WebGPURenderer: Premultiply alpha for clear color.

[Mugen87]

Related issue: -

Description

Semi transparent clear values do not work as expected with WebGPURenderer.

WebGLRenderer (correct): https://jsfiddle.net/uq8wbo26/1/
WebGPURenderer (incorrect): https://jsfiddle.net/v70u128x/

That's because WebGPURenderer does not premultiply the clear alpha with the clear color. The PR fixes that for both backends.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
685.1 kB (169.6 kB)	685.1 kB (169.6 kB)	+0 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
462 kB (111.4 kB)	462 kB (111.4 kB)	+0 B

[Mugen87]

@WestLangley We've used to control this behavior in WebGLRenderer with the premultipliedAlpha renderer parameter.

three.js/src/renderers/webgl/WebGLState.js

Lines 36 to 40 in 2d6bf90

	if ( premultipliedAlpha === true ) {
	r *= a; g *= a; b *= a;
	}

Do you remember the use case when premultipliedAlpha should set to false when creating WebGLRenderer? WebGPURenderer does not currently not have such a parameter.

[WestLangley]

Do you remember the use case when premultipliedAlpha should set to false when creating WebGLRenderer?

In the renderer constructor, premultilpiedAlpha: true is a hint to the compositor that the drawing buffer contains RGB values that have been multiplied by alpha.

The three.js NormalBlending function results in a premultiplied buffer, so users should be advised not to change this.

As far as I know, a use case for premultilpiedAlpha: false has not been articulated.

WebGPURenderer does not currently not have such a parameter.

Only indirectly.

three.js/src/renderers/webgpu/WebGPUBackend.js

Line 109 in 2d6bf90

const alphaMode = parameters.alpha ? 'premultiplied' : 'opaque';

[Mugen87]

In this case, I'll add a check in WebGPUBackend to only premultiply alpha when the alpha mode is premultiplied.

The WebGL backend does not have an alpha mode so it's always assumed to be premultiplied.