How can i read data by glReadPixels? - opengl-es-2.0

Question

I'm working on openGL es on Android.
Now i meet a problem. I defined a float array, which is used to pass to fragment shader.
float[] data = new float[texWidth*texHeight];
// test data
for (int i = 0; i < data.length; i++) {
data[i] = 0.123f;
}
1. initTexture:
glGenTextures...
glBindTexture...
glTexParameteri...
FloatBuffer fb = BufferUtils.array2FloatBuffer(data);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texWidth, texHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, fb);
2.FBO:
glGenBuffers...
glBindFramebuffer...
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texId, 0);
3.onDrawFrame:
glUseProgram(mProgram);...
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);...
IntBuffer fb = BufferUtils.iBufferAllocateDirect(texWidth*texHeight);
glReadPixels(0, 0, texWidth, texHeight, GL_RGBA, GL_UNSIGNED_BYTE, fb);
System.out.println(Integer.toHexString(fb.get(0)));
System.out.println(Integer.toHexString(fb.get(1)));
System.out.println(Integer.toHexString(fb.get(2)));
fragment shader:
precision mediump float;
uniform sampler2D sTexture;
varying vec2 vTexCoord;
void main()
{
tex = texture2D(sTexture, vTexCoord.st);
vec4 color = tex;
gl_FragColor = color;
}
So, how can i get the float data(0.123f, which i defined before) whith glReadPixels? Now what i get is ff000000(ABGR), so i suspect shader doesn't get the data through this way. Can someone tell me why and how can i deal with it? i am a newbie on it and really appreciate it.

Answer 1

Your main problem happens before glReadPixels(). The primary issue is with the way you use glTexImage2D():
FloatBuffer fb = BufferUtils.array2FloatBuffer(data);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texWidth, texHeight, 0,
GL_RGBA, GL_UNSIGNED_BYTE, fb);
The GL_UNSIGNED_BYTE value for the 8th argument specifies that the data passed in consists of unsigned bytes. However, the values in your buffer are floats. So your float values are interpreted as bytes, which can't possibly end well because they are completely different formats, with different sizes and memory layouts.
Now, you might be tempted to do this instead:
FloatBuffer fb = BufferUtils.array2FloatBuffer(data);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texWidth, texHeight, 0,
GL_RGBA, GL_FLOAT, fb);
This would work in desktop OpenGL, which supports implicit format conversions as part of specifying texture data. But it is not supported in OpenGL ES. In ES 2.0, GL_FLOAT is not even a legal value for the format argument. In ES 3.0, it is legal, but only for internal formats that actually store floats, like GL_RGBA16F or GL_RGBA32F. It is an error to use it in combination with the GL_RGBA internal format (3rd argument).
So unless you use float textures in ES 3.0 (which consume much more memory), you need to convert your original data to bytes. If you have float values between 0.0 and 1.0, you can do that by multiplying them by 255, and rounding to the next integer.
Then you can read them back also as bytes with glReadPixels(), and should get the same values again.