From ba7ea27ddee4398afa98a0e45b9e227bbcfae906 Mon Sep 17 00:00:00 2001 From: skal Date: Sun, 8 Mar 2026 11:36:04 +0100 Subject: fix: negate Y in perspective() to correct rasterized 3D orientation MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The fullscreen post-process VS uses Y-up UVs (uv.y=0 = bottom), so textureSample() Y-flips any rasterized offscreen texture. SDF effects author their content Y-down and look correct after the flip. Rasterized effects (RotatingCube, Hybrid3D) must pre-flip their geometry: - mat4::perspective(): m[5] = -t (negated Y scale) - Pipelines with cullMode=Back: frontFace = WGPUFrontFace_CW (Y-flip reverses winding, so CW becomes the visible face) - Remove incorrect transposes from GlobalUniforms::make(), ObjectData::make(), and Uniforms::make() — mini_math is column-major, no transpose needed for GPU upload - Document the convention in doc/3D.md under "Rasterized 3D and the Y-flip rule" handoff(Gemini): Y-flip rule now documented; all rasterized 3D pipelines must follow it. Co-Authored-By: Claude Sonnet 4.6 --- doc/3D.md | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) (limited to 'doc/3D.md') diff --git a/doc/3D.md b/doc/3D.md index c27b034..dbcd6f9 100644 --- a/doc/3D.md +++ b/doc/3D.md @@ -24,6 +24,23 @@ - WebGPU framebuffer: y-down (pixel row 0 at top) - `uv.y = 0` → NDC `y = -1` → bottom of framebuffer ✓ +### Rasterized 3D and the Y-flip rule + +The fullscreen post-process VS (`fullscreen_uv_vs.wgsl`) uses **Y-up UVs** (`uv.y=0` = bottom of screen). +`textureSample(tex, s, uv)` with those UVs samples the **top** of the input texture at the bottom of +the screen — a Y-flip. SDF effects are authored Y-down and appear correct after this flip. Rasterized +effects (RotatingCube, Hybrid3D) render into an offscreen texture and are consumed by the post-process +chain, so they must **pre-flip** their geometry to compensate: + +- **`mat4::perspective()`** uses `m[5] = -t` (negated Y scale) — renders geometry upside-down into the + texture so the post-process chain flips it right-side up. +- **Rasterized pipelines** with `cullMode = Back` must set `frontFace = WGPUFrontFace_CW` — the Y-flip + reverses winding order on screen, so CW becomes the visible (front) face. +- Pipelines with `cullMode = None` (SDF proxy hull, skybox) need no winding change. + +**Do not** set `m[5] = +t` or use `frontFace = WGPUFrontFace_CCW` (default) in rasterized 3D +pipelines — the result will be upside-down with back-faces culled. + ## Core Concept Hybrid SDF/rasterization pipeline with physics and collision detection. -- cgit v1.2.3