Deferred shading

In the field of 3D computer graphics, deferred shading is a screen-space shading technique. It is called deferred because no shading is actually performed in the first pass of the vertex and pixel shaders: instead shading is "deferred" until a second pass.

On the first pass of a deferred shader, only data that is required for shading computation is gathered. Positions, normals, and materials for each surface are rendered into the geometry buffer (G-buffer) using "render to texture". After this, a pixel shader computes the direct and indirect lighting at each pixel using the information of the texture buffers in screen space.

Screen space directional occlusion can be made part of the deferred shading pipeline to give directionality to shadows and interreflections.

The primary advantage of deferred shading is the decoupling of scene geometry from lighting. Only one geometry pass is required and each light is only computed for those pixels that it actually affects. This gives the ability to render many lights in a scene without a significant performance-hit. There are some other advantages claimed for the approach. These advantages may include simpler management of complex lighting resources, ease of managing other complex shader resources, and the simplification of the software rendering pipeline.

One key disadvantage of deferred rendering is the inability to handle transparency within the algorithm, although this problem is a generic one in Z-buffered scenes and it tends to be handled by delaying and sorting the rendering of transparent portions of the scene. Depth peeling can be used to achieve order-independent transparency in deferred rendering, but at the cost of additional batches and g-buffer size. Modern hardware, supporting DirectX 10 and later, is often capable of performing batches fast enough to maintain interactive frame rates. When order-independent transparency is desired (commonly for consumer applications) deferred shading is no less effective than forward shading using the same technique.

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