Memory Bandwidth Requirements of Tile-Based Rendering

Because mobile phones are omnipresent and equipped with displays, they are attractive platforms for rendering 3D images. However, because they are powered by batteries, a graphics accelerator for mobile phones should dissipate as little energy as possible. Since external memory accesses consume a significant amount of power, techniques that reduce the amount of external data traffic also reduce the power consumption. A technique that looks promising is tile-based rendering. This technique decomposes a scene into tiles and renders the tiles one by one. This allows the color components and z values of one tile to be stored in small, on-chip buffers, so that only the pixels visible in the final scene need to be stored in the external frame buffer. However, in a tile-based renderer each triangle may need to be sent to the graphics accelerator more than once, since it might overlap more than one tile. In this paper we measure the total amount of external data traffic produced by conventional and tile-based renderers using several representative OpenGL benchmark scenes. The results show that employing a tile size of 32×32 pixels generally yields the best trade-off between the amount of on-chip memory and the amount of external data traffic. In addition, the results show that overall, a tile-based architecture reduces the total amount of external data traffic by a factor of 1.96 compared to a traditional architecture.