docs(3d): document set_direct_render() in Y-flip rule section

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/3D.md b/doc/3D.md
index dbcd6f9..c468fed 100644
--- a/doc/3D.md
+++ b/doc/3D.md
@@ -41,6 +41,13 @@ chain, so they must **pre-flip** their geometry to compensate:
 **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.
 
+**Exception — direct-to-surface rendering** (no post-process chain): call
+`renderer.set_direct_render(true)` **before** `renderer.init()`. This selects standard
+`WGPUFrontFace_CCW` winding and un-negates Y in the projection matrix, producing a
+correct upright image when the output goes straight to the swap-chain surface
+(e.g. `test_3d_render`). Effects that feed into the post-process chain must **not**
+set this flag.
+
 ## Core Concept
 
 Hybrid SDF/rasterization pipeline with physics and collision detection.