The next step in the pipeline determines the color of each pixel based on the polygons behind them, and is called rasterization. This step depends both on the number of polygons and on how many pixels each polygon covers. Unlike the previous steps, it depends strongly on the number of pixels on the screen. If rasterization is slow, reducing the resolution of the image can speed up this process.

Rasterization determines which polygons are closest to the camera at each pixel. When visible polygons are transparent, more calculations are required to look for the polygons that are behind them. Long ago, visible polygons were determined by sorting the polygons and rendering them from back to front. The closer polygons were essentially "painted" over the ones behind them. But modern graphics hardware has replaced those algorithms with z-buffering, which calculates polygon depths at each pixel between the near and far clipping planes as integers and keeps track of the closest one, as discussed in the previous steps.