Global illumination is a general name for a group of algorithms used to give more realistic lighting to a 3D scene. These algorithm's allow objects to be lit not only from light rays directly from the light source but also by rays that have bounced off of other surfaces. As seen in the image above the white surfaces are tinted with colour cause by the light rays that have bounced off of the coloured green and red walls.
Ray tracing is a technique that can be used to achieve more realistic lighting in a scene. Ray tracing is basically trying to simulate rays of light. Ray tracing is done by tracing rays of light from a light source and checking to see how it interacts with the objects in the scene and render accordingly.
Shadows:
Shadows are caused by absence of light in an area. In a 3D scene this is usually because there is an object in-between the light source and the object creating an obstruction for the light.
Shadow Maps:
The algorithm for shadow mapping that we went over in class required 2 rendering passes. During the first pass you would render the scene from the light's viewpoint, draw blockers and store nearest z in the z-buffer and the resulting depth buffer is the shadow map. The second pass is done from the observer's viewpoint and you compare the depth of each pixel against the shadow map to determine which pixels are in shadows.
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