diff options
Diffstat (limited to 'src/pkg/image/jpeg/reader.go')
| -rw-r--r-- | src/pkg/image/jpeg/reader.go | 17 |
1 files changed, 10 insertions, 7 deletions
diff --git a/src/pkg/image/jpeg/reader.go b/src/pkg/image/jpeg/reader.go index d9adf6e587..eb6b0c93b6 100644 --- a/src/pkg/image/jpeg/reader.go +++ b/src/pkg/image/jpeg/reader.go @@ -74,7 +74,9 @@ const ( comMarker = 0xfe // COMment. ) -// Maps from the zig-zag ordering to the natural ordering. +// unzig maps from the zig-zag ordering to the natural ordering. For example, +// unzig[3] is the column and row of the fourth element in zig-zag order. The +// value is 16, which means first column (16%8 == 0) and third row (16/8 == 2). var unzig = [blockSize]int{ 0, 1, 8, 16, 9, 2, 3, 10, 17, 24, 32, 25, 18, 11, 4, 5, @@ -101,7 +103,7 @@ type decoder struct { nComp int comp [nColorComponent]component huff [maxTc + 1][maxTh + 1]huffman - quant [maxTq + 1]block + quant [maxTq + 1]block // Quantization tables, in zig-zag order. b bits tmp [1024]byte } @@ -264,6 +266,7 @@ func (d *decoder) processSOS(n int) error { for j := 0; j < d.comp[i].h*d.comp[i].v; j++ { // TODO(nigeltao): make this a "var b block" once the compiler's escape // analysis is good enough to allocate it on the stack, not the heap. + // b is in natural (not zig-zag) order. b = block{} // Decode the DC coefficient, as specified in section F.2.2.1. @@ -282,7 +285,7 @@ func (d *decoder) processSOS(n int) error { b[0] = dc[i] * qt[0] // Decode the AC coefficients, as specified in section F.2.2.2. - for k := 1; k < blockSize; k++ { + for zig := 1; zig < blockSize; zig++ { value, err := d.decodeHuffman(&d.huff[acTable][scan[i].ta]) if err != nil { return err @@ -290,20 +293,20 @@ func (d *decoder) processSOS(n int) error { val0 := value >> 4 val1 := value & 0x0f if val1 != 0 { - k += int(val0) - if k > blockSize { + zig += int(val0) + if zig > blockSize { return FormatError("bad DCT index") } ac, err := d.receiveExtend(val1) if err != nil { return err } - b[unzig[k]] = ac * qt[k] + b[unzig[zig]] = ac * qt[zig] } else { if val0 != 0x0f { break } - k += 0x0f + zig += 0x0f } } |