image/draw: fold TestClipWithNilMP into TestClip.

https://go-review.googlesource.com/#/c/1876/ introduced a new
TestClipWithNilMP test, along with a code change that fixed a panic,
but the existing TestClip test already contained almost enough machinery
to cover that bug.

There is a small code change in this CL, but it is a no-op: (*x).y is
equivalent to x.y for a pointer-typed x, but the latter is cleaner.

Change-Id: I79cf6952a4999bc4b91f0a8ec500acb108106e56
Reviewed-on: https://go-review.googlesource.com/2304Reviewed-by: Dave Cheney <dave@cheney.net>

src/image/draw/clip_test.go

@@ -139,7 +139,19 @@ var clipTests = []clipTest{
 		image.Pt(20, 0),
 		image.Pt(20, 0),
 	},
-	// TODO(nigeltao): write more tests.
+	{
+		"clip sr and mr",
+		image.Rect(0, 0, 100, 100),
+		image.Rect(0, 0, 100, 100),
+		image.Rect(23, 23, 55, 86),
+		image.Rect(44, 44, 87, 58),
+		image.Pt(10, 10),
+		image.Pt(11, 11),
+		false,
+		image.Rect(33, 33, 45, 47),
+		image.Pt(43, 43),
+		image.Pt(44, 44),
+	},
 }
 
 func TestClip(t *testing.T) {
@@ -149,12 +161,12 @@ func TestClip(t *testing.T) {
 	for _, c := range clipTests {
 		dst := dst0.SubImage(c.dr).(*image.RGBA)
 		src := src0.SubImage(c.sr).(*image.RGBA)
-		var mask image.Image
-		if !c.nilMask {
-			mask = mask0.SubImage(c.mr)
-		}
 		r, sp, mp := c.r, c.sp, c.mp
-		clip(dst, &r, src, &sp, mask, &mp)
+		if c.nilMask {
+			clip(dst, &r, src, &sp, nil, nil)
+		} else {
+			clip(dst, &r, src, &sp, mask0.SubImage(c.mr), &mp)
+		}
 
 		// Check that the actual results equal the expected results.
 		if !c.r0.Eq(r) {
@@ -191,13 +203,3 @@ func TestClip(t *testing.T) {
 		}
 	}
 }
-
-func TestClipWithNilMP(t *testing.T) {
-	src := image.NewRGBA(image.Rect(0, 0, 100, 100))
-	// dst must be smaller than src for clipping to occur
-	dst := image.NewRGBA(image.Rect(50, 50, 100, 100))
-	r := image.Rect(0, 0, 100, 100)
-	sp := image.ZP
-	// issue 9177: floydSteinberg.Draw passes nil for mp, which used to cause clip to panic
-	clip(dst, &r, src, &sp, nil, nil)
-}

src/image/draw/draw.go

@@ -81,11 +81,11 @@ func clip(dst Image, r *image.Rectangle, src image.Image, sp *image.Point, mask
 	if dx == 0 && dy == 0 {
 		return
 	}
-	(*sp).X += dx
-	(*sp).Y += dy
+	sp.X += dx
+	sp.Y += dy
 	if mp != nil {
-		(*mp).X += dx
-		(*mp).Y += dy
+		mp.X += dx
+		mp.Y += dy
 	}
 }