glReadPixels alpha value issue

Question

I am having issues reading back the alpha channel from my OpenGL texture on Android. I call glReadPixels this way:

glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

and just as a test, I made a fragment shader looking like this:

gl_FragColor = vec4(0.5, 0.5, 0.5, 0.5);

For each pixels, I get back { 64, 64, 64, -1 }.

I've tried all sorts of things to work out why the alpha is not returned the same way as the RGB values, including this before each render:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
//Tried various blend functions
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

But no matter what I do, it comes back -1.

Is this an Android limitation or what am I missing?

Just as a note, I am using an off-screen FBO if it makes any difference.

EDIT: If I do this:

int[] bits= new int[4];
glGetIntegerv(GL_RED_BITS, bits, 0);
glGetIntegerv(GL_GREEN_BITS, bits, 1);
glGetIntegerv(GL_BLUE_BITS, bits, 2);
glGetIntegerv(GL_ALPHA_BITS, bits, 3);

I get back { 8, 8, 8, 0 }.

Answer 1

Worked out what the problem was. My render buffer storage was set to a 24 bit mode. When changing it to a 32 bit mode, it started working:

glRenderbufferStorage(GL_RENDERBUFFER, GLES11Ext.GL_RGBA8_OES, w, h);

Answer 2

Alpha channels control blending, they're usually lost the moment they are used. The data that was in the alpha channel has been used to blend your colours with the colours that were already in the data set. You cannot use:

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

and get your alpha channel back. I'd note also that this is why you're getting 64 rather than 128 back for your RGB values, each original value (128) has been blended 50/50 with the existing black to produce an RGB of 64.