Question

I load a texture with grayscale image using GL_LUMINANCE format.

when i use float NP = texture2D(input1, textureCoordinate).r; in the fragment shader the program fails.

is it possible to read the pixel grayscale value or am i limited to work with 4 channels only?

Answer 1

Sure. Here is some GL code the loads a “height map” (grays levels stored as float‘s).
The texture is stored in unit 0.

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, textureNames[0]);
...
glTexImage2D(GL_TEXTURE_2D, 0, GL_LUMINANCE,
             heightMap.W, heightMap.H, 0,
             GL_LUMINANCE, GL_FLOAT, &heightMap.elev[0]);

Here is the pertinent code from my vertex shader. The uniform HeightMap is 0 (unit 0).

 float z = texture2D(HeightMap, TexCoord0).x * ElevScale + ElevShift;

Works like a charm.