glRead/drawpixels over FBO

OpenGL OpenGL: Advanced Coding
1

I want to read all the pixel of my offscreen render, but when I call the glreadpixels, it returns to me only the window pixel value, and not the offscreen pixel’s value.

First I think that it can be for an incorrect FBO setup, but I had not found any error.

So I want to know if glreadpixel read all the buffer or not…

2

Be sure you call glBindFramebuffer with valid FBO before glReadPixels.

It works fine for me (NV cards).

Beware: glBindFramebufferEXT( GL_FRAMEBUFFER_EXT, 0)
will switch back to reading from Window.

3

I’ve call glbindframebuffer before glreadpixels, but it read nonsense value…this is my config:
FOR GLREADPIXELS:

glReadPixels(0, 0, dimX, dimY, GL_RGBA, GL_FLOAT, imagePixels);

AND THIS IS THE TEXTURE FBO:

glBindTexture(GL_TEXTURE_2D, color_tex);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, dimX, dimY, 0,
GL_RGBA8, GL_FLOAT, NULL);

	glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_2D, color_tex, 0);

somethings wrong?

4
  1. BindFBO(true)
  2. glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);
  3. glReadPixels()
  4. BindFBO(false)

Also, when you Bind a texture make sure which texture unit is active or just say glActiveTexture(GL_TEXTURE0) or some other texture unit. These are common mistake easily overlooked.

You dont’ seem to initialize the contents of the texture.

5

What did you mean with “context of the texture”?

I follow your istruction and I make a little step over, but my offscreen render seems to working wrong. Maybe for this “context of the texture”?

6

Your FBO is probably not complete.
After glFramebufferTexture2D()
call following to be sure the FBO is OK.

  GLenum status; 
  status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT); 
  if(status != GL_FRAMEBUFFER_COMPLETE_EXT) {
    // report error
  }
7

I think FBO isn’t complete because in

 glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, dimX, dimY, 0, GL_RGBA8, GL_FLOAT, NULL);

the second GL_RGBA8 isn’t a correct value but GL_RGBA is.

8

My FBO seems to be ok, this is my problem:

Render off-screen

  glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,mRenderer.fb);

glDrawElements( GL_TRIANGLES, kNumVertices, GL_UNSIGNED_INT, &mTriBufferIndices[0] ); 

 glBindFramebufferEXT(GL_FRAMEBUFFER_EXT,fb);
glReadPixels(0, 0, dimX, dimY, GL_RGBA, GL_FLOAT, imagePixels);
    1. Draw the pixel

but that pixel are incomplete, in fact in the offscreen rectangle there’s only the window clear color without the 3D image. So when I call gldrawelements with previosly the glbindframebufferEXT, the 3D object seems to disappear…while if I use only the gldrawelements without the glbindframebufferEXT the 3D object is rendered on the window(but maybe not in the offscreen buffer).

Any idea? thanks…