vp8: skip filtering for all-zero-DC macroblock residuals.

This makes the Go code match the libwebp C code's output on
blue-purple-pink-large.*-filter.lossy.webp

Also make the various WEBP benchmarks all decode a similar image,
the image at http://blog.golang.org/gophercon/image01.jpg, to make
it more meaningful to e.g. compare the simple filter's numbers with
the normal filter's numbers.

Also fix a "go vet" warning in webp/decode.go.

The test data was generated by:
wget http://blog.golang.org/gophercon/image01.jpg -O blue-purple-pink-large.jpeg
convert blue-purple-pink-large.jpeg blue-purple-pink-large.png
cwebp -lossless blue-purple-pink-large.png -o blue-purple-pink-large.lossless.webp
cwebp -q 80 -f 0 blue-purple-pink-large.png -o blue-purple-pink-large.no-filter.lossy.webp
cwebp -q 80 -strong blue-purple-pink-large.png -o blue-purple-pink-large.normal-filter.lossy.webp
cwebp -q 80 -nostrong blue-purple-pink-large.png -o blue-purple-pink-large.simple-filter.lossy.webp
dwebp -pgm blue-purple-pink-large.no-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.no-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.normal-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.normal-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.simple-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.simple-filter.lossy.webp.ycbcr.png && rm tmp.pgm

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/106230044

vp8/reconstruct.go

@@ -266,8 +266,9 @@ func (d *Decoder) parseResiduals4(r *partition, plane int, context uint8, quant
 	return 1
 }
 
-// parseResiduals parses the residuals.
+// parseResiduals parses the residuals and returns whether inner loop filtering
-func (d *Decoder) parseResiduals(mbx, mby int) {
+// should be skipped for this macroblock.
+func (d *Decoder) parseResiduals(mbx, mby int) (skip bool) {
 	partition := &d.op[mby&(d.nOP-1)]
 	plane := planeY1SansY2
 	quant := &d.quant[d.segment]
@@ -332,6 +333,11 @@ func (d *Decoder) parseResiduals(mbx, mby int) {
 	d.upMB[mbx].nzMask = uint8(unzMask)
 	d.nzDCMask = nzDCMask
 	d.nzACMask = nzACMask
+
+	// Section 15.1 of the spec says that "Steps 2 and 4 [of the loop filter]
+	// are skipped... [if] there is no DCT coefficient coded for the whole
+	// macroblock."
+	return nzDCMask == 0 && nzACMask == 0
 }
 
 // reconstructMacroblock applies the predictor functions and adds the inverse-
@@ -384,7 +390,8 @@ func (d *Decoder) reconstructMacroblock(mbx, mby int) {
 	}
 }
 
-// reconstruct reconstructs one macroblock.
+// reconstruct reconstructs one macroblock and returns whether inner loop
+// filtering should be skipped for it.
 func (d *Decoder) reconstruct(mbx, mby int) (skip bool) {
 	if d.segmentHeader.updateMap {
 		if !d.fp.readBit(d.segmentHeader.prob[0]) {
@@ -411,9 +418,7 @@ func (d *Decoder) reconstruct(mbx, mby int) (skip bool) {
 	d.parsePredModeC8()
 	// Parse the residuals.
 	if !skip {
-		// TODO(nigeltao): make d.parseResiduals return a bool, and change this to
+		skip = d.parseResiduals(mbx, mby)
-		// skip = d.parseResiduals(mbx, mby)
-		d.parseResiduals(mbx, mby)
 	} else {
 		if d.usePredY16 {
 			d.leftMB.nzY16 = 0

webp/decode.go

@@ -65,7 +65,7 @@ func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
 		return m, image.Config{}, nil
 	}
 
-	r = &io.LimitedReader{r, int64(dataLen)}
+	r = &io.LimitedReader{R: r, N: int64(dataLen)}
 	if configOnly {
 		c, err := vp8l.DecodeConfig(r)
 		return nil, c, err

webp/decode_test.go

@@ -33,6 +33,9 @@ func hex(x []byte) string {
 func TestDecodeVP8(t *testing.T) {
 	testCases := []string{
 		"blue-purple-pink",
+		"blue-purple-pink-large.no-filter",
+		"blue-purple-pink-large.simple-filter",
+		"blue-purple-pink-large.normal-filter",
 		"video-001",
 		"yellow_rose",
 	}
@@ -40,17 +43,20 @@ func TestDecodeVP8(t *testing.T) {
 	for _, tc := range testCases {
 		f0, err := os.Open("../testdata/" + tc + ".lossy.webp")
 		if err != nil {
-			t.Fatal(err)
+			t.Errorf("%s: Open WEBP: %v", tc, err)
+			continue
 		}
 		defer f0.Close()
 		img0, err := Decode(f0)
 		if err != nil {
-			t.Fatal(err)
+			t.Errorf("%s: Decode WEBP: %v", tc, err)
+			continue
 		}
 
