WebGPURenderer: Fix premultiplied alpha with clear colors. (#29538)

src/renderers/common/Background.js

@@ -111,8 +111,6 @@ class Background extends DataMap {
 
 		if ( renderer.autoClear === true || forceClear === true ) {
 
-			_clearColor.multiplyScalar( _clearColor.a );
-
 			const clearColorValue = renderContext.clearColorValue;
 
 			clearColorValue.r = _clearColor.r;
@@ -120,6 +118,18 @@ class Background extends DataMap {
 			clearColorValue.b = _clearColor.b;
 			clearColorValue.a = _clearColor.a;
 
+			// premultiply alpha
+
+			if ( renderer.backend.isWebGLBackend === true || renderer.alpha === true ) {
+
+				clearColorValue.r *= clearColorValue.a;
+				clearColorValue.g *= clearColorValue.a;
+				clearColorValue.b *= clearColorValue.a;
+
+			}
+
+			//
+
 			renderContext.depthClearValue = renderer._clearDepth;
 			renderContext.stencilClearValue = renderer._clearStencil;
 

src/renderers/webgl-fallback/WebGLBackend.js

@@ -433,9 +433,17 @@ class WebGLBackend extends Backend {
 
 		if ( descriptor === null ) {
 
+			const clearColor = this.getClearColor();
+
+			// premultiply alpha
+
+			clearColor.r *= clearColor.a;
+			clearColor.g *= clearColor.a;
+			clearColor.b *= clearColor.a;
+
 			descriptor = {
 				textures: null,
-				clearColorValue: this.getClearColor()
+				clearColorValue: clearColor
 			};
 
 		}
@@ -450,13 +458,23 @@ class WebGLBackend extends Backend {
 
 		if ( clear !== 0 ) {
 
-			const clearColor = descriptor.clearColorValue || this.getClearColor();
+			let clearColor;
 
-			// premultiply alpha
+			if ( descriptor.clearColorValue ) {
 
-			clearColor.r *= clearColor.a;
-			clearColor.g *= clearColor.a;
-			clearColor.b *= clearColor.a;
+				clearColor = descriptor.clearColorValue;
+
+			} else {
+
+				clearColor = this.getClearColor();
+
+				// premultiply alpha
+
+				clearColor.r *= clearColor.a;
+				clearColor.g *= clearColor.a;
+				clearColor.b *= clearColor.a;
+
+			}
 
 			if ( depth ) this.state.setDepthMask( true );