Different alpha values in volumetric data often correspond to different materials in the volume being rendered. To help analyze the volume data, a non-linear transfer function can be applied to the texels, highlighting particular classes of volume data. This transformation function can be applied through one of OpenGL's lookup tables. The SGI_texture_color_table extension applies a lookup table to texels values during texturing, after the texel value is filtered.
Since filtering adjusts the texel component values, a more accurate method is to apply the lookup table to the texel values before the textures are filtered. If the EXT_color_table table extension is available, then a colortable in the pixel path can be used to process the texel values while the texture is loaded. If lookup tables aren't available, the processing can be done to the volume data by the application, before loading the texture.
If the paletted texture extension (EXT_paletted_texture) is available and the 3D texture can be stored simply as color table indices, it is possible to rapidly change the resulting texel component values by changing the color table.