 		m0, ok := img0.(*image.YCbCr)
 		if !ok || m0.SubsampleRatio != image.YCbCrSubsampleRatio420 {
-			t.Fatal("decoded WEBP image is not a 4:2:0 YCbCr")
+			t.Errorf("%s: decoded WEBP image is not a 4:2:0 YCbCr", tc)
+			continue
 		}
 		// w2 and h2 are the half-width and half-height, rounded up.
 		w, h := m0.Bounds().Dx(), m0.Bounds().Dy()
@@ -58,12 +64,14 @@ func TestDecodeVP8(t *testing.T) {
 
 		f1, err := os.Open("../testdata/" + tc + ".lossy.webp.ycbcr.png")
 		if err != nil {
-			t.Fatal(err)
+			t.Errorf("%s: Open PNG: %v", tc, err)
+			continue
 		}
 		defer f1.Close()
 		img1, err := png.Decode(f1)
 		if err != nil {
-			t.Fatal(err)
+			t.Errorf("%s: Open PNG: %v", tc, err)
+			continue
 		}
 
 		// The split-into-YCbCr-planes golden image is a 2*w2 wide and h+h2 high
@@ -73,11 +81,13 @@ func TestDecodeVP8(t *testing.T) {
 		//   BBRR
 		// See http://www.fourcc.org/yuv.php#IMC4
 		if got, want := img1.Bounds(), image.Rect(0, 0, 2*w2, h+h2); got != want {
-			t.Fatalf("bounds0: got %v, want %v", got, want)
+			t.Errorf("%s: bounds0: got %v, want %v", tc, got, want)
+			continue
 		}
 		m1, ok := img1.(*image.Gray)
 		if !ok {
-			t.Fatal("decoded PNG image is not a Gray")
+			t.Errorf("%s: decoded PNG image is not a Gray", tc)
+			continue
 		}
 
 		planes := []struct {
@@ -101,14 +111,14 @@ func TestDecodeVP8(t *testing.T) {
 				}
 				nDiff++
 				if nDiff > 10 {
-					t.Errorf("%s plane: more rows differ", plane.name)
+					t.Errorf("%s: %s plane: more rows differ", tc, plane.name)
 					break
 				}
 				for i := range got {
 					diff[i] = got[i] - want[i]
 				}
-				t.Errorf("%s plane: m0 row %d, m1 row %d\ngot %s\nwant%s\ndiff%s",
+				t.Errorf("%s: %s plane: m0 row %d, m1 row %d\ngot %s\nwant%s\ndiff%s",
-					plane.name, j, y, hex(got), hex(want), hex(diff))
+					tc, plane.name, j, y, hex(got), hex(want), hex(diff))
 			}
 		}
 	}
@@ -117,6 +127,7 @@ func TestDecodeVP8(t *testing.T) {
 func TestDecodeVP8L(t *testing.T) {
 	testCases := []string{
 		"blue-purple-pink",
+		"blue-purple-pink-large",
 		"gopher-doc.1bpp",
 		"gopher-doc.2bpp",
 		"gopher-doc.4bpp",
@@ -197,7 +208,7 @@ loop:
 }
 
 func benchmarkDecode(b *testing.B, filename string) {
-	data, err := ioutil.ReadFile("../testdata/" + filename + ".webp")
+	data, err := ioutil.ReadFile("../testdata/blue-purple-pink-large." + filename + ".webp")
 	if err != nil {
 		b.Fatal(err)
 	}
@@ -213,6 +224,7 @@ func benchmarkDecode(b *testing.B, filename string) {
 	}
 }
 
-func BenchmarkDecodeVP8SimpleFilter(b *testing.B) { benchmarkDecode(b, "blue-purple-pink.lossy") }
+func BenchmarkDecodeVP8NoFilter(b *testing.B)     { benchmarkDecode(b, "no-filter.lossy") }
-func BenchmarkDecodeVP8NormalFilter(b *testing.B) { benchmarkDecode(b, "yellow_rose.lossy") }
+func BenchmarkDecodeVP8SimpleFilter(b *testing.B) { benchmarkDecode(b, "simple-filter.lossy") }
-func BenchmarkDecodeVP8L(b *testing.B)            { benchmarkDecode(b, "yellow_rose.lossless") }
+func BenchmarkDecodeVP8NormalFilter(b *testing.B) { benchmarkDecode(b, "normal-filter.lossy") }
+func BenchmarkDecodeVP8L(b *testing.B)            { benchmarkDecode(b, "lossless") }