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$Date: 1998/10/01 23:30:18 $ $Revision: 1.11 $
can be implemented in parallel with SGIX_pixel_texture_lod
The pixel_texture extension causes the "convert to fragment"
stage of the pixel pipeline to derive the fragment's texture
coordinates from the pixel group's color components.
At this stage in the pixel pipeline, the color components have
been converted into the framebuffer format. These
framebuffer format values are used to produce texture coordinates
of exactly the same value. The internal representation of
texture coordinates is intentionally left vague in the spec
so implementations can choose whatever they like.
This works well when the frame buffer format for
a color component is a fixed point value in the range [0,1].
However, if SGIX_color_range is supported, the framebuffer
format can be floating point, and the values are not
a linear ramp. This means that the texture must be
large and sparsely populated in order to ensure that
each texture coordinate value reaches a unique texel.
This extension allows the application to specify that
the color component be treated like an unsigned integer
with the same number of bits as are in the framebuffer format,
which makes each texel in the texture accessible through
the texture lookup equations with a texture whose size
is 2^n in each dimension, where n is the number of bits in
the framebuffer format.
The application must know the exact frambuffer format
in order to define the texture correctly.
For example, this approach allows the implementation of
a bit-preserving 1D lookup table for the s10e5 bali pixel format.
It specifies a mapping of the s10e5 pixel format to a texture
coordinate index in a manner that allows one texel to be
mapped to each of the 64K bit patterns possible in the
s10e5 format.
The motivation is to allow the shader toolkit to implement
functions, e.g., cos(x), with a copy pixels
and pixel texture enabled in a way that preserves as
much accuracy as possible.
IP Status
Silicon Graphics has filed for patent protection for some of the
techniques described in this extension document.
* Could we define a separate scale factor so that we don't
necessarily have to specify that we convert to the framebuffer
There isn't much need to mention the framebuffer format
here; we just need to specify how we're going to interpret
the bits. However, the application must use a texture
that corresponds to the framebuffer format.
* To get to the exact center of a texel, we need to add
0.5 to the calculation after the conversion to unsigned
int and before the divide by 2^m-1.
This is not necessary because the equations for calculating
the 2x2 square of texels when TEXTURE_MIN_FILTER is LINEAR
do it. (Section 3.8 in the spec.)
New Procedures and Functions
New Tokens
Accepted by the <pname> parameter of PixelTexGenParameteriSGIX and
Accepted by the <param> parameter of PixelTexGenParameteriSGIX and
Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)
Additions to Chapter 3 of the 1.0 Specification (Rasterization)
This extension, similar to the pixel_texture.spec modifies the
"Conversion to Fragments" subsection of section 3.6.3 (Rasterization
of Pixel Rectangles) of the GL Specification. Immediately following
the text added by the pixel_texture.spec, insert the following:
If the PixelTexGenParameterSGIX param COLOR_TO_TEXTURE_COORD_SGIX
is set to COLOR_BIT_PATTERN_SGIX, the mapping from r,g,b,a
to s,t,r,q is altered.
At this stage, final color processing has been performed and the
r,g,b,a values are in the form that matches the framebuffer.
To map the red color value to the s texture coordinate the
m bits of red, interpreted as an integer in the range [0,2^m-1], are
divided by 2^m-1 to obtain an s coordinate in the range [0,1].
Green and Blue are mapped similarly. The q texture coordinate
is set to one if a divide by q is in effect. If TEXTURE_4D_SGIS
is enabled, Alpha is mapped to q in the same way Red is
mapped to s.
If the PixelTexGenParameterSGIX param COLOR_TO_TEXTURE_COORD_SGIX
is set to COLOR_VALUE_SGIX, the r,g,b,a values are used
to produce floating point values for s,t,r,q.
Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame Buffer)
Additions to Chapter 5 of the 1.0 Specification (Special Functions)
Additions to Chapter 6 of the 1.0 Specification (State and State Requests)
Additions to the GLX Specification
New State
The following is added to Table 6.16. Pixels.
Get Value Get Command Type Initial Value Attrib
--------- ----------- ---- ------------- ------
New Implementation Dependent State