Custom directional shadow maps

[mate-h]

Objective

While I was working on terrain rendering, I realized that I needed to make use of custom shadow maps that raymarches the heightfield towards the light. For the terrain material I use apply_pbr_lighting. Using this approach, we better support custom shadow maps while still being able to use the pbr lighting code.

Use cases:

• terrain rendering system optimized shadowing
• cloud shadowing in outdoor scenes

Solution

• pbr_input now includes a new field called directional_shadow_factor which defaults to 1.0
• In the custom material code the user would:

Given some function terrain_shadow_factor that samples a custom shadow map.

var pbr_input: PbrInput = pbr_input_new();
// ...

// set a custom shadow factor
pbr_input.directional_shadow_factor = terrain_shadow_factor(world_position.xyz);

vec4 out_color = apply_pbr_lighting(pbr_input);

Testing

• Tested with https://github.com/mate-h/planetary_terrain_renderer/tree/bevy-main

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Showcase

Context: the custom shadow map translates to a texture binding in my material, output of a compute pass that ray-marches the terrain heightmap towards the light centered on the camera, where R is the highest occluding elevation and G is ray length. and at fragment shade time the shadow factor value is computed to get cheap penumbras on the terrain.
At first I tried wiring it into the regular cascaded shadow maps but that resulted in quite a bad performance so this is essentially an optimization. This technique based on the Red Dead Redemption 2 Siggraph talk from 2019.

Custom shadow map example:

[kfc35]

fyi main has the fix for the ci error