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Use bias pattern for 4:4:0 (h1v2) fancy upsampling

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This commit modifies h1v2_fancy_upsample() so that it uses an ordered
dither pattern, similar to that of h2v1_fancy_upsample(), rounding up or
down the result for alternate pixels rather than always rounding down.
This ensures that the decompression error pattern for a 4:4:0 JPEG image
will be similar to the rotated decompression error pattern for a 4:2:2
JPEG image.  Thus, the final result will be similar regardless of
whether a 4:2:2 JPEG image is rotated or transposed before or after
decompression.

Closes #356
This commit is contained in:
Jonathan Wright
2019-05-09 13:46:53 +01:00
committed by DRC
parent ec5adb83dd
commit 509c2680aa
2 changed files with 17 additions and 5 deletions
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8
ChangeLog.md
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@@ -13,6 +13,14 @@ instruction exception, in rare cases, on CPUs that lack support for CPUID leaf
07H (or on which the maximum CPUID leaf has been limited by way of a BIOS
setting.)
3. The 4:4:0 (h1v2) fancy (smooth) chroma upsampling algorithm in the
decompressor now uses a similar bias pattern to that of the 4:2:2 (h2v1) fancy
chroma upsampling algorithm, rounding up or down the upsampled result for
alternate pixels rather than always rounding down. This ensures that the final
result will be similar regardless of whether a 4:2:2 JPEG image is rotated or
transposed prior to decompression (in the frequency domain) or after
decompression (in the spatial domain.)
2.0.2
=====

14
jdsample.c
View File

@@ -8,6 +8,7 @@
* Copyright (C) 2010, 2015-2016, D. R. Commander.
* Copyright (C) 2014, MIPS Technologies, Inc., California.
* Copyright (C) 2015, Google, Inc.
* Copyright (C) 2019, Arm Limited.
* For conditions of distribution and use, see the accompanying README.ijg
* file.
*
@@ -315,9 +316,9 @@ h1v2_fancy_upsample(j_decompress_ptr cinfo, jpeg_component_info *compptr,
JSAMPARRAY output_data = *output_data_ptr;
JSAMPROW inptr0, inptr1, outptr;
#if BITS_IN_JSAMPLE == 8
int thiscolsum;
int thiscolsum, bias;
#else
JLONG thiscolsum;
JLONG thiscolsum, bias;
#endif
JDIMENSION colctr;
int inrow, outrow, v;
@@ -327,15 +328,18 @@ h1v2_fancy_upsample(j_decompress_ptr cinfo, jpeg_component_info *compptr,
for (v = 0; v < 2; v++) {
/* inptr0 points to nearest input row, inptr1 points to next nearest */
inptr0 = input_data[inrow];
if (v == 0) /* next nearest is row above */
if (v == 0) { /* next nearest is row above */
inptr1 = input_data[inrow - 1];
else /* next nearest is row below */
bias = 1;
} else { /* next nearest is row below */
inptr1 = input_data[inrow + 1];
bias = 2;
}
outptr = output_data[outrow++];
for (colctr = 0; colctr < compptr->downsampled_width; colctr++) {
thiscolsum = GETJSAMPLE(*inptr0++) * 3 + GETJSAMPLE(*inptr1++);
*outptr++ = (JSAMPLE)((thiscolsum + 1) >> 2);
*outptr++ = (JSAMPLE)((thiscolsum + bias) >> 2);
}
}
inrow++;