Question

I am using OpenGL ES 2.0 and GLSL to draw objects.
Then I want to read pixel from whatever my fragment shader draws on my screen.
I am blending two grayscale textures and at the end of my fragment shader, I have:

gl_FragColor = vec4(something...blending textures);

I know that gl_FragColor means the final color will be written in the frame buffer.
(According to http://nehe.gamedev.net/article/glsl_an_introduction/25007/)
Given that, I did something like

GLubyte *pixels = new GLubyte[320*240];
glReadPixels(0, 0, 320,240, GL_LUMINANCE, GL_UNSIGNED_BYTE, pixels);

After drawing image using that, at the first, I get same as the original texture1 and then get the black empty color. Can someone help me what’s wrong? Do I need to do something with FBO? I am a kinda lost…:(

Answer 1

Per the canonical ES 2.0 documentation for glReadPixels, the format parameter:

Specifies the format of the pixel data. The following symbolic values
are accepted: GL_ALPHA, GL_RGB, and GL_RGBA.

So GL_LUMINANCE is not a supported read format in GL ES.

Beyond that, assuming you have a working frame buffer of some sort and you’re applying your fragment shader by rendering a suitably placed piece of geometry to the frame buffer, glReadPixels should be the correct thing to use.