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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 }.

BlueVoodoo
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  • This looks like you're using the NDK, or native OpenGL library. If so, please tag it as well. – m0skit0 Jan 30 '13 at 12:08
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    Is `buffer` actually a `GLubyte` array? Because it's not possible for a `GLubyte` to have a negative number. So unless you're doing it wrong, you cannot possibly get `{ 64, 64, 64, -1 }` back. – Nicol Bolas Jan 30 '13 at 12:16
  • @NicolBolas That's fine. They don't need to be unsigned for what I need to do. They are put in a Java ByteBuffer. – BlueVoodoo Jan 30 '13 at 12:20
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    @BlueVoodoo: No, it's not fine. How can you expect to get accurate values when you and OpenGL are in disagreement over how to interpret the data? If you are reading unsigned bytes, you need to *interpret them* as unsigned bytes. It's not optional. – Nicol Bolas Jan 30 '13 at 12:27
  • @NicolBolas - Yes, I understand the concern but in my case they are turned into signed bytes via the ByteBuffer. For what I neeed to to, this is fine. I just need the alpha channel to be returned correctly and my problem will be solved. The rest of the code in the pipeline already works. – BlueVoodoo Jan 30 '13 at 12:37

2 Answers2

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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.

Jack Aidley
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  • Right, I didn't even pay attention to the fact that the array should be 128. If I remove those two lines, I get back { -128, -128, -128, -1 } so the alpha is still not correct. – BlueVoodoo Jan 30 '13 at 12:17
  • The alpha is correct. It's _lost_ when you draw. With blending off, you've now told it that it should ignore the alpha channel, it's still not going to be preserved. – Jack Aidley Jan 30 '13 at 12:23
  • What can be done to resolve this? Any way it can be preserved or will it always be lost when drawing not matter what setting I use? – BlueVoodoo Jan 30 '13 at 12:28
  • What are you actually trying to do? The whole point of alpha is that it is treated differently to the RGB channels. – Jack Aidley Jan 30 '13 at 13:21
  • The short answer is I just need it to be preserved as I depend on having the alpha channel values when I read the buffer back. It seems other poeple have managed to get it working, so it shouldn't be impossible at least. – BlueVoodoo Jan 30 '13 at 13:33
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    While much of this answer is good, it is not universally correct. If your framebuffer contains alpha biplanes (specified when you create your surface), then the alpha component is preserved, and can be retrieved like another other color component. – radical7 Jan 30 '13 at 14:55
  • @radical7 can you please elaborate on your comment? I have a related problem and you already answered half of it: https://stackoverflow.com/questions/68777261/opengles-alpha-value-modifies-rgb-values-with-glreadpixels – Dávid Tóth Aug 13 '21 at 19:03
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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);
BlueVoodoo
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