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POStutter when updating Video Texture using multiple Pixel Buffer Objects
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  1. POStutter when updating Video Texture using multiple Pixel Buffer Objects
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    copied!<p>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:</p> <pre><code>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, &amp;(_buffer[0]) ); Util::GLErrorAssert(); } // Reset the dirty flag after updating _isDirty = false; } </code></pre> <p>In the decoding thread, I simply update _currentBuffer and set the _isDirty flag to true. This function is called in the render thread.</p> <p>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 <a href="https://docs.google.com/file/d/0B_3UZN1ZieQgX1dRTFFZemZFUDA">here</a>. The more number of PBOs I use, the worse the stutter becomes.</p> <p>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. </p> <p>I am looking for some pointers/hints as to what could be going wrong.</p>
 

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