XXX needs more work.

Name

    SGIX_pixel_texture_lod

Name Strings

    GL_SGIX_pixel_texture_lod

Version

    $Date: 1998/07/22 18:07:16 $ $Revision: 1.9 $

Number

    128

Dependencies

    GL_SGIS_pixel_texture
    can be implemented in parallel with pixel_texture_bits

Overview

    The pixel_texture extension provides a way to derive the
    texture coordinates from RGBA pixel groups.  This extension
    introduces a way to derive the lod (lambda) from the
    pixel groups as well.

Issues

    * We don't operate in "bits" mode here. Extended Range
      and Precision (erp) values are just erp values 
      and old fashioned [0,1] values are interpeted as [0,1] values; 
      this makes lambda uninteresting except for the erp case.
      should we bother to define lambda in a way that is interesting
      for non extended range and precision?

    * Is it possible to compute lambda in a pixel field in a way that
      makes this extension useful?

      Seems like you can always do the shift and subtract trick and then
      use blend function and lookup tables to compute the lod in a reasonably
      efficient manner.

    * Do we need to implicitly scale the lambda by the number of lods?

    * Need to make sure this extension isn't too had to implement in
      the hardware (bali).

    * This extension is intended to be used with the color components
      of the fragments derived from the current raster position.  Can
      we leave that part of it orthogonal and let the application
      set that explicitly using PIXEL_FRAGMENT_{RGB,ALPHA}_SOURCE_SGIX?

New Procedures and Functions

    None

New Tokens

    Accepted by the <pname> parameter of PixelTexGenParameterSGIX:

	PIXEL_TEX_GEN_LAMBDA_SOURCE_SGIX
	
Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)

    None

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 PIXEL_TEX_GEN_SGIX is enabled and the PixelTexGenSGIX 
    pname PIXEL_TEX_GEN_LAMBDA_SOURCE_SGIX is set to ALPHA, 
    the mapping from r,g,b,a to s,t,r,q is altered.

    The alpha color component value becomes the lambda value, the log-base-2
    of the ratio of the projected texel_area and pixel area. The boundary
    of minification to magnification occurs at lambda = 0, with values
    less than 0 corresponding to minification and values greater than
    0 corresponding to magnification.  The alpha color component value
    becomes the texture coordinate value.  The Q texture coordinate 
    is set to 1.

    If PIXEL_TEX_GEN_SGIX is enabled and the PixelTexGenSGIX 
    pname PIXEL_TEX_GEN_LAMBDA_SOURCE_SGIX is set to ZERO, 
    lambda is set to zero.

Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations
and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.0 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.0 Specification (State and State Requests)

    None

Additions to the GLX Specification

    XXX

Errors

    None

New State

    The following is added to Table 6.16. Pixels.

    Get Value				Get Command			Type	Initial Value			Attrib
    ---------				-----------			----	-------------			------
    PIXEL_TEX_GEN_LAMBDA_SOURCE_SGIX	GetPixelTexGenParameterivSGIX	Z2	ZERO				pixel

New Implementation Dependent State

    None