The PowerVR chipset uses a method of 3D rendering known as tile-based deferred rendering (often abbreviated as TBDR). As the polygon generating program feeds triangles to the PowerVR (driver) it stores them in memory in a triangle strip or an indexed format.
Unlike other architectures, polygon rendering is (usually) not performed until all polygon information has been collated for the current frame. Furthermore, the expensive operations of texturing and shading of pixels (or fragments) is delayed, whenever possible, until the surface visible at a pixel is determined — hence rendering is deferred.
In order to render, the display is split into rectangular sections in a grid pattern. Each section is known as a tile. Associated with each tile is a list of the triangles that visibly overlap that tile. Each tile is rendered in turn to produce the final image.
Tiles are rendered using a process similar to ray-casting. Rays are cast onto the triangles associated with the tile and a pixel is rendered from the triangle closest to the camera. The PowerVR hardware typically calculates the depths associated with each polygon for one tile row in 1 cycle.
This method has the advantage that, unlike a more traditional z-buffered rendering pipeline, no calculations need to be made to determine what a polygon looks like in an area where it is obscured by other geometry. It also allows for correct rendering of partially transparent polygons, independent of the order in which they are processed by the polygon producing application. (This capability was only implemented in Series 2 and one MBX variant. It is generally not included for lack of API support and cost reasons.) More importantly, as the rendering is limited to one tile at a time, the whole tile can be in fast onchip memory, which is flushed to video memory before processing the next tile.
Under normal circumstances, each tile is visited just once per frame.
PowerVR is not the only pioneer of tile based deferred rendering, but the only one to successfully bring a TBDR solution to market. Microsoft also conceptualised the idea with their abandoned Talisman project. Gigapixel, a company that developed IP for tile-based deferred 3D graphics, was purchased by 3dfx, who were subsequently purchased by Nvidia. Nvidia currently has no official plans to pursue tile-based rendering.
Intel uses a similar concept in their integrated graphics solutions. However, their method, coined zone rendering, does not perform full hidden surface removal (HSR) and deferred texturing, therefore wasting fillrate and texture bandwidth on pixels that are not visible in the final image.
Recent advances in hierarchical Z-buffering have effectively incorporated ideas previously only used in deferred rendering, including the idea of being able to split a scene into tiles and of potentially being able to accept or reject tile sized pieces of polygon.
and also, no reason to push new hardware to the market.
