3D Fundamentals Tutorial 11

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Things that are behind get covered up by things that are in front.


The tutorial video is on YouTube here.

  • The need for occlusion in scenes with multiple objects 0:24
  • We used culling of triangles when drawing convex shapes
  • But this doen't solve our problem with occlusion (e.g. multiple meshes in the same scene)
  • One could considers the painter's algorithm, but that's inefficient and of limited use
  • Solution: the z-buffer: simple, elegant & performant 2:27
  • Essentially sorts triangles based on their depth at the pixel level
  • Uses two 2D-arrays (for x,y coordinates): a back-buffer (of colors, in gfx in our case) as a render target, and a z-buffer (of floats) to store depth
  • We initialize the z-buffer with infinity
  • Whenever we want to write to the back-buffer, we check to see if z-value of the pixel we are drawing is smaller (and set the z-buffer in that case)
  • This is a special sorting case 6:26
  • Implementation in code 7:06
  • The ZBuffer class
  • Changes to the Pipeline class, if (zb.TestAndSet(x,y,z)) before the PutPixel call
  • If done well, Z-testing can reduce rendering time 9:54
  • Works best if geometries are sorted and drawn from front to back (more Z-test rejections)
  • This is contrary to intuition and old/other techniques (e.g. in 2D: drawing sprites and tilemaps)
  • Scene definitions and demo 10:58
  • Differences with Hardware APIs 11:50
  • These provide more options for configuring the z-buffer, such as depth comparison operators, choice of formats/types
  • These typically store 1/z in the buffer, requiring corrections for depth precision (high in the near plane, low in the far plane)
  • Dealing with translucent pixels and depth 13:58
  • You will need to store the order of all the translucent pixels up until the first opaque one
  • Provide more flexibility to reject pixels & have more control (general purpose bufffers)
  • E.g. for UIs, HUDs, mirrors, shadow effects etc.
  • Next up: vertex shaders and working towards dynamic lighting effects 15:00


The GitHub repository for the tutorial code is here.

Paper on depth precision

See also