image/png: do not apply filters if level is NoCompression

PNG filters are applied to get better compression ratio.
It does not make sense to apply them if we are not going
to compress.

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/137830043

diff --git a/src/pkg/image/png/writer.go b/src/pkg/image/png/writer.go
index 703aeec0a227b8fb1db3a20c41d0a2720163b2ee..b1a70c79d3f83b54b465a449697c8d328ba168c1 100644 (file)
--- a/src/pkg/image/png/writer.go
+++ b/src/pkg/image/png/writer.go
@@ -417,7 +417,10 @@ func writeImage(w io.Writer, m image.Image, cb int, level int) error {
                }
 
                // Apply the filter.
-               f := filter(&cr, pr, bpp)
+               f := ftNone
+               if level != zlib.NoCompression {
+                       f = filter(&cr, pr, bpp)
+               }
 
                // Write the compressed bytes.
                if _, err := zw.Write(cr[f]); err != nil {

diff --git a/src/pkg/image/png/writer_test.go b/src/pkg/image/png/writer_test.go
index 6a872e2749493d5bc76103bd8107fef749e039a7..d67a815698fd0e75358b92b0257e88a45f0ee574 100644 (file)
--- a/src/pkg/image/png/writer_test.go
+++ b/src/pkg/image/png/writer_test.go
@@ -81,20 +81,23 @@ 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 {
+       if err := (&Encoder{}).Encode(&b1, m); err != nil {
                t.Fatal(err)
        }
-
-       e2.CompressionLevel = NoCompression
-       if err := e2.Encode(&b2, m); err != nil {
+       noenc := &Encoder{CompressionLevel: NoCompression}
+       if err := noenc.Encode(&b2, m); err != nil {
                t.Fatal(err)
        }
 
        if b2.Len() <= b1.Len() {
                t.Error("DefaultCompression encoding was larger than NoCompression encoding")
        }
+       if _, err := Decode(&b1); err != nil {
+               t.Error("cannot decode DefaultCompression")
+       }
+       if _, err := Decode(&b2); err != nil {
+               t.Error("cannot decode NoCompression")
+       }
 }
 
 func TestSubImage(t *testing.T) {