feat: basic shadow map + renderdoc labels

destrum/assets_src/shaders/shadows.glsl

@@ -0,0 +1,34 @@
+#ifndef SHADOWS_GLSL
+#define SHADOWS_GLSL
+
+// expects bindless.glsl to be included before this file (it has no include guard)
+
+// Returns 1.0 when fully lit, 0.0 when fully in shadow
+float calculateDirectionalShadow(vec3 fragPos, vec3 n, mat4 lightViewProj, uint shadowMapID, float normalOffset)
+{
+    // pushing the sample point along the normal hides most of the shadow acne
+    vec4 lightClip = lightViewProj * vec4(fragPos + n * normalOffset, 1.0);
+    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) {
+        return 1.0;
+    }
+
+    vec2 texelSize = 1.0 / vec2(textureSize(sampler2DShadow(textures[shadowMapID], samplers[SHADOW_SAMPLER_ID]), 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));
+        }
+    }
+
+    // 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;
+}
+
+#endif // SHADOWS_GLSL