forge-grass-rendering

Add grass rendering to an SDL GPU project using `forge_scene.h`. Implements instanced segmented grass blades with wind animation, density-based LOD rings, and tile-based terrain LOD with vertex shader geomorphing. ## When to use - Adding vegetation to a 3D scene - Implementing instanced rendering with per-instance attributes - Adding LOD systems for both terrain and foliage - Split-screen or multi-viewport rendering ## Key API calls - `forge_shapes_plane()` — generate tile meshes at different resolutions - `forge_scene_upload_buffer_deferred()` — per-frame instance buffer upload - `forge_scene_create_shader()` — create custom shaders alongside forge_scene - `SDL_SetGPUViewport()` / `SDL_SetGPUScissor()` — split-screen viewports - `SDL_DrawGPUIndexedPrimitives()` — instanced draw with per-instance buffer - `forge_noise_fbm2d()` — procedural heightmap generation - `forge_hash_wang()` / `forge_hash_to_float()` — deterministic placement variation ## Correct order 1. Generate procedural heightmap via fBm noise 2. Upload heightmap as R32_FLOAT GPU texture 3. Create tile meshes at multiple LOD resolutions (both triangle and line IBs) 4. Create grass blade meshes at multiple segment counts 5. Create pipelines: terrain fill/line/shadow + grass fill/line/shadow 6. Per frame: a. Select terrain tile LODs based on camera distance b. Place grass instances with distance/frustum/slope culling c. Upload instance buffers via deferred upload d. Shadow pass: draw terrain tiles + near grass into shadow map e. Main pass: set left viewport → draw terrain + grass filled; set right viewport → draw terrain + grass wireframe f. Reset viewport for UI ## Grass blade geometry Each blade is a vertical segmented strip with `2*(N+1)` vertices and `2*N` triangles. The `height_t` attribute (0 at base, 1 at tip) controls wind weighting and tapering. ```c for (int i = 0; i <= segments; i++) { float t = (float)i / (float)segments; float half_w = 0.5f * (1.0f - t * GRASS_BLADE_TIP_TAPER); verts[2*i+0] = (GrassVertex){ {-half_w, t, 0}, t }; verts[2*i+1] = (GrassVertex){ { half_w, t, 0}, t }; } ``` ## Per-instance data ```c typedef struct { float position[3]; /* world-space base position */ float rotation; /* Y-axis rotation (radians) */ float scale[2]; /* width, height */ float color[3]; /* RGB tint */ float _pad; /* alignment to 40 bytes */ } GrassInstance; ``` ## Wind animation (vertex shader) ```hlsl float wind_weight = height_t * height_t; /* quadratic: tip bends most */ float phase = time * wind_speed + world.x * 0.7 + world.z * 0.3; float wind_offset = sin(phase) * wind_strength * wind_weight; ``` ## Terrain geomorphing (vertex shader) ```hlsl float2 snapped_uv = round(global_uv / coarse_cell_size) * coarse_cell_size; float2 morphed_uv = lerp(global_uv, snapped_uv, morph_factor); float morphed_x = lerp(world_x, snapped_x, morph_factor); float h = height_tex.SampleLevel(smp, morphed_uv, 0).r; ``` ## Common mistakes - **Forgetting half-aspect projection** — split-screen viewports are half the screen width, so the projection matrix must use `half_w / height` aspect - **Using TRIANGLELIST pipeline for wireframe** — the right viewport needs a LINELIST pipeline with a separate line index buffer (each triangle edge as two indices) - **Grass shadow without wind** — the shadow vertex shader must apply the same wind animation as the main shader, or shadows will not match blade positions - **Missing morph factor** — without geomorphing, terrain tile LOD switches cause visible popping; morph_factor must ramp 0→1 in the last 20% of each LOD distance band - **Forgetting to reset viewport for UI** — after right-viewport draws, reset viewport and scissor to full screen before forge_scene_end_main_pass ## Reference See [Lesson 50 — Grass Rendering](../../../lessons/gpu/50-grass-rendering/) for the full implementation.