234 lines
12 KiB
Markdown
234 lines
12 KiB
Markdown
# HD-2D Engine
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A small C++20 / OpenGL 4.5 engine that recreates the look of Square Enix's
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**HD-2D** style (Octopath Traveler, Triangle Strategy, Live A Live, Dragon
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Quest III HD-2D): a 3D diorama world where the ground is skinned in pixel art,
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characters and small props (flowers, grass tufts, tall grass) are billboarded
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pixel-art sprites, and scenery like trees, bushes and rocks are real **3D
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models with higher-resolution painted textures**. Lighting, shadows, ambient
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occlusion, fog, god rays, water, fire, rain and particles are all rendered as
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high-resolution 3D effects.
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## What makes HD-2D look like HD-2D
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| Trait | How this engine does it |
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| --- | --- |
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| 3D world built from pixel-art textures | Tile-based terraced terrain (`Terrain`) with pixel-art material layers, organically jittered material borders at texel granularity, grass overhangs on cliffs, baked corner AO |
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| Crisp pixels at any distance | "Sharp bilinear" sampling (`pixelart.glsl`): hard texel edges when magnified, antialiased + mipmapped when minified (no `GL_NEAREST` shimmer) |
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| Upright 2D sprites in 3D | Characters and small props are camera-facing (Y-axis) billboards (`BillboardBatch`), planted by their lowest opaque pixel row |
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| 3D scenery with painted textures | Trees, bushes, rocks and the stone bridge are instanced glTF models (`ModelLibrary`, `ModelBatch`) built in Blender: leaf-card canopies with alpha-tested 1K leaf textures, rounded custom normals for soft stylized shading, sun translucency, wind sway from per-vertex weights, emissive materials (glowing coals) |
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| Grounded objects | Two kinds of ambient occlusion: half-res SSAO (depth-reconstructed normals, depth-aware blur, multiplied into the opaque scene before water) and a baked **ground contact occlusion** map (`GroundOcclusion`) that puts a dark core at every trunk / bush / rock base plus a soft canopy shade, applied to terrain, sprites and models near the ground |
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| Nothing hides the player | Tall scenery (tree canopies) between the camera and the player is dithered away around the sprite, only above the sprite's head; low props opt out per batch (`ModelBatch::occluderFade = false`, used for the bridge, rocks and bushes) |
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| Sprites cast/receive real shadows | Shadow pass renders sprites as **sun-facing** cards so silhouettes are never edge-on; receivers sample their own sun-facing card to avoid self-shadowing |
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| Miniature / diorama feel | Narrow FOV (~30°) high camera + strong depth-of-field with an extra tilt-shift band |
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| Soft, warm, glowing light | HDR pipeline, Karis-averaged mip-chain bloom, ACES tonemap, lift/gain grading, vignette, subtle chromatic aberration and grain |
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| Fog and god rays | Half-res ray-marched height fog; every step samples the sun shadow map and a sparse "canopy" beam pattern, giving physically shadowed light shafts; depth-aware upsample |
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| Dappled forest light | Animated canopy cookie projected along the sun direction onto every surface |
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| Beautiful water | Planar reflections, depth-aware refraction, per-channel absorption, analytic-derivative wave normals, shore distance field driving foam + ripples, strong HDR sun glints plus a field of twinkling point sparkles that bloom and depth of field turn into bokeh, caustics on the bottom |
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| Living foliage | Instanced swaying grass with wind gusts; tall grass and bushes bend and rustle away from the player (vertex shader, Pokémon-style patches); leaf flutter scales with wind strength |
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| High-res particles | Fireflies (also feed point lights) and dust motes that only glow inside sun shafts |
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| Fire | Brazier fires (`ParticleSystem::addFire`): a compact, bowl-filling mass of nearly opaque, noise-eroded flame licks shaded per pixel from a hot yellow core through orange to red tips (drawn oldest-first so the core sits on top), turbulent embers, lit smoke puffs, a glow halo, and a flickering point light that also scatters into the fog |
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| Weather | `Weather` blends the clear look into a storm preset (weak grey sun, overcast sky, thick low fog, gusty wind): procedural world-space rain streaks (up to 18k, one draw call), ground splashes, raindrop ripple rings on the water, wet darker glossy surfaces, ambient fireflies fade out |
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References: Unreal Engine's [Octopath Traveler spotlight](https://www.unrealengine.com/spotlights/octopath-traveler-s-hd-2d-art-style-and-story-make-for-a-jrpg-dream-come-true),
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the [Octopath Traveler II interview](https://www.unrealengine.com/developer-interviews/octopath-traveler-ii-builds-a-bigger-bolder-world-in-its-stunning-hd-2d-style),
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the Unreal Fest Europe 2019 talk ["The Fusion of Nostalgia and Novelty"](https://www.unrealengine.com/events/unreal-fest-europe-2019/the-fusion-of-nostalgia-and-novelty-in-the-development-of-octopath-traveler),
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and [The Verge on HD-2D's evolution](https://www.theverge.com/entertainment/840080/square-enix-hd2d-games-octopath-dragon-quest).
