WebGLRenderer: bugfix when reading pixel in MRT (#32506)

src/renderers/WebGLRenderer.js

@@ -2988,6 +2988,10 @@ class WebGLRenderer {
 					const textureFormat = texture.format;
 					const textureType = texture.type;
 
+					// when using MRT, select the correct color buffer for the subsequent read command
+
+					if ( renderTarget.textures.length > 1 ) _gl.readBuffer( _gl.COLOR_ATTACHMENT0 + textureIndex );
+
 					if ( ! capabilities.textureFormatReadable( textureFormat ) ) {
 
 						error( 'WebGLRenderer.readRenderTargetPixels: renderTarget is not in RGBA or implementation defined format.' );
@@ -3006,10 +3010,6 @@ class WebGLRenderer {
 
 					if ( ( x >= 0 && x <= ( renderTarget.width - width ) ) && ( y >= 0 && y <= ( renderTarget.height - height ) ) ) {
 
-						// when using MRT, select the correct color buffer for the subsequent read command
-
-						if ( renderTarget.textures.length > 1 ) _gl.readBuffer( _gl.COLOR_ATTACHMENT0 + textureIndex );
-
 						_gl.readPixels( x, y, width, height, utils.convert( textureFormat ), utils.convert( textureType ), buffer );
 
 					}
@@ -3070,6 +3070,11 @@ class WebGLRenderer {
 					const textureFormat = texture.format;
 					const textureType = texture.type;
 
+					// when using MRT, select the correct color buffer for the subsequent read command
+
+					if ( renderTarget.textures.length > 1 ) _gl.readBuffer( _gl.COLOR_ATTACHMENT0 + textureIndex );
+
+
 					if ( ! capabilities.textureFormatReadable( textureFormat ) ) {
 
 						throw new Error( 'THREE.WebGLRenderer.readRenderTargetPixelsAsync: renderTarget is not in RGBA or implementation defined format.' );
@@ -3086,10 +3091,6 @@ class WebGLRenderer {
 					_gl.bindBuffer( _gl.PIXEL_PACK_BUFFER, glBuffer );
 					_gl.bufferData( _gl.PIXEL_PACK_BUFFER, buffer.byteLength, _gl.STREAM_READ );
 
-					// when using MRT, select the correct color buffer for the subsequent read command
-
-					if ( renderTarget.textures.length > 1 ) _gl.readBuffer( _gl.COLOR_ATTACHMENT0 + textureIndex );
-
 					_gl.readPixels( x, y, width, height, utils.convert( textureFormat ), utils.convert( textureType ), 0 );
 
 					// reset the frame buffer to the currently set buffer before waiting