code review 4435051: image/jpeg: add an encoder.

src/pkg/image/jpeg/writer_test.go

Index: src/pkg/image/jpeg/writer_test.go
===================================================================
new file mode 100644
--- /dev/null
+++ b/src/pkg/image/jpeg/writer_test.go
@@ -0,0 +1,87 @@
+// 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 jpeg
+
+import (
+	"bytes"
+	"image"
+	"image/png"
+	"os"
+	"testing"
+)
+
+var testCase = []struct {
+	filename  string
+	quality   int
+	tolerance int64
+}{
+	{"../testdata/video-001.png", 1, 24 << 8},
+	{"../testdata/video-001.png", 20, 12 << 8},
+	{"../testdata/video-001.png", 60, 8 << 8},
+	{"../testdata/video-001.png", 80, 6 << 8},
+	{"../testdata/video-001.png", 90, 4 << 8},
+	{"../testdata/video-001.png", 100, 2 << 8},
+}
+
+func delta(u0, u1 uint32) int64 {
+	d := int64(u0) - int64(u1)
+	if d < 0 {
+		return -d
+	}
+	return d
+}
+
+func readPng(filename string) (image.Image, os.Error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return png.Decode(f)
+}
+
+func TestWriter(t *testing.T) {
+	for _, tc := range testCase {
+		// Read the image.
+		m0, err := readPng(tc.filename)
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		// Encode that image as JPEG.
+		buf := bytes.NewBuffer(nil)
+		err = Encode(buf, m0, &Options{Quality: tc.quality})
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		// Decode that JPEG.
+		m1, err := Decode(buf)
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		// Compute the average delta in RGB space.
+		b := m0.Bounds()
+		var sum, n int64
+		for y := b.Min.Y; y < b.Max.Y; y++ {
+			for x := b.Min.X; x < b.Max.X; x++ {
+				c0 := m0.At(x, y)
+				c1 := m1.At(x, y)
+				r0, g0, b0, _ := c0.RGBA()
+				r1, g1, b1, _ := c1.RGBA()
+				sum += delta(r0, r1)
+				sum += delta(g0, g1)
+				sum += delta(b0, b1)
+				n += 3
+			}
+		}
+		// Compare the average delta to the tolerance level.
+		if sum/n > tc.tolerance {
+			t.Errorf("%s, quality=%d: average delta is too high", tc.filename, tc.quality)
+			continue
+		}
+	}
+}