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## Building
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Requirements: CMake ≥ 3.20, a C++20 compiler, OpenGL 4.5, `libepoxy`.
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GLFW, GLM, stb and cgltf are used from the system when found and fetched
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otherwise; Dear ImGui is always fetched.
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```sh
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# Arch / CachyOS
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sudo pacman -S cmake ninja glfw glm libepoxy stb
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cmake -S . -B build -G Ninja
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ninja -C build
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./build/hd2d_demo
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```
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The asset path is baked in at configure time (`HD2D_ASSET_DIR`), so the demo can
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be run from any working directory.
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## Demo controls
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| Input | Action |
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| --- | --- |
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| WASD / arrows | Move the sprite (camera-relative) |
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| Q / E, right-drag | Rotate / orbit camera |
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| Mouse wheel | Zoom |
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| F1 | Toggle the tuning panels (sun, sky, AO, fog, god rays, water, DoF, bloom, grading, weather) |
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| R | Toggle heavy rain (5 s transition) |
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| F5 | Hot-reload shaders |
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| F12 | Screenshot to `screenshots/` |
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| Esc | Quit |
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Scriptable captures (useful for look-dev and regression checks):
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```sh
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./build/hd2d_demo --screenshot out.png --frames 60 --pos 25,43.6 \
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--sun -35,38 --yaw 20 --dist 12 --move 1,0 --rain 1 --set godRays=6 --set dof=0
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```
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`--rain 0..1` starts the capture already raining. `--set` accepts:
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`volumetrics dof bloom reflections occluderFade ssao groundAO ssaoRadius
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ssaoIntensity fogDensity fogHeightFalloff godRays shaftStrength shaftCoverage
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shaftScale cookieStrength exposure bloomStrength tiltShift focusRange`.
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## Layout
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```
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engine/include/hd2d/ public headers
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engine/src/ engine implementation
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assets/shaders/ GLSL (common.glsl = shared UBO + lighting helpers)
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assets/textures/ PixelLab-generated pixel art (terrain tiles, grass, flowers)
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assets/sprites/ character sprites
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assets/models/ scenery.glb (trees, bushes, rocks, bridge) + src/ painted textures
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demo/ meadow demo scene + main loop
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tools/pixellab.py pixel-art generator, driven by tools/assets.json
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tools/scenery_textures.py paints the high-res scenery textures
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tools/blender_scenery.py builds + exports the 3D scenery in Blender
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docs/ASSET_GUIDE.md how to make sprites, tiles, textures and models (for humans and AI agents)
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AGENTS.md build/verify workflow and code rules for AI agents
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```
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### Frame pipeline (`Renderer::render`)
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```mermaid
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flowchart LR
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A[Sun shadow map<br/>terrain, 3D models, sun-facing sprite cards] --> B[Planar reflection<br/>half res, clipped]
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B --> C[HDR scene<br/>sky, terrain, models, billboards]
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C --> C2[SSAO<br/>half res, multiplied in]
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C2 --> D[Water<br/>reads color/depth copies]
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D --> E[Volumetric fog + god rays<br/>half res ray march, fire haze]
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E --> F[Fog composite<br/>depth-aware upsample]
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F --> F2[Rain streaks]
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F2 --> G[Particles<br/>fire, smoke, fireflies]
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G --> H[Depth of field<br/>tilt-shift]
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H --> I[Bloom mip chain]
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I --> J[Tonemap + grade]
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```
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All per-frame data lives in one std140 uniform block (`FrameUBO` in
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`Renderer.cpp` ⇄ `Frame` in `common.glsl`); keep them in sync.
