JPEG decoder now handles RST (restart) markers.

R=r
CC=golang-dev
https://golang.org/cl/181075

diff --git a/src/pkg/image/jpeg/reader.go b/src/pkg/image/jpeg/reader.go
index 7e640d9bac535a9fb880f7386b2c9822bb06a19a..22c7a1fb06e80e7d66402efc0c34760fb26c267d 100644 (file)
--- a/src/pkg/image/jpeg/reader.go
+++ b/src/pkg/image/jpeg/reader.go
@@ -55,6 +55,9 @@ const (
        dhtMarker   = 0xc4 // Define Huffman Table.
        dqtMarker   = 0xdb // Define Quantization Table.
        sosMarker   = 0xda // Start Of Scan.
+       driMarker   = 0xdd // Define Restart Interval.
+       rst0Marker  = 0xd0 // ReSTart (0).
+       rst7Marker  = 0xd7 // ReSTart (7).
        app0Marker  = 0xe0 // APPlication specific (0).
        app15Marker = 0xef // APPlication specific (15).
        comMarker   = 0xfe // COMment.
@@ -82,6 +85,7 @@ type decoder struct {
        r             Reader
        width, height int
        image         *image.RGBA
+       ri            int // Restart Interval.
        comps         [nComponent]component
        huff          [maxTc + 1][maxTh + 1]huffman
        quant         [maxTq + 1][blockSize]int
@@ -265,6 +269,7 @@ func (d *decoder) processSOS(n int) os.Error {
        mxx := (d.width + 8*int(h0) - 1) / (8 * int(h0))
        myy := (d.height + 8*int(v0) - 1) / (8 * int(v0))
 
+       mcu, expectedRST := 0, uint8(rst0Marker)
        var allZeroes [blockSize]int
        var dc [nComponent]int
        for my := 0; my < myy; my++ {
@@ -319,12 +324,47 @@ func (d *decoder) processSOS(n int) os.Error {
                                } // for j
                        } // for i
                        d.convertMCU(mx, my, int(d.comps[0].h), int(d.comps[0].v))
+                       mcu++
+                       if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy {
+                               // A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input,
+                               // but this one assumes well-formed input, and hence the restart marker follows immediately.
+                               _, err := io.ReadFull(d.r, d.tmp[0:2])
+                               if err != nil {
+                                       return err
+                               }
+                               if d.tmp[0] != 0xff || d.tmp[1] != expectedRST {
+                                       return FormatError("bad RST marker")
+                               }
+                               expectedRST++
+                               if expectedRST == rst7Marker+1 {
+                                       expectedRST = rst0Marker
+                               }
+                               // Reset the Huffman decoder.
+                               d.b = bits{}
+                               // Reset the DC components, as per section F.2.1.3.1.
+                               for i := 0; i < nComponent; i++ {
+                                       dc[i] = 0
+                               }
+                       }
                } // for mx
        } // for my
 
        return nil
 }
 
+// Specified in section B.2.4.4.
+func (d *decoder) processDRI(n int) os.Error {
+       if n != 2 {
+               return FormatError("DRI has wrong length")
+       }
+       _, err := io.ReadFull(d.r, d.tmp[0:2])
+       if err != nil {
+               return err
+       }
+       d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
+       return nil
+}
+
 // Decode reads a JPEG formatted image from r and returns it as an image.Image.
 func Decode(r io.Reader) (image.Image, os.Error) {
        var d decoder
@@ -379,6 +419,8 @@ func Decode(r io.Reader) (image.Image, os.Error) {
                        err = d.processDQT(n)
                case marker == sosMarker: // Start Of Scan.
                        err = d.processSOS(n)
+               case marker == driMarker: // Define Restart Interval.
+                       err = d.processDRI(n)
                case marker >= app0Marker && marker <= app15Marker || marker == comMarker: // APPlication specific, or COMment.
                        err = d.ignore(n)
                default: