Note about "Deferred Shading"

Deferred rendering

• Problems of Lighting
  • It may waste time on fragments which not not actually visible.
  • It may waste time on objects which is out of the reach of the light.
  • The complexity is high: N objects X M lights. It becomes worse if there is heavy pixel shading.
• Key points in Deferred Rendering
  • Lighting calculations would be performed in image space to reduce complexity.
  • Only ran on fragments that ended up as pixels in the final back buffer.
  • It is a way to make real-time lighting a kind of post-processing effect.
  • Deferred rendering is a multi-pass rendering algorithm.
    • G-Buffer, storing info like Position, Normal, Tex which are needed in Lighting Pass.
  • It may be implemented as frame buffer with multiple attachments in OpenGL.
• Positives of Deferred Shading
  • Easy for screen space ray-tracing.
  • Easy for applying shadow.
  • Easy for Area Lighting.
• Negatives in Deferred Rendering
  • This post summarize the defects of Deferred Shading.
  • massive bandwidth consuming by G-Buffer.
  • Hard for Alpha blend, for example transparent objects, so how UE4 handle this issue ?
  • Hard for MSAA.
  • Material limitation by G-Buffer.

Frame Buffer in OpenGL

• OpenGL FBOs support multiple rendering targets
  • That is, a single fragment shader can write to multiple colour attachments.
  • The number of attachments supported is dependent on graphics hardware, may be as many as 8 attachments.
  • The beauty of multiple render targets is that whatever information you think you might need, you can get on a per-pixel basis, and store into a texture.
  • Example of rendering to multiple target
    • Store scene’s normal on a per-pixel basis, in order to process them later.
    • Write fragments above a certain luminance to a separate render target in order to blur them later.
  • Constraints of OpenGL FBO's attachments
    • Attachments cannot be of a compressed type.
    • All of the attachments must be of the same dimensions.
  • Tutorial-Post-Processing@research.ncl.ac.uk

Reference

• Tutorial-Deferred-Rendering@research.ncl.ac.uk
• Tutorial-Post-Processing@research.ncl.ac.uk