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Understanding 3D rendering step by step with 3DMark11
by Damien Triolet
Published on January 27, 2012

Stage 1: clearing memory buffers
The first stage in any 3D rendering is the least interesting and consists in resetting the memory buffer zones, known as Render Targets (RTs) to which the GPU writes data. Without this the data defining the previous image will interfere with the new image to be computed.

In certain types of rendering, RTs can be shared between several successive images, to accumulate information for example. Here of course they aren’t reset. 3DMark 11 doesn’t however share any data beween successive images, which is a requirement for maximum efficiency in a multi-GPU set up.

Resetting all these buffers basically means stripping all the values they contain back to zero, which corresponds to a black image. Recent GPUs carry out this process very rapidly, depending on the size of the memory buffers.

When the rendering is initialised, 3DMark 11 resets 7 RTs very rapidly: 0.1ms or 0.1% of the rendering time. Later five very large RTs dedicated to shadows will also have to be reset, taking the total time taken up with this thankless task to 1.4ms, or 1.1% of the overall rendering time.

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Stage 2: filling the g-buffer  

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