Severe artifact for quality <= 54

[akobel]

Hi,

encoding the following image with -quality 54 or less yields a severe artifact at the "ü" character; with -quality 55 or higher, everything is as expected.

The issue occurs with the current git HEAD, 3.2 (the mozjpeg-opt from Arch Linux' user repository), and 3.3.1 (built with an adapted build script). The build instructions for the Arch packages are pretty straightforward: unpack the release tarball and call autoreconf -fiv, configure --with-jpeg8 and make. My build from git HEAD was a plain mkdir build; cd build; cmake ..; make on an Arch Linux machine; CMake output is attached. The make log was clean. As far as I know, I have only Arch's standard repositories' versions of any relevant libraries, freshly updated.

This is a crop from a larger file, where the same problem occurs at several positions; but all have the same structure, that is, a sharp black vertical line in one block. I didn't investigate with rotations etc.

Input: (PNG instead of PNM for the sake of uploading):

cjpeg-static -quality 54 -outfile moz54.jpg in.pnm:

cjpeg-static -quality 55 -outfile moz55.jpg in.pnm:

build.log

[tigermonkey]

Hi, I run into this bug as well. It seems to be related to overshoot deringing and the SIMD-optimized DCT functions.

mozcjpeg -quality 50 block.pnm >with-artifact.jpg
mozcjpeg -quality 50 -noovershoot block.