png: make the encoder configurable

In order to support different compression levels, make the
encoder type public, and add an Encoder method to it.

Fixes #8499.

LGTM=nigeltao
R=nigeltao, ruiu
CC=golang-codereviews
https://golang.org/cl/129190043

diff --git a/src/pkg/image/png/writer.go b/src/pkg/image/png/writer.go
index 5c232b760a50ca014512312b7f27fd592c797972..703aeec0a227b8fb1db3a20c41d0a2720163b2ee 100644 (file)
--- a/src/pkg/image/png/writer.go
+++ b/src/pkg/image/png/writer.go
@@ -14,7 +14,13 @@ import (
        "strconv"
 )
 
+// Encoder configures encoding PNG images.
+type Encoder struct {
+       CompressionLevel CompressionLevel
+}
+
 type encoder struct {
+       enc    *Encoder
        w      io.Writer
        m      image.Image
        cb     int
@@ -24,6 +30,15 @@ type encoder struct {
        tmp    [4 * 256]byte
 }
 
+type CompressionLevel int
+
+const (
+       DefaultCompression CompressionLevel = iota
+       NoCompression
+       BestSpeed
+       BestCompression
+)
+
 // Big-endian.
 func writeUint32(b []uint8, u uint32) {
        b[0] = uint8(u >> 24)
@@ -255,8 +270,11 @@ func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
        return filter
 }
 
-func writeImage(w io.Writer, m image.Image, cb int) error {
-       zw := zlib.NewWriter(w)
+func writeImage(w io.Writer, m image.Image, cb int, level int) error {
+       zw, err := zlib.NewWriterLevel(w, level)
+       if err != nil {
+               return err
+       }
        defer zw.Close()
 
        bpp := 0 // Bytes per pixel.
@@ -419,18 +437,41 @@ func (e *encoder) writeIDATs() {
        }
        var bw *bufio.Writer
        bw = bufio.NewWriterSize(e, 1<<15)
-       e.err = writeImage(bw, e.m, e.cb)
+       e.err = writeImage(bw, e.m, e.cb, levelToZlib(e.enc.CompressionLevel))
        if e.err != nil {
                return
        }
        e.err = bw.Flush()
 }
 
+// This function is required because we want the zero value of
+// Encoder.CompressionLevel to map to zlib.DefaultCompression.
+func levelToZlib(l CompressionLevel) int {
+       switch l {
+       case DefaultCompression:
+               return zlib.DefaultCompression
+       case NoCompression:
+               return zlib.NoCompression
+       case BestSpeed:
+               return zlib.BestSpeed
+       case BestCompression:
+               return zlib.BestCompression
+       default:
+               return zlib.DefaultCompression
+       }
+}
+
 func (e *encoder) writeIEND() { e.writeChunk(nil, "IEND") }
 
-// Encode writes the Image m to w in PNG format. Any Image may be encoded, but
-// images that are not image.NRGBA might be encoded lossily.
+// Encode writes the Image m to w in PNG format. Any Image may be
+// encoded, but images that are not image.NRGBA might be encoded lossily.
 func Encode(w io.Writer, m image.Image) error {
+       var e Encoder
+       return e.Encode(w, m)
+}
+
+// Encode writes the Image m to w in PNG format.
+func (enc *Encoder) Encode(w io.Writer, m image.Image) error {
        // Obviously, negative widths and heights are invalid. Furthermore, the PNG
        // spec section 11.2.2 says that zero is invalid. Excessively large images are
        // also rejected.
@@ -440,6 +481,7 @@ func Encode(w io.Writer, m image.Image) error {
        }
 
        var e encoder
+       e.enc = enc
        e.w = w
        e.m = m
 

diff --git a/src/pkg/image/png/writer_test.go b/src/pkg/image/png/writer_test.go
index 3116fc9ff94e467612083d0cd8e1c9a2e3cc6121..6a872e2749493d5bc76103bd8107fef749e039a7 100644 (file)
--- a/src/pkg/image/png/writer_test.go
+++ b/src/pkg/image/png/writer_test.go
@@ -40,11 +40,7 @@ func encodeDecode(m image.Image) (image.Image, error) {
        if err != nil {
                return nil, err
        }
-       m, err = Decode(&b)
-       if err != nil {
-               return nil, err
-       }
-       return m, nil
+       return Decode(&b)
 }
 
 func TestWriter(t *testing.T) {
@@ -81,6 +77,26 @@ func TestWriter(t *testing.T) {
        }
 }
 
+func TestWriterLevels(t *testing.T) {
+       m := image.NewNRGBA(image.Rect(0, 0, 100, 100))
+
+       var b1, b2 bytes.Buffer
+       var e1, e2 Encoder
+
+       if err := e1.Encode(&b1, m); err != nil {
+               t.Fatal(err)
+       }
+
+       e2.CompressionLevel = NoCompression
+       if err := e2.Encode(&b2, m); err != nil {
+               t.Fatal(err)
+       }
+
+       if b2.Len() <= b1.Len() {
+               t.Error("DefaultCompression encoding was larger than NoCompression encoding")
+       }
+}
+
 func TestSubImage(t *testing.T) {
        m0 := image.NewRGBA(image.Rect(0, 0, 256, 256))
        for y := 0; y < 256; y++ {