OpenGL® provides tools like blending, lighting, materials, and GLSL to achieve effects like fog, edge detection, and reflective surfaces. Anti-aliasing reduces pixellation, while shadows and particle systems must be explicitly programmed. Truly reflective surfaces are computationally expensive but add realism to a scene.
There are a number of OpenGL® effects that can be achieved through the use of blending, surface normal scaling, materials, and lighting. OpenGL® doesn’t directly support many special effects, as some graphics cards do, but it does provide tools such as the OpenGL® shader language (GLSL) to allow you to produce some OpenGL® effects with relative ease. Basic effects like fog, edge detection, and lighting tricks can be used to provide a more stylistic rendering of the scene, while effects like truly reflective surfaces, volumetric shadows, and dynamically animated textures can be used to reveal a very realistic scene.
One of the most basic OpenGL® effects that can be found in many applications is anti-aliasing. This involves taking a series of pixels that have been rendered and then calculating the spaces between them to create intermediate steps and colors to surround them. The end result is a scene that can look a little blurry at times, with a softness added to any hard edges that have been drawn. Most graphics hardware supports accelerated anti-aliasing, which reduces annoying pixellation on the screen.
Many programmers who are just starting out are surprised to learn that shadows within a scene are actually one of the OpenGL® special effects that must be explicitly programmed and do not occur by default. This effect can be achieved by rendering the outline of a model into a stencil buffer and then projecting that stencil onto a plane placed in a scene at the appropriate angle. Through more complex blending techniques, it is also possible to create shadows that are more realistic in their depth.
The lighting model used by OpenGL® does not actually calculate the bounce of light rays within a scene in a way that is consistent with the physical world. This has led to the development of truly reflective surfaces as one of the most popular OpenGL® effects. This type of effect essentially renders the scene from a different angle onto an image and then uses that image as a texture for the reflective surface. While computationally expensive, it can help emulate realism in a scene.
One of the most generic and most used OpenGL® effects that can be done with shaders is a particle system. This is a way of applying algorithms, usually fractal algorithms, to elements in a scene to provide objects or other elements that might appear to move or exhibit organic qualities, even though those qualities are actually the result of a static formula. Particle systems can be used in OpenGL® to simulate a burning fire, the movement of birds, or even the shape of clouds in the sky.
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