Stutter when updating Video Texture using multiple Pixel Buffer Objects - opengl

Question

I have an application that decodes a video file using FFMPEG (in a separate thread) and renders this texture using PBOs in another. All the PBO do-hickey happens in the following function:
void DynamicTexture::update()
{
if(!_isDirty)
{
return;
}
/// \todo Check to make sure that PBOs are supported
if(_usePbo)
{
// In multi PBO mode, we keep swapping between the PBOs
// We use one PBO to actually set the texture data that we will upload
// and the other we use to update/modify. Once modification is complete,
// we simply swap buffers
// Unmap the PBO that was updated last so that it can be released for rendering
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, _pboIds[_currentPboIndex]);
glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER);
Util::GLErrorAssert();
// bind the texture
glBindTexture(GL_TEXTURE_2D, _textureId);
Util::GLErrorAssert();
// copy pixels from PBO to texture object
// Use offset instead of pointer.
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, _width, _height,
(_channelCount==4)?GL_RGBA:GL_RGB,
GL_UNSIGNED_BYTE, 0);
Util::GLErrorAssert();
// Now swap the pbo index
_currentPboIndex = (++_currentPboIndex) % _numPbos;
// bind PBO to update pixel values
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, _pboIds[_currentPboIndex]);
Util::GLErrorAssert();
// map the next buffer object into client's memory
// Note that glMapBuffer() causes sync issue.
// If GPU is working with this buffer, glMapBuffer() will wait(stall)
// for GPU to finish its job
GLubyte* ptr = (GLubyte*)glMapBuffer(GL_PIXEL_UNPACK_BUFFER, GL_WRITE_ONLY);
Util::GLErrorAssert();
if(ptr)
{
// update data directly on the mapped buffer
_currentBuffer = ptr;
Util::GLErrorAssert();
}
else
{
printf("Unable to map PBO!");
assert(false);
}
// It is good idea to release PBOs with ID 0 after use.
// Once bound with 0, all pixel operations behave normal ways.
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
Util::GLErrorAssert();
// If a callback was registered, call it
if(_renderCallback)
{
(*_renderCallback)(this);
}
}
else
{
glBindTexture(GL_TEXTURE_2D, _textureId);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0,
_width, _height, (_channelCount==4)?GL_RGBA:GL_RGB,
GL_UNSIGNED_BYTE,
&(_buffer[0])
);
Util::GLErrorAssert();
}
// Reset the dirty flag after updating
_isDirty = false;
}
In the decoding thread, I simply update _currentBuffer and set the _isDirty flag to true. This function is called in the render thread.
When I use a single PBO, i.e. when _numPbos=1 in the above code, then the rendering works fine without any stutter. However, when I use more than one PBO, there is a visible stutter in the video. You can find a sample of me rendering 5 videos with _numPbos=2 here. The more number of PBOs I use, the worse the stutter becomes.
Theoretically, the buffer that I am updating and the buffer than I am using for render are different, so there should be no glitch of this sort. I want to use double/triple buffering so as to increase rendering performance.
I am looking for some pointers/hints as to what could be going wrong.

Answer 1

I dont know, if it is your problem, but after you call this:
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, _pboIds[_currentPboIndex]);
glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER);
Util::GLErrorAssert();
You are calling
glBindTexture
But you are still operating with buffer at index _currentPboIndex.
In my code, I have two indices - index and nextIndex
In init I set
index = 0;
nextIndex = 1;
Than my update pipeline is like this:
index = (index + 1) % 2;
nextIndex = (nextIndex + 1) % 2;
uint32 textureSize = sizeof(RGB) * width * height;
GL_CHECK( glBindBuffer(GL_PIXEL_UNPACK_BUFFER, pbo[nextIndex]) );
GL_CHECK( glBufferData(GL_PIXEL_UNPACK_BUFFER, textureSize, 0, GL_STREAM_DRAW_ARB) );
GL_CHECK( gpuDataPtr = glMapBufferRange(GL_PIXEL_UNPACK_BUFFER, 0, textureSize, GL_MAP_WRITE_BIT | GL_MAP_INVALIDATE_BUFFER_BIT) );
//update data gpuDataPtr
GL_CHECK( glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER_ARB) );
//bind texture
GL_CHECK( glBindBufferARB(GL_PIXEL_UNPACK_BUFFER, pbo[index]) );
GL_CHECK( glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0,
width, height, glFormat, GL_UNSIGNED_BYTE, 0) );
GL_CHECK( glBindBufferARB(type, 0) );