then c is the interpolated
	value of the fog coordinate for this fragment. ...

Dependencies on EXT_cull_vertex
    If screen coordinates are enabled and EXT_cull_vertex is supported,
    then vertex culling is never performed regardless of whether
    CULL_VERTEX_EXT is enabled or not.

Errors

    None.

New State

    Get Value		Get Command    Type    Initial Value Attribute
    ---------		-----------    ----    ------------- ---------
    SCREEN_COORDINATES	IsEnabled     1* x B   False	      enable

    INVERTED_SCREEN_W	IsEnabled     1* x B   False	      enable

New Implementation Dependent State

    None.