image: add a decoding test for common file formats.

The test image was converted from doc/video-001.png using the
convert command line tool (ImageMagick 6.5.7-8) at -quality 100.

R=r, nigeltao_gnome
CC=golang-dev
https://golang.org/cl/4259047

src/pkg/Makefile

@@ -172,7 +172,6 @@ NOTEST=\
 	go/token\
 	hash\
 	http/pprof\
-	image\
 	image/jpeg\
 	net/dict\
 	rand\

src/pkg/image/decode_test.go

+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package image_test
+
+import (
+	"bufio"
+	"image"
+	"os"
+	"testing"
+
+	// TODO(nigeltao): implement bmp, gif and tiff decoders.
+	_ "image/jpeg"
+	_ "image/png"
+)
+
+const goldenFile = "testdata/video-001.png"
+
+type imageTest struct {
+	filename  string
+	tolerance int
+}
+
+var imageTests = []imageTest{
+	//{"testdata/video-001.bmp", 0},
+	// GIF images are restricted to a 256-color palette and the conversion
+	// to GIF loses significant image quality.
+	//{"testdata/video-001.gif", 64<<8},
+	// JPEG is a lossy format and hence needs a non-zero tolerance.
+	{"testdata/video-001.jpeg", 8 << 8},
+	{"testdata/video-001.png", 0},
+	//{"testdata/video-001.tiff", 0},
+}
+
+func decode(filename string) (image.Image, string, os.Error) {
+	f, err := os.Open(filename, os.O_RDONLY, 0400)
+	if err != nil {
+		return nil, "", err
+	}
+	defer f.Close()
+	return image.Decode(bufio.NewReader(f))
+}
+
+func delta(u0, u1 uint32) int {
+	d := int(u0) - int(u1)
+	if d < 0 {
+		return -d
+	}
+	return d
+}
+
+func withinTolerance(c0, c1 image.Color, tolerance int) bool {
+	r0, g0, b0, a0 := c0.RGBA()
+	r1, g1, b1, a1 := c1.RGBA()
+	r := delta(r0, r1)
+	g := delta(g0, g1)
+	b := delta(b0, b1)
+	a := delta(a0, a1)
+	return r <= tolerance && g <= tolerance && b <= tolerance && a <= tolerance
+}
+
+func TestDecode(t *testing.T) {
+	golden, _, err := decode(goldenFile)
+	if err != nil {
+		t.Errorf("%s: %v", goldenFile, err)
+	}
+loop:
+	for _, it := range imageTests {
+		m, _, err := decode(it.filename)
+		if err != nil {
+			t.Errorf("%s: %v", it.filename, err)
+			continue loop
+		}
+		b := golden.Bounds()
+		if !b.Eq(m.Bounds()) {
+			t.Errorf("%s: want bounds %v got %v", it.filename, b, m.Bounds())
+			continue loop
+		}
+		for y := b.Min.Y; y < b.Max.Y; y++ {
+			for x := b.Min.X; x < b.Max.X; x++ {
+				if !withinTolerance(golden.At(x, y), m.At(x, y), it.tolerance) {
+					t.Errorf("%s: at (%d, %d), want %v got %v", it.filename, x, y, golden.At(x, y), m.At(x, y))
+					continue loop
+				}
+			}
+		}
+	}
+}