diff options
Diffstat (limited to 'src/pkg/image/jpeg/idct.go')
| -rw-r--r-- | src/pkg/image/jpeg/idct.go | 80 |
1 files changed, 34 insertions, 46 deletions
diff --git a/src/pkg/image/jpeg/idct.go b/src/pkg/image/jpeg/idct.go index b387dfdff..46fcaecb7 100644 --- a/src/pkg/image/jpeg/idct.go +++ b/src/pkg/image/jpeg/idct.go @@ -37,6 +37,10 @@ package jpeg * */ +const blockSize = 64 // A DCT block is 8x8. + +type block [blockSize]int32 + const ( w1 = 2841 // 2048*sqrt(2)*cos(1*pi/16) w2 = 2676 // 2048*sqrt(2)*cos(2*pi/16) @@ -55,9 +59,7 @@ const ( r2 = 181 // 256/sqrt(2) ) -// idct performs a 2-D Inverse Discrete Cosine Transformation, followed by a -// +128 level shift and a clip to [0, 255], writing the results to dst. -// stride is the number of elements between successive rows of dst. +// idct performs a 2-D Inverse Discrete Cosine Transformation. // // The input coefficients should already have been multiplied by the // appropriate quantization table. We use fixed-point computation, with the @@ -67,33 +69,34 @@ const ( // For more on the actual algorithm, see Z. Wang, "Fast algorithms for the // discrete W transform and for the discrete Fourier transform", IEEE Trans. on // ASSP, Vol. ASSP- 32, pp. 803-816, Aug. 1984. -func idct(dst []byte, stride int, src *block) { +func idct(src *block) { // Horizontal 1-D IDCT. for y := 0; y < 8; y++ { + y8 := y * 8 // If all the AC components are zero, then the IDCT is trivial. - if src[y*8+1] == 0 && src[y*8+2] == 0 && src[y*8+3] == 0 && - src[y*8+4] == 0 && src[y*8+5] == 0 && src[y*8+6] == 0 && src[y*8+7] == 0 { - dc := src[y*8+0] << 3 - src[y*8+0] = dc - src[y*8+1] = dc - src[y*8+2] = dc - src[y*8+3] = dc - src[y*8+4] = dc - src[y*8+5] = dc - src[y*8+6] = dc - src[y*8+7] = dc + if src[y8+1] == 0 && src[y8+2] == 0 && src[y8+3] == 0 && + src[y8+4] == 0 && src[y8+5] == 0 && src[y8+6] == 0 && src[y8+7] == 0 { + dc := src[y8+0] << 3 + src[y8+0] = dc + src[y8+1] = dc + src[y8+2] = dc + src[y8+3] = dc + src[y8+4] = dc + src[y8+5] = dc + src[y8+6] = dc + src[y8+7] = dc continue } // Prescale. - x0 := (src[y*8+0] << 11) + 128 - x1 := src[y*8+4] << 11 - x2 := src[y*8+6] - x3 := src[y*8+2] - x4 := src[y*8+1] - x5 := src[y*8+7] - x6 := src[y*8+5] - x7 := src[y*8+3] + x0 := (src[y8+0] << 11) + 128 + x1 := src[y8+4] << 11 + x2 := src[y8+6] + x3 := src[y8+2] + x4 := src[y8+1] + x5 := src[y8+7] + x6 := src[y8+5] + x7 := src[y8+3] // Stage 1. x8 := w7 * (x4 + x5) @@ -123,14 +126,14 @@ func idct(dst []byte, stride int, src *block) { x4 = (r2*(x4-x5) + 128) >> 8 // Stage 4. - src[8*y+0] = (x7 + x1) >> 8 - src[8*y+1] = (x3 + x2) >> 8 - src[8*y+2] = (x0 + x4) >> 8 - src[8*y+3] = (x8 + x6) >> 8 - src[8*y+4] = (x8 - x6) >> 8 - src[8*y+5] = (x0 - x4) >> 8 - src[8*y+6] = (x3 - x2) >> 8 - src[8*y+7] = (x7 - x1) >> 8 + src[y8+0] = (x7 + x1) >> 8 + src[y8+1] = (x3 + x2) >> 8 + src[y8+2] = (x0 + x4) >> 8 + src[y8+3] = (x8 + x6) >> 8 + src[y8+4] = (x8 - x6) >> 8 + src[y8+5] = (x0 - x4) >> 8 + src[y8+6] = (x3 - x2) >> 8 + src[y8+7] = (x7 - x1) >> 8 } // Vertical 1-D IDCT. @@ -186,19 +189,4 @@ func idct(dst []byte, stride int, src *block) { src[8*6+x] = (y3 - y2) >> 14 src[8*7+x] = (y7 - y1) >> 14 } - - // Level shift by +128, clip to [0, 255], and write to dst. - for y := 0; y < 8; y++ { - for x := 0; x < 8; x++ { - c := src[y*8+x] - if c < -128 { - c = 0 - } else if c > 127 { - c = 255 - } else { - c += 128 - } - dst[y*stride+x] = uint8(c) - } - } } |