The process of finding all the pixels in an image that are occupied by a geometric primitive is called rasterization, so object-order rendering can also be called rendering by rasterization. The sequence of operations that is required, starting with objects and ending by updating pixels in the image, is known as the graphics pipeline. Any graphics system has one or more types of “primitive object” that it can handle directly, and more complex objects are converted into these “primitives.” Rasterization is the central operation in object-order graphics, and the rasterizer is central to any graphics pipeline. One problem with the minimal 3D pipeline is that in order to get occlusion relationships correct—to get nearer objects in front of farther away objects—primitives must be drawn in back-to-front order. So far the application sending triangles into the pipeline is responsible for setting the color; the rasterizer just interpolates the colors and they are written directly into the output image.