Shadow Mapping

Shadow Mapping with Directional Light

Workflow

Render the scene from light’s perspective, values in z-buffer is the shadow map.
Render the scene from camera’s perspective. Check if current fragment is occluded by using shadow map.
PCF, set a bias for shadow acne, etc.

Implementation

Render the scene from light’s perspective. This demo has 1 and only 1 directional light as light source.
- Since it’s directional light, orthographic projection is used to calculate projection matrix.
- View projection is calculated based on light source’s position and orientation.
- Model matrix doesn’t change.
  
  Visualized Shadow Map:
- The darker the region the smaller z value is.
- The size of shadow map depends on the size of depth map, in this case, the size of texture. Left down part of above picture is the shadow map.
Render the scene from camera’s perspective. Using Shadow Map to calculate shadow.
1. In vertex shader, transform vertices from local space to “light source” clip space by applying PVM from part 1 and pass the coordinates to fragment shader.
  
  vs_out.lightSpaceFragPos = lightProjection * lightView * model * vec4(aPos, 1.0f);
2. Since OpenGL does perspective division after vertex shader. The “light source” clip space coordinate should perform perspective division manually.
  
  vec3 projCoord = fs_in.lightSpaceFragPos.xyz / fs_in.lightSpaceFragPos.w; /* clip space to NDC */
3. Now coordinates are in NDC, in the range of [-1.0, 1.0]. Coordinates in depth buffer is in the range of [0.0, 1.0]. Mapping coords from [-1.0, 1.0] to [0.0, 1.0]
  
  projCoord = projCoord * 0.5f + 0.5f;
4. Fianlly, sampling values from shadow map.
  
  float closestDepth = texture(depthMap, projCoord.xy).r;
5. Compare values in depth buffer with the fragment’s z value. If fragment’s z value is larger, it means it was occluded which should be in the shadow.
  
  float currentDepth = projCoord.z;
  
  float shadow = currentDepth > closestDepth;
  
  Result:
PCF, set a bias for shadow acne, etc.
- The picture above has black lines, it’s called shadow acne. In short, low resolution of shadow map causes this.
- One of the solutions is add a bias to fragment’s z value.
  
  After bias applied:
- The picture above also has zig zag edges.
- Applying percentage-closer filtering(PCF), this demo simply samples surrounding texels and average the result. This produces relative soft edges.
  
  PCF:

Resource

https://learnopengl.com/Advanced-Lighting/Shadows/Shadow-Mapping

https://learnopengl.com/Advanced-Lighting/Shadows/Point-Shadows

https://www.youtube.com/watch?v=6Ofj9xppNew

Shadow Mapping with Directioanl Light

Workflow

Implementation

Resource

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