Improved performance for protected canvas part "input"

Fixes #517

lib/modifiedCanvasAPI.js

@@ -226,17 +226,26 @@
 		if (!this || canvasSizeShouldBeFaked(this.canvas, prefs)){
 			notify("fakedInput");
 			let oldImageData;
+			let x, y, width, height;
+			const border = 4;
 			try {
 				// "this" is not trustable - it may be not a context
-				oldImageData = getImageData(window, this).imageData;
+				const measurement = window.CanvasRenderingContext2D.prototype.measureText.call(this, args[0]);
+				width = Math.ceil(measurement.actualBoundingBoxRight + measurement.actualBoundingBoxLeft) + border;
+				height = Math.ceil(measurement.actualBoundingBoxDescent + measurement.actualBoundingBoxAscent) + border;
+				x = Math.max(0, args[1] - width);
+				y = Math.max(0, args[2] - height);
+				width = Math.min(3 * width, this.canvas.width - x);
+				height = Math.min(3 * height, this.canvas.height - y);
+				oldImageData = window.CanvasRenderingContext2D.prototype.getImageData.call(this, x, y, width, height);
 			}
 			catch (error){
 				// nothing to do here
 			}
 			// if "this" is not a correct context the next line will throw an error
 			const ret = original.call(this, ...args);
-			const newImageData = getImageData(window, this).imageData;
+			const newImageData = window.CanvasRenderingContext2D.prototype.getImageData.call(this, x, y, width, height);
-			this.putImageData(randomMixImageData(window, oldImageData, newImageData), 0, 0);
+			this.putImageData(randomMixImageData(window, oldImageData, newImageData), x, y);
 			return ret;
 		}
 		else {

releaseNotes.txt

@@ -4,6 +4,7 @@ Version 1.5:
 	
 	new features:
 		- added {empty}, {false} and {undefined} to webGL preference parameters (i.e. VENDOR, RENDERER, UNMASKED_VENDOR_WEBGL and UNMASKED_RENDERER_WEBGL)
+		- improved performance for protected canvas part "input"
 	
 	fixes:
 		- protection of DOM manipulations on unloaded windows may break websites