Direct Light vs. Ambient Light
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4/8/2010
Although both direct and ambient light illuminate objects in a scene, they are independent of one another, they have very different effects, and they require that you work with them in completely different ways.
Direct light is just that: direct. Direct light always has direction and color, and it is a factor for shading algorithms, such as Gouraud shading. Different types of lights emit direct light in different ways, creating special attenuation effects. You create a set of light parameters for direct light by calling the IDirect3DMobileDevice::SetLight method. For more information, see Lights.
Ambient light is effectively everywhere in a scene. You can think of it as a general level of light that fills an entire scene, regardless of the objects and their locations in that scene. Ambient light, being everywhere, has no position or direction, only color and intensity. Set the ambient light level with a call to the IDirect3DMobileDevice::SetRenderState method, specifying D3DMRS_AMBIENT (see D3DMRENDERSTATETYPE) as the State parameter, and the desired RGBA color as the Value parameter.
Ambient light color takes the form of an RGBA value, where each component is an integer value from 0 to 255. This is unlike most color values in Microsoft® Direct3D®. For more information, see Color Values for Lights and Materials.
You can use the D3DMCOLOR_RGBA macro to generate RGBA values. The red, green, and blue components combine to make the final color of the ambient light. The alpha component controls the transparency of the color. When using hardware acceleration or RGB emulation, the alpha component is ignored.