feat: add spot light shadows with a tilled buffer

destrum/assets_src/shaders/shadows.glsl

@@ -1,7 +1,30 @@
 #ifndef SHADOWS_GLSL
 #define SHADOWS_GLSL
 
-// expects bindless.glsl to be included before this file (it has no include guard)
+// expects bindless.glsl and lights.glsl to be included before this file
+
+// Returns how much of a 3x3 area around uv is lit, every tap stays inside [minUV, maxUV]
+float sampleShadowPCF(uint shadowMapID, vec2 uv, float depth, vec2 minUV, vec2 maxUV)
+{
+    vec2 texelSize = 1.0 / vec2(textureSize(sampler2DShadow(textures[shadowMapID], samplers[SHADOW_SAMPLER_ID]), 0));
+
+    float shadowed = 0.0;
+    for (int x = -1; x <= 1; x++) {
+        for (int y = -1; y <= 1; y++) {
+            vec2 sampleUV = clamp(uv + vec2(x, y) * texelSize, minUV, maxUV);
+            shadowed += sampleTexture2DShadow(shadowMapID, vec3(sampleUV, depth));
+        }
+    }
+
+    // the shadow sampler compares with GREATER_OR_EQUAL (it was set up for reversed Z),
+    // with regular depth that returns 1 when the fragment is behind the occluder
+    return 1.0 - shadowed / 9.0;
+}
+
+bool isOutsideShadowMap(vec2 uv, float depth)
+{
+    return any(lessThan(uv, vec2(0.0))) || any(greaterThan(uv, vec2(1.0))) || depth > 1.0;
+}
 
 // Returns 1.0 when fully lit, 0.0 when fully in shadow
 float calculateDirectionalShadow(vec3 fragPos, vec3 n, mat4 lightViewProj, uint shadowMapID, float normalOffset)
@@ -11,24 +34,37 @@ float calculateDirectionalShadow(vec3 fragPos, vec3 n, mat4 lightViewProj, uint
     vec3 lightNDC = lightClip.xyz / lightClip.w;
     vec2 uv = lightNDC.xy * 0.5 + 0.5;
 
-    // outside the shadow map counts as lit
-    if (any(lessThan(uv, vec2(0.0))) || any(greaterThan(uv, vec2(1.0))) || lightNDC.z > 1.0) {
+    if (isOutsideShadowMap(uv, lightNDC.z)) {
         return 1.0;
     }
 
-    vec2 texelSize = 1.0 / vec2(textureSize(sampler2DShadow(textures[shadowMapID], samplers[SHADOW_SAMPLER_ID]), 0));
+    return sampleShadowPCF(shadowMapID, uv, lightNDC.z, vec2(0.0), vec2(1.0));
+}
 
-    // 3x3 PCF, every tap is already bilinearly filtered by the compare sampler
-    float shadowed = 0.0;
-    for (int x = -1; x <= 1; x++) {
-        for (int y = -1; y <= 1; y++) {
-            shadowed += sampleTexture2DShadow(shadowMapID, vec3(uv + vec2(x, y) * texelSize, lightNDC.z));
-        }
+float calculateSpotShadow(vec3 fragPos, vec3 n, vec3 lightPos, SpotShadow spotShadow,
+                          uint shadowIndex, uint atlasID, uint tilesPerRow)
+{
+    float normalOffset = distance(fragPos, lightPos) * spotShadow.normalOffsetPerMeter;
+    vec4 lightClip = spotShadow.lightViewProj * vec4(fragPos + n * normalOffset, 1.0);
+    if (lightClip.w <= 0.0) {
+        return 1.0;
     }
 
-    // the shadow sampler compares with GREATER_OR_EQUAL (it was set up for reversed Z),
-    // with regular depth that returns 1 when the fragment is behind the occluder
-    return 1.0 - shadowed / 9.0;
+    vec3 lightNDC = lightClip.xyz / lightClip.w;
+    vec2 tileUV = lightNDC.xy * 0.5 + 0.5;
+    if (isOutsideShadowMap(tileUV, lightNDC.z)) {
+        return 1.0;
+    }
+
+    vec2 tile = vec2(shadowIndex % tilesPerRow, shadowIndex / tilesPerRow);
+    float tileScale = 1.0 / float(tilesPerRow);
+    vec2 atlasTexelSize = 1.0 / vec2(textureSize(sampler2DShadow(textures[atlasID], samplers[SHADOW_SAMPLER_ID]), 0));
+
+    vec2 atlasUV = (tile + tileUV) * tileScale;
+    vec2 tileMinUV = tile * tileScale + atlasTexelSize;
+    vec2 tileMaxUV = (tile + 1.0) * tileScale - atlasTexelSize;
+
+    return sampleShadowPCF(atlasID, atlasUV, lightNDC.z, tileMinUV, tileMaxUV);
 }
 
 #endif // SHADOWS_GLSL