hShadertoy

Experimental Shadertoy.com importer for Houdini 21 Copernicus

https://youtu.be/ULpn8tGFsRI

What is this?

"You know what would be cool, if you could import a Shadertoy shader to Houdini COPs, do you think it's possible?"

It is. hShadertoy fetches a shader straight from Shadertoy.com and rebuilds it as a native Copernicus network — including multi-pass buffers, cubemaps and textures. The GLSL is translated to OpenCL on the fly. Madness! Never been done!

How it works

1. Editor — a Shadertoy mini-IDE on the hShadertoy shelf. Browse, fetch by URL/ID, import (Shadertoy API JSON in/out).
2. Builder — creates the hShadertoy HDA and wires every renderpass up.
3. Transpiler — translates each GLSL pass to OpenCL for Copernicus.

Shadertoy → Houdini mapping:

• GLSL fragment shaders → OpenCL COP nodes
• Buffer passes → Block Begin/End
• Cubemaps packed/unpacked to 2D image maps (volumes on the todo list)
• Shadertoy textures ship with the HDA — no downloads needed
• iMouse → viewer state / Mouse CHOP (todo) · Webcam → Live Video COP (todo) · Audio → Audio In CHOP (todo)

Requirements

• Houdini 21.0+ (Copernicus)
• A free Shadertoy API key: https://www.shadertoy.com/howto
• Three Python packages installed into Houdini's own Python (step 2 below)

Installation

1. Download and unpack (or clone) to a local directory, e.g. C:/dev/hShadertoy

2. Install the Python deps into Houdini's Python (plain pip may target the wrong interpreter):

   "C:/Program Files/Side Effects Software/Houdini 21.0.XXX/python311/python.exe" -m pip install --user tree-sitter tree-sitter-glsl curl-cffi
   

3. Copy houdini/packages/hShadertoy.json to your packages directory, e.g. $HOUDINI_USER_PREF_DIR/packages

4. Edit it — fill in the first three values and set "enable": true:

   { "HSHADERTOY_ROOT": "C:/dev/hShadertoy" },
   { "HSHADERTOY_HOUDINI": "C:/dev/hShadertoy/houdini" },
   { "SHADERTOY_API_KEY": "YOUR_API_KEY" },
   ...
   "enable": true
   

5. Start Houdini → hShadertoy shelf → Editor → paste a shader URL → import. That's it.

Limitations

• Not every shader transpiles (yet). Typical image shaders work well; heavy preprocessor tricks are the main holdouts. On a random sample of 999 Shadertoy shaders, ~56% currently compile end-to-end — and that number grows weekly with the ongoing fix campaign.
• dFdx(), dFdy(), fwidth() are passthrough functions (no derivatives in OpenCL) — shaders relying on them will look different.
• No mipmaps in COPs: texture() bias/LOD arguments are accepted but ignored.
• Sound passes, webcam, video and keyboard input aren't wired up yet.
• iTime & friends used in global initializers evaluate once (constant).
• Only shaders published as Public + API can be fetched.

Development

New contributors: start with .claude/skills/onboarding/SKILL.md — it maps the repo, the rules and the workflows. The forward plan lives in docs/handover/ROADMAP.md.

Pipeline

1. houdini/toolbar/hShadertoy.shelf
2. houdini/scripts/python/hshadertoy/gui/editor.py
3. houdini/scripts/python/hshadertoy/builder/builder.py
4. houdini/scripts/python/hshadertoy/transpiler/transpile_glsl.py
5. src/glsl_to_opencl
6. houdini/otls/hShadertoy.hda - hShadertoy::shadertoy
7. magic!

Transpiler spec

• src/glsl_to_opencl/GLSL_TO_OPENCL_SPEC.md

Testing

pip install -r requirements.txt              # dev deps (pytest, pyopencl, ...)
python -m pytest tests/unit/ -q              # unit suite — the regression gate
python tests/transpile.py <file>.glsl        # transpile one shader -> .header.cl/.kernel.cl
python tests/compilecl.py --header <file>.header.cl <file>.kernel.cl   # compile it

Compile options live in tests/build_options.json; extract the ones for your machine by setting HOUDINI_OCL_REPORT_BUILD_LOGS = 2 in houdini.env and reading the HDA's OpenCL build log.