first pass

examples/jsm/renderers/webgl/WebGLBackend.js

@@ -51,6 +51,12 @@ class WebGLBackend extends Backend {
 
 	}
 
+	async getArrayBufferAsync( attribute ) {
+
+		return await this.attributeUtils.getArrayBufferAsync( attribute );
+
+	}
+
 	beginRender( renderContext ) {
 
 		const { gl } = this;

examples/jsm/renderers/webgl/utils/WebGLAttributeUtils.js

@@ -1,3 +1,5 @@
+import { arrayBuffer } from "../../../nodes/Nodes";
+
 class WebGLAttributeUtils {
 
 	constructor( backend ) {
@@ -114,6 +116,76 @@ class WebGLAttributeUtils {
 
 	}
 
+	async getArrayBufferAsync( attribute ) {
+
+		const bufferAttribute = attribute.isInterleavedBufferAttribute ? attribute.data : attribute;
+		const { bufferType, bufferGPU, bytesPerElement } = backend.get( bufferAttribute );
+
+		const length = attribute.array.length * bytesPerElement;
+
+		gl.bindBuffer( bufferType, bufferGPU );
+
+		const writeBuffer = gl.createBuffer();
+
+		gl.bindBuffer(gl.COPY_WRITE_BUFFER, writeBuffer );
+		gl.bufferData(gl.COPY_WRITE_BUFFER, dest.byteLength, gl.STREAM_READ);
+
+		gl.copyBufferSubData( bufferType, gl.COPY_WRITE_BUFFER, 0, 0, length );
+
+		await this._clientWaitAsync();
+
+		const dstBuffer = new attribute.array.constructor( length );
+
+		gl.getBufferSubData( gl.COPY_WRITE_BUFFER, 0, dstBuffer );
+
+		gl.deleteBuffer( writeBuffer );
+
+		return dstBuffer;
+
+	}
+
+	_clientWaitAsync() {
+
+		const { gl } = this;
+
+		const sync = gl.fenceSync( gl.SYNC_GPU_COMMANDS_COMPLETE, 0 );
+
+		gl.flush();
+
+		return new Promise( ( resolve, reject ) => {
+
+			function test() {
+
+				const res = gl.clientWaitSync( sync, gl.SYNC_FLUSH_COMMANDS_BIT, 0 );
+
+				if ( res === gl.WAIT_FAILED) {
+
+					gl.deleteSync( sync );
+
+					reject();
+					return;
+
+				}
+
+				if ( res === gl.TIMEOUT_EXPIRED) {
+
+					requestAnimationFrame( test );
+					return;
+
+				}
+
+				gl.deleteSync( sync );
+
+				resolve();
+
+			}
+
+			test();
+
+		} );
+
+	}
+
 }
 
 export default WebGLAttributeUtils;