Replacement for GL_LUMINANCE, GL_LUMINANCE_ALPHA
image, opengl, opengl-3
Solution
If you scroll up a bit on that wiki page you'll see `GL_RED/GL_R*` and `GL_RG*` for 1- and 2-channel images.
`glTexImage2D()`:
`GL_RED`: Each element is a single red component. The GL converts it to floating point and assembles it into an RGBA element by attaching 0 for green and blue, and 1 for alpha...
`GL_RG`: Each element is a red/green double. The GL converts it to floating point and assembles it into an RGBA element by attaching 0 for blue, and 1 for alpha...
As GuyRT pointed out if you have `EXT_texture_swizzle/ARB_texture_swizzle` and/or OpenGL version >= 3.3 you can use swizzle mask to expand out/rearrange your texture color components.
Problem
For memory-efficient realtime embedded graphics I have traditionally used DDS images encoded using `GL_LUMINANCE` when I need lossless grayscale images (or `GL_LUMINANCE_ALPHA` when there is alpha needed). However, I have just discovered that these file formats were deprecated in OpenGL 3 and removed from 3.1. Is there a replacement image format for lossless grayscale-only data that is 8 bits per pixel or less (or 16bpp when alpha is involved)?