diff options
Diffstat (limited to 'src/pkg/image/jpeg/scan.go')
| -rw-r--r-- | src/pkg/image/jpeg/scan.go | 21 |
1 files changed, 13 insertions, 8 deletions
diff --git a/src/pkg/image/jpeg/scan.go b/src/pkg/image/jpeg/scan.go index a69ed1748..559235d51 100644 --- a/src/pkg/image/jpeg/scan.go +++ b/src/pkg/image/jpeg/scan.go @@ -141,25 +141,30 @@ func (d *decoder) processSOS(n int) error { for j := 0; j < d.comp[compIndex].h*d.comp[compIndex].v; j++ { // The blocks are traversed one MCU at a time. For 4:2:0 chroma // subsampling, there are four Y 8x8 blocks in every 16x16 MCU. + // // For a baseline 32x16 pixel image, the Y blocks visiting order is: // 0 1 4 5 // 2 3 6 7 // - // For progressive images, the DC data blocks (zigStart == 0) are traversed - // as above, but AC data blocks are traversed left to right, top to bottom: + // For progressive images, the interleaved scans (those with nComp > 1) + // are traversed as above, but non-interleaved scans are traversed left + // to right, top to bottom: // 0 1 2 3 // 4 5 6 7 + // Only DC scans (zigStart == 0) can be interleaved. AC scans must have + // only one component. // - // To further complicate matters, there is no AC data for any blocks that - // are inside the image at the MCU level but outside the image at the pixel - // level. For example, a 24x16 pixel 4:2:0 progressive image consists of - // two 16x16 MCUs. The earlier scans will process 8 Y blocks: + // To further complicate matters, for non-interleaved scans, there is no + // data for any blocks that are inside the image at the MCU level but + // outside the image at the pixel level. For example, a 24x16 pixel 4:2:0 + // progressive image consists of two 16x16 MCUs. The interleaved scans + // will process 8 Y blocks: // 0 1 4 5 // 2 3 6 7 - // The later scans will process only 6 Y blocks: + // The non-interleaved scans will process only 6 Y blocks: // 0 1 2 // 3 4 5 - if zigStart == 0 { + if nComp != 1 { mx0, my0 = d.comp[compIndex].h*mx, d.comp[compIndex].v*my if h0 == 1 { my0 += j |