Christian announced this Sunday morning on the mesa-dev mailing list that the first implementation of XvMC iDCT is now working. The pure matrix multiplication with iDCT is working but the luma/chroma scaling and clamping code is still a bit buggy. It's also inefficient and causing issues for some GPUs when driving the video playback at higher resolutions due to the GPU calculations hitting a bottleneck. This work also led to discovering a few bugs in the Radeon DRM command checker and also required introducing support for signed normalized color buffers in the R600g driver.
The inverse discrete cosine transform (iDCT) is one of the processes (along with motion compensation, which the XvMC state tracker already utilizes) for offloading these tasks from the CPU to the GPU. A discrete cosine transform (DCT) expresses a sequence of finitely many data points in terms of a sum of cosine functions oscillating at different frequencies. For more information on iDCT, see Wikipedia.
Kudos to this German developer for tackling XvMC Gallium3D support for ATI Radeon hardware after previously adding HDMI audio support to the Radeon DRM driver and the now-defunct RadeonHD DDX driver.
As already said on Facebook, we have benchmarks coming soon of this Gallium3D driver compared to its classic Mesa R600 driver and also AMD's proprietary Catalyst driver.