OpenGL.GL.SGIS.pixel_texture

OpenGL extension SGIS.pixel_texture

This module customises the behaviour of the OpenGL.raw.GL.SGIS.pixel_texture to provide a more Python-friendly API

Overview (from the spec)

The geometry rasterization and pixel pipeline "convert to fragment" stages each produce fragments. The fragments are processed by a unified per fragment pipeline that begins with the application of the texture to the fragment color. Because the pixel pipeline shares the per fragment processing with the geometry pipeline, the fragments produced by the pixel pipeline must have the same fields as the ones produced by the geometry pipeline. When pixel groups are being converted to fragments, the parts of the fragment that aren't derived from the pixel groups are taken from the associated values in the current raster position.

A fragment consists of x and y window coordinates and their associated color value, depth value, and texture coordinates. In the 1.1 OpenGL specification, when the pixel group is RGBA the fragment color is always derived from the pixel group, and the depth value and texture coordinates always come from the raster position.

This extension provides a way to specify how the texture coordinates of the fragments can be derived from RGBA pixel groups. When this option is enabled, the source of the fragment color value when the pixel group is RGBA can be specified to come from either the raster position or the pixel group.

Deriving the fragment texture coordinates from the pixel group effectively converts a color image into a texture coordinate image. The multidimensional texture mapping lookup logic also makes this extension useful for implementing multidimensional color lookups. Multidimensional color lookups can be used to implement very accurate color space conversions.

Deriving texture coordinates from the pixel groups in the pixel pipeline introduces a problem with the lambda parameter in the texture mapping equations. When texture coordinates are being taken from the current raster position texture coordinates, the texture coordinate values don't change from pixel to pixel, and the equation for calculating lambda always produces zero. Enabling pixel_texture introduces changes in the texture coordinates from pixel to pixel which are not necessarily meaningful for texture lookups. This problem is addressed by specifying that lambda is always set to zero when pixel_texture is enabled.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/SGIS/pixel_texture.txt