init

engine/include/hd2d/Billboard.h

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+#pragma once
+#include "hd2d/GL.h"
+#include "hd2d/Texture.h"
+
+#include <glm/glm.hpp>
+#include <vector>
+
+namespace hd2d {
+
+// One upright pixel-art sprite. `pos` is where the lowest opaque pixel row
+// touches the ground; sprites always face the camera around the Y axis.
+struct BillboardInstance {
+    glm::vec3 pos{0.0f};
+    float sway = 0.0f;       // wind / player-push response (0 = rigid)
+    glm::vec2 size{1.0f};    // world units of the full texture quad
+    float flip = 0.0f;       // 1 = mirror horizontally
+    float tint = 1.0f;       // brightness multiplier (breaks up repetition)
+};
+
+// Instanced batch of billboards sharing one texture.
+class BillboardBatch {
+public:
+    BillboardBatch() = default;
+    BillboardBatch(const BillboardBatch&) = delete;
+    BillboardBatch& operator=(const BillboardBatch&) = delete;
+    ~BillboardBatch();
+
+    bool loadTexture(const std::string& path) { return texture.load(path); }
+    void upload();  // call after editing `instances`
+    void draw() const;
+
+    // World size that keeps the texture at `pixelsPerUnit` texel density.
+    glm::vec2 sizeForDensity(float pixelsPerUnit) const {
+        return glm::vec2(texture.size()) / pixelsPerUnit;
+    }
+
+    Texture texture;
+    std::vector<BillboardInstance> instances;
+    bool castShadows = true;
+    bool inReflections = true;
+    float emissive = 0.0f;  // unlit contribution, for glowing sprites
+
+private:
+    GLuint vao_ = 0, vbo_ = 0;
+    size_t capacity_ = 0;
+    GLsizei count_ = 0;
+};
+
+}  // namespace hd2d