I want to take screenshot of current frame in OpenGL for further processing and I'm trying to improve the performance of glReadPixels by using PBO to asynchronously read framebuffers.
I'm under the impression that glReadPixels after GL_PIXEL_PACK_BUFFER is bound to buffer should return immediately, but it actually takes similar or even more time than not using PBO.
Here are samples of my codes:
// Setup PBO
GLES30.glGenBuffers(nPbo, pboIndex, 0);
for(int i=0;i<nPbo; i++){
GLES30.glBindBuffer (GL_PIXEL_PACK_BUFFER, pboIndex[i]);
GLES30.glBufferData(GL_PIXEL_PACK_BUFFER, size, null,GL_STREAM_READ);
}
GLES30.glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
......
// For each frame, trigger async transfer of framebuffer to PBO.
// Note that I don't even map the PBO to memory yet
GLES30.glBindBuffer (GL_PIXEL_PACK_BUFFER, pboIndex[index]);
// The following is a JNI method to overload glReadPixels in GLES20.glReadPixels,
// to allow passing int offset to the last param in order to use PBO,
// and slowdown (around 500ms on my device) happens here
GLES3PBOReadPixelsFix.glReadPixelsPBO(0, 0, mWidth, mHeight, GLES30.GL_RGBA, GLES30.GL_UNSIGNED_BYTE, 0);
GLES30.glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
Based on this article, the cause of the slowdown could be due to conversion between internal format, which may be GL_BGRA, and pixel transfer format, which is GL_RGBA in my code. Changing the transfer format to GL_RGB will reduce the latency of glReadPixels to around 100ms, but when I map the buffer with GLES30.glMapBufferRange the output frame doesn't look rendered correctly. I also tried the GL_BGRA format in GLES11Ext but it will cause GL_INVALID_OPERATION in glReadPixel.
Is there any other way to make glReadPixels on Android return immediately so that PBO can improve performance?
As Reto has suggested, it turns out to be an implementation specific issue. The GPU that I was originally testing with is Adreno 306. When I test the same codes on Samsung Note 4 (Adreno 420), it works as expected. So it's always worthwhile to test on different devices and GPUs for such types of issues.