Immediate Mode Rendering: the status quo
Imagination Technology has been in the tile based rendering market for quite some time. Sold under the PowerVR chip name, the first true tile based rendering system that Imagination created was the PowerVR Series 2 chip. The PowerVR Series 2 marked a significant deviation from the traditional immediate mode rendering which has been the mainstay of 3D graphics for over 40 years.
In immediate mode rendering, a video card is programed to render each polygon in a scene without any knowledge of what lies in the scene as a whole. Taking 4 sets of coordinates (x, y, z, and a color value), every forward facing polygon in a scene must be rendered, shaded, and textured. A z-buffer then checks the forward (z) location of the current polygon against every other polygon in the scene. If the z-value of the current polygon is greater than the z-value of the polygons stored in the z-buffer, the current polygon is stored in the frame buffer. If the z-buffer determines that the polygon is actually behind another polygon, the fully shaded and textured polygon that is not visible is thrown out.
Since the check against the z-buffer occurs after the pixel is already shaded and textured, many pixels that will never be visible in the final scene are fully rendered, shaded and textured only to be thrown out. This process, which results in many pixels being processed that will never be seen or used, is called overdraw, and occurs in nearly every scene rendered on an immediate mode renderer. Typical estimates place the current overdraw in many 3D games today at anywhere from 2 to 4, with an average around 3. This means that on average 2 pixels are actually rendered and then thrown out, with only the topmost pixel being displayed on screen.
As you can imagine, this form of rendering is extremely inefficient. Since fully shaded and textured pixels are thrown out, a good portion of the work an immediate mode rendering video card performs is actually in vain, as the pixels will never see the light of day. It is a direct result of this that current video cards are suffering memory bandwidth limitations, as the data for pixels that will never be used have to travel over the already busy memory bus.