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engine/include/hd2d/GroundOcclusion.h

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+#pragma once
+#include "hd2d/GL.h"
+
+#include <glm/glm.hpp>
+#include <vector>
+
+namespace hd2d {
+
+class ModelBatch;
+
+// Baked contact ambient occlusion on the ground under 3D scenery: a dark,
+// tight core at a tree trunk / bush / rock base plus a wide, soft canopy
+// shadow under tall models. Sampled by terrain, sprite and model shaders, so
+// grass tufts and bushes growing at a tree's foot darken too. Rebuild after
+// moving instances.
+class GroundOcclusion {
+public:
+    GroundOcclusion() = default;
+    GroundOcclusion(const GroundOcclusion&) = delete;
+    GroundOcclusion& operator=(const GroundOcclusion&) = delete;
+    ~GroundOcclusion();
+
+    // Covers the rectangle [min, max] (world xz). Uses every batch whose
+    // `groundOcclusion` strength is > 0.
+    void build(glm::vec2 min, glm::vec2 max, const std::vector<ModelBatch*>& batches, int texelsPerUnit = 8);
+
+    GLuint texture() const { return tex_; }
+    glm::vec4 rect() const { return rect_; }  // matches uGroundAO in common.glsl
+
+private:
+    GLuint tex_ = 0;
+    glm::vec4 rect_{0.0f};
+};
+
+}  // namespace hd2d