How do you blend multiple shadow maps and cascades to achieve smooth and consistent shadows?

1 What are shadow maps and cascades?

Shadow maps are a way of projecting shadows from a light source onto the scene. They work by rendering the scene from the light's perspective and storing the depth values of each pixel in a texture. Then, in the main rendering pass, you can compare the depth of each pixel from the camera's perspective with the corresponding depth from the light's perspective. If the camera's depth is greater than the light's depth, it means that the pixel is in shadow. Otherwise, it is lit. Shadow map cascading is a technique that divides the view frustum into several regions, each with its own shadow map. The regions closer to the camera have higher resolution shadow maps, while the regions farther away have lower resolution shadow maps. This way, you can avoid wasting memory and processing power on distant shadows that are barely visible.

2 How to create multiple shadow maps?

To create multiple shadow maps, you need to split the view frustum into different segments, usually based on a logarithmic scale. For each segment, you need to calculate a new projection matrix for the light that covers the segment's bounds. Then, you need to render the scene from the light's perspective using this matrix and store the depth values in a separate texture. You can use a texture array or a texture atlas to store multiple shadow maps in one texture. You also need to keep track of the split distances and the projection matrices for each segment, as you will need them later for blending.

3 How to blend multiple shadow maps?

To blend multiple shadow maps, you need to determine which segment each pixel belongs to based on its distance from the camera. You can use a simple if-else statement or a binary search to find the segment index. Then, you need to sample the corresponding shadow map using the projection matrix for that segment. However, this can create hard edges between the segments, as the shadow map resolution changes abruptly. To avoid this, you can use a technique called percentage-closer filtering (PCF), which samples multiple neighboring pixels from the shadow map and averages their depth values. This creates a smoother transition between the segments and reduces aliasing.

4 How to adjust the cascade parameters?

The number and size of the segments can affect the quality and performance of your shadows. If you have too many segments, you will need more memory and processing power to render and sample them. If you have too few segments, you will lose detail and accuracy in your shadows. Therefore, you need to find a balance between the two. You can adjust the cascade parameters by changing the split scheme, the near and far planes, and the bias values. The split scheme determines how the view frustum is divided into segments. You can use a uniform, linear, or logarithmic scheme, or a combination of them. The near and far planes determine the range of distances that are covered by the shadow maps. You can set them based on your scene's scale and depth range. The bias values are used to avoid shadow acne, which is a phenomenon where self-shadowing artifacts appear due to numerical errors. You can set them based on your shadow map resolution and depth precision.

5 How to optimize the shadow rendering?

Rendering and sampling multiple shadow maps can be expensive, especially in complex scenes with large view frustums. To optimize shadow rendering and improve performance, you can use a variety of techniques, such as frustum and occlusion culling, level of detail (LOD) to reduce geometry and texture complexity, and different filtering methods like variance shadow maps (VSM), exponential shadow maps (ESM), or moment shadow maps (MSM) to store more information than just depth values in the shadow maps. These methods allow you to apply blur and anti-aliasing filters to your shadow maps without sacrificing accuracy.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?