Fixed the Texture pattern

artist/patterns/texture.go

@@ -10,42 +10,58 @@ type Texture struct {
 }
 
 // Draw tiles the pattern's canvas within the given bounds. The minimum
-// points of the pattern's canvas and the destination canvas will be lined up.
+// point of the pattern's canvas will be lined up with the minimum point of the
+// bounding rectangle.
 func (pattern Texture) Draw (destination canvas.Canvas, bounds image.Rectangle) {
-	drawBounds := bounds.Canon().Intersect(destination.Bounds())
-	if drawBounds.Empty() { return }
-	
+	dstBounds := bounds.Canon().Intersect(destination.Bounds())
+	if dstBounds.Empty() { return }
+
 	dstData, dstStride := destination.Buffer()
 	srcData, srcStride := pattern.Buffer()
 	srcBounds := pattern.Bounds()
 
-	dstPoint := image.Point { }
-	srcPoint := drawBounds.Min.Sub(bounds.Min).Add(srcBounds.Min)
+	// offset is a vector that is added to points in destination space to
+	// convert them to points in source space
+	offset := srcBounds.Min.Sub(bounds.Min)
+
+	// calculate the starting position in source space
+	srcPoint := dstBounds.Min.Add(offset)
 	srcPoint.X = wrap(srcPoint.X, srcBounds.Min.X, srcBounds.Max.X)
 	srcPoint.Y = wrap(srcPoint.Y, srcBounds.Min.Y, srcBounds.Max.Y)
-	
-	for dstPoint.Y = drawBounds.Min.Y; dstPoint.Y < drawBounds.Max.Y; dstPoint.Y ++ {
-		srcPoint.X = srcBounds.Min.X
-		dstPoint.X = drawBounds.Min.X
+	srcStartPoint := srcPoint
+
+	// for each row
+	dstPoint := image.Point { }
+	for dstPoint.Y = dstBounds.Min.Y; dstPoint.Y < dstBounds.Max.Y; dstPoint.Y ++ {
+		srcPoint.X = srcStartPoint.X
+		dstPoint.X = dstBounds.Min.X
 		dstYComponent := dstPoint.Y * dstStride
 		srcYComponent := srcPoint.Y * srcStride
-		
+
+		// for each pixel in the row
 		for {
+			// draw pixel
 			dstIndex := dstYComponent + dstPoint.X
 			srcIndex := srcYComponent + srcPoint.X
 			dstData[dstIndex] = srcData[srcIndex]
 
+			// increment X in source space. wrap to start if out of
+			// bounds.
 			srcPoint.X ++
 			if srcPoint.X >= srcBounds.Max.X {
 				srcPoint.X = srcBounds.Min.X
 			}
 
+			// increment X in destination space. stop drawing this
+			// row if out of bounds.
 			dstPoint.X ++
-			if dstPoint.X >= drawBounds.Max.X {
+			if dstPoint.X >= dstBounds.Max.X {
 				break
 			}
 		}
 
+		// increment row in source space. wrap to start if out of
+		// bounds.
 		srcPoint.Y ++
 		if srcPoint.Y >= srcBounds.Max.Y {
 			srcPoint.Y = srcBounds.Min.Y