13.2.3.1 Pixel Maps

Pixel maps are a feature of base OpenGL which allow certain look-up operations to be performed. OpenGL maintains tables which map:

• The red channel to the red channel (GL_PIXEL_MAP_R_TO_R)
• The green channel to the green channel (GL_PIXEL_MAP_G_TO_G)
• The blue channel to the blue channel (GL_PIXEL_MAP_B_TO_B)
• The alpha channel to the alpha channel (GL_PIXEL_MAP_A_TO_A)
• Color indices to color indices (GL_PIXEL_MAP_I_TO_I)
• Stencil indices to stencil indices (GL_PIXEL_MAP_S_TO_S)
• Color indices to RGBA values (GL_PIXEL_MAP_I_TO_R, GL_PIXEL_MAP_I_TO_G, GL_PIXEL_MAP_I_TO_B, and GL_PIXEL_MAP_I_TO_A)

Tables that map color indices to RGBA values are used automatically whenever an image with a color index format is transferred to a destination which requires an RGBA image. For example, performing a glDrawPixels() of a color index image to an RGBA framebuffer would result in application of the I to RGBA pixel maps. Other tables are enabled with the commands glPixelTransferGL_MAP_COLOR, 1(GL_MAP_COLOR, 1) and glPixelTransferGL_MAP_STENCIL, 1(GL_MAP_STENCIL, 1).

Pixel maps are defined using the glPixelMap() command and queried using the glGetPixelMap() command. Details on the use of these commands may be found in [12]. The sizes of the pixel maps are not tied together in any way. For example, the R to R pixel map does not need to be the same size as the G to G pixel map.

Each system provides a constant, GL_MAX_PIXEL_MAP_TABLE, which gives the maximum size of a pixel map which may be defined.