In deferred rendering (aka deferred shading), the lighting computation is performed only for visiblefragments, using multi-pass rendering. 

• In the first pass, the geometry is rendered to a geometry buffer, or “g-buffer,” providing colour, normals, depth, etc. for visible fragments.
• In a second pass, lighting is computed only for the fragments in the g-buffer, hence the name deferred rendering.

Source: Wikipedia

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