Render Back-Ends

Raster processors or render back-ends (RBEs) in ATI’s terminology have never been a weak spot of the Radeon HD architecture, but the RV770 features certain improvements in this area, too. The number of these units hasn’t changed. The chip contains four raster back-ends equivalent to 16 classic ROPs.

The developer’s goal was to increase performance when performing full-screen antialiasing and increase efficiency when processing the Z-buffer/stencil buffer. The number of appropriate subunits has been doubled in the latter case.

As the result of the modernization, the scene fill rate with enabled FSAA has doubled for both 32-bit and 64-bit color while the number of Z/stencil values processed per clock cycle has increased from 32 to 64, which is more than the G92 can do when using FSAA. In other words, ATI has outperformed the opponent from the aspect where its solutions have always been on the losing side!

The render back-ends of the RV770 support classic fixed multisampling modes including the interesting mode that combines classic MSAA with edge antialiasing to achieve a level of antialiasing equivalent to 12-24x MSAA. This mode was announced back in the Radeon HD 2000.

The CFAA modes introduced earlier employed the wide and narrow tent filters to sample subpixels outside the pixel without counting in the edges of polygons. This improved the overall image quality but also made the image fuzzy. The CFAA edge detect mode helps avoid that fuzziness.

The programmable sampling filter of this mode is set up in such a way as to sample subpixels in the vicinity of a polygon edge only. This improves the quality of antialiasing, especially on small details such as hanging cables, but without the undesirable fuzziness typical of the less intellectual CFAA algorithms. The graphics memory usage is the same as with ordinary 4x/8x MSAA modes. Note that the new CFAA mode has become available for the owners of earlier Radeon HD cards.