Lecture #04. Triangle rasterization

Computer graphics in Game development

Ivan Belyavtsev

10.09.2022

Rendering pipeline

Rasterization

After vertex shader stage we have an NDC coordinates of each vertex of the triangle.

Next steps:

  • Convert NDC to screen space coordinates
  • Find the triangle bounding rectangle
  • Clip the triangle to screen space size
  • For each pixel inside the bounding rectangle decide which pixel inside the triangle or not
  • For each pixel inside the triangle compute a color with a pixel shader [1]

Convert NDC to screen space coordinates

Could be done in homogeneous coordinates or cartesian

\[M_{ss} = \left[\begin{array}{cccc} width & 0 & 0 & 0 \\ 0 & - height & 0 & 0 \\ 0 & 0 & 1 & 0 \\ x & height + y & 0 & 1 \end{array}\right]\]

where \(x\) and \(y\) are offsets from the screen space \((0, 0)\) [1]

Bounding rectangle

How to find whether the pixel is inside the triangle - Edge function

Let’s take two points \((X, Y)\) and \((X+dx, Y+dy)\) on an edge

Then, define an edge function:

\[E(x, y) = (x - X)dy - (y - Y)dx\]

[2]

Edge function values

If \(E(x, y)>0\) then \((x, y)\) on the “right” side

If \(E(x, y)<0\) then \((x, y)\) on the “left” side

If \(E(x, y)=0\) then \((x, y)\) on the edge

[2]

Edge function view

From: https://www.cs.drexel.edu/~david/Classes/Papers/comp175-06-pineda.pdf [2]

Clockwise vs counter-clockwise

From: https://www.scratchapixel.com/lessons/3d-basic-rendering/rasterization-practical-implementation/rasterization-stage [3]

Another meaning of the edge function

Lets compare \[E(x, y) = (x - X)dy - (y - Y)dx\]

and \[(u_1, u_2, 0) \times (v_1, v_2, 0) = (0, 0, u_1v_1-u_2v_1)\]

The edge function value is a twice area of the triangle

Barycentric coordinates

Let \(u = \frac{S_{CAP}}{S_{ABC}}\), \(v = \frac{S_{BCP}}{S_{ABC}}\), \(w = \frac{S_{ABP}}{S_{ABC}}\)

Therefore, \(u+v+w=1\), and \(P=uA+vB+wC\). The barycentric could be used for attribute interpolation [4]

From: https://www.scratchapixel.com/lessons/3d-basic-rendering/ray-tracing-rendering-a-triangle/barycentric-coordinates

Pixel shader

Input: interpolated vertex attributes

Output: result pixel color

The pixel shader should be provided by graphics programmer [5]

Lab: 1.05 Triangle rasterization

  1. Implement cg::renderer::rasterizer::edge_function method
  2. Implement pixel_shader lambda for the instance of cg::renderer::rasterizer
  3. Add Rasterization and Pixel shader stages to draw method of cg::renderer::rasterizer
  4. Make sure that you are getting correct image as a result

References

1.
Satran M., Jacobs M. The Direct3D transformation pipeline [Electronic resource]. 2019. URL: https://docs.microsoft.com/en-us/windows/win32/dxtecharts/the-direct3d-transformation-pipeline.
2.
Pineda J. A parallel algorithm for polygon rasterization // Proceedings of the 15th annual conference on computer graphics and interactive techniques. 1988. P. 17–20.
3.
Rasterization: A practical implementation [Electronic resource]. 2015. URL: https://www.scratchapixel.com/lessons/3d-basic-rendering/rasterization-practical-implementation/rasterization-stage.
4.
5.
Satran M. et al. Pixel shader stage [Electronic resource]. 2020. URL: https://docs.microsoft.com/en-us/windows/win32/direct3d11/pixel-shader-stage.
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