The OpenGL® depth buffer is a useful tool for 3D graphics programmers, but it can be optimized by setting clipping planes correctly, clearing the buffer, and avoiding depth issues. For 2D graphics, the depth buffer can help with tasks like compensating for tiles and creating fade effects. It’s best to use the 16-bit version of the buffer for faster rendering and wider compatibility.
The OpenGL® depth buffer is one of the most misunderstood, complex, and ultimately useful tools available to a three-dimensional (3D) graphics programmer. There are several ways the buffer can be optimized to increase the frame rate for a program, including setting the near and far clipping planes correctly. Other tips include clearing the buffer between renders and avoiding scene comps which can cause depth issues by placing objects too close together. Some two-dimensional (2D) graphics tricks can be done easily and efficiently using the OpenGL® depth buffer. Even the graphics card you use can sometimes be an issue with depth, so using the correct setting can help increase speed and reduce unnecessary processing cycles.
One of the first issues that can affect the performance of the OpenGL® depth buffer, also called the Z buffer after the letter that traditionally denotes the Cartesian depth plane, is the placement of the near and far clipping planes. These planes define the boundaries of what should and shouldn’t be rendered in a scene, and their values indicate the distance from the viewer where clipping should start and end, respectively. An intuitive thought would be to start rendering where the viewer is, setting the plane close to zero, but this is actually incorrect. In fact, OpenGL® does not allow you to reset the neighboring plane. If the neighbor plane value is very small, such as a fraction of one, the renderer may not show anything or may fail to sort the depth buffer correctly.
This occurs because the closer an object is to the viewer, the more accurately OpenGL® calculates the object’s position. As the distance from the viewer approaches zero, the calculated accuracy increases exponentially. This slows rendering time and can lead to strange graphical artifacts and other problems, other than that the level of precision calculated is rarely needed.
When rendering 2D graphics, the OpenGL® depth buffer can help simplify some tasks. Using the depth buffer to slightly compensate for the quads that are used as tiles in a composition can help elements in a tiled scene move smoothly without generating weird effects in the image caused by two overlapping polygons in the same plane. Similarly, prepared elements can be hidden out of sight or behind the far clipping plane so that they can be brought into the scene quickly, possibly even using transforms and rotations to give a special fade in or fade out effect.
Finally, while the OpenGL® depth buffer can support different hardware buffer sizes, it is best to use the 16-bit version. This is because, in most scenes, using a 32-bit buffer can slow down render time. Also, not all graphics cards support 32-bit depth buffers, so going down to the lowest common denominator means more people will be able to run the 3D program as it was written.
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