WebGPURenderer: Setting receiveShadow to true produces shader error with flat shaded objects.

[Mugen87]

Description

While working on ShadowMapViewer I have noticed that flat shaded objects that receive shadow produce a WGSL error which is logged in the browser console as a warning.

Error while parsing WGSL: :87:33 error: built-in cannot be used by vertex pipeline stage
normalFlat = normalize( cross( dpdx( varyings.v_positionView ), - dpdy( varyings.v_positionView ) ) );
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The screen also gets white so nothing is rendered.

Commenting out mesh.receiveShadow = true; or disable flat shading fixes the issue.

Reproduction steps

1. Open https://jsfiddle.net/fcxk984g/
2. Check browser console.

Live example

• r168: https://jsfiddle.net/fcxk984g/
• dev: https://jsfiddle.net/qntd2ev0/

Version

r168

Device

Desktop, Mobile, Headset

Browser

Chrome

OS

MacOS

[Mugen87]

According to the WGSL spec, derivative functions can only be used in a fragment shader stage.

https://www.w3.org/TR/WGSL/#derivative-builtin-functions

So it seems the TSL must avoid to use dFdx() and dFdy() in code assigned to vertex node.

[Mugen87]

This seems to be fixed in dev 🎉.