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### Using the engine in your own project
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```cpp
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hd2d::Window window; window.create(1600, 900, "My Game");
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hd2d::Renderer renderer; renderer.init("assets/shaders", window.width(), window.height());
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hd2d::Terrain terrain; /* init, setLevel/setMaterial, textures.load(...), build() */
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hd2d::BillboardBatch hero; hero.loadTexture("assets/sprites/hero.png");
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hero.instances.push_back({{10, 0, 10}, 0.0f, hero.sizeForDensity(32.0f)});
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hero.upload();
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hd2d::ModelLibrary models; models.load("assets/models/scenery.glb");
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hd2d::ModelBatch oaks(models.get("oak_a"));
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oaks.instances.push_back({{14, 0, 12}, /*yaw*/ 0.7f, /*scale*/ glm::vec3(1.0f), /*tint*/ 1.0f});
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oaks.upload();
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hd2d::Scene scene;
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scene.terrain = &terrain;
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scene.billboards = {&hero};
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scene.models = {&oaks};
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hd2d::GroundOcclusion contactAO; // dark contact shadow at tree bases
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oaks.groundOcclusion = 1.0f;
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contactAO.build({0, 0}, {64, 64}, {&oaks});
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scene.groundOcclusion = &contactAO;
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hd2d::ParticleSystem particles; // fire: flames, embers, smoke, glow, light
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particles.addFire({12, 1.5f, 12});
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scene.particles = &particles;
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scene.volumetricLights = 1; // the fire light also glows in the fog
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hd2d::Weather weather; // weather.setRain(1.0f) for a storm
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hd2d::Camera cam; cam.target = {10, 0.7f, 10};
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float time = 0.0f;
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while (window.pollEvents()) {
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time += 1.0f / 60.0f;
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particles.update(1.0f / 60.0f, time);
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scene.lights.clear();
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particles.collectLights(scene.lights, 12);
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weather.update(1.0f / 60.0f, scene, renderer.settings);
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renderer.render(scene, cam, time);
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window.swapBuffers();
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}
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```
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`demo/DemoScene.cpp` is the full reference: terrain generation, prop
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scattering, bridge walking, fires from model sockets, contact AO, water +
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shore field, particles, weather, player movement and collision.
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## Assets
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The full guide to making sprites, terrain tiles, high-res textures and 3D
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models (scale, conventions, PixelLab manifest fields, Blender helpers, how to
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check results) is in [`docs/ASSET_GUIDE.md`](docs/ASSET_GUIDE.md). Summary:
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### 3D scenery (Blender)
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Trees, bushes, rocks and the stone bridge (with braziers) are generated
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procedurally so they can be tweaked and rebuilt at any time:
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```sh
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python3 tools/scenery_textures.py # paints assets/models/src/*.png (numpy + Pillow)
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blender -b -P tools/blender_scenery.py # builds the models, writes assets/models/scenery.glb
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```
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`blender_scenery.py` can also be run inside a live Blender session (e.g. through
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the Blender MCP) with `exec(open(".../tools/blender_scenery.py").read())`; it
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only touches its own `HD2D_Scenery` collection and lays the models out side by
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side for inspection. Any glTF works with `ModelLibrary`; the engine relies on
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a few conventions:
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- every root node with a mesh becomes a model, looked up by node name;
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- `COLOR_0.rgb` multiplies the base color (tint / baked AO), `COLOR_0.a` is the
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wind weight (0 rigid, 1 fully swaying);
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- materials named `foliage*` are alpha-tested, two-sided and get wrap lighting,
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translucency and soft self-shadowing; others honor glTF `alphaMode` and
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`doubleSided`;
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- foliage looks best with custom normals pointing away from the canopy center;
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- emissive materials (glTF `emissiveFactor` / `emissiveTexture` /
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`KHR_materials_emissive_strength`) glow and bloom;
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- child empties of a model node become named **sockets** (`Model::sockets`,
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model space; `ModelInstance::toWorld` places them). The bridge exports
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`fire_0` / `fire_1` in its braziers and the demo spawns fires there.
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### Pixel art (PixelLab)
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`tools/pixellab.py` generates pixel-art textures through the
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[PixelLab API](https://api.pixellab.ai/v2/llms.txt). Put your key in `.env`
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(see `.env.example`); assets are declared in `tools/assets.json`.
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```sh
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python3 tools/pixellab.py --list # what exists
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python3 tools/pixellab.py # generate missing assets
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python3 tools/pixellab.py tall_grass # regenerate one asset
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python3 tools/pixellab.py --balance
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```
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- `"kind": "image"` uses `/create-image-pixflux` (transparent sprites).
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- `"kind": "tiles"` uses `/create-tiles-pro` (seamless top-down tiles); every
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variation is saved as `<name>_<n>.png`. A `"tile_id"` re-downloads an existing
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set without paying again.
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Pixel-art texel density is 32 px per world unit; the 256 px test sprite is 4×
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upscaled 64 px art, so it is rendered at 128 px per unit.
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## Roadmap
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The demo covers the rendering core. Natural next steps for a full engine:
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sprite-sheet animation + 4/8-direction facing, a scene/level file format and
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editor, more weather (snow, thunderstorms with lightning, wind-blown leaves),
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time-of-day presets, cascaded shadows for large maps, audio, and an
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entity/component layer for gameplay.
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