Procedure for debugging glReadPixels

So my glReadPixel call:

  glPixelStorei(GL_PACK_ALIGNMENT, 1);
  GLfloat lebuf[128 * 128 * 4];
  glReadPixels(0, 0, 128, 128, GL_RGBA, GL_FLOAT, lebuf);

just puts 1.0 values in the lebuf array. This is just after finishing drawing the page, and the resultant result is a "white" image.

checking the GL errors indicate that there's nothing wrong.

what could have possibly 开发者_开发问答gone wrong?

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Make sure glReadBuffer(GL_FRONT) is set before glReadPixels. If it is not, you could be reading from a different buffer, such as the back buffer when double buffering.

And of course, ensure that your capture area - 128x128 - is not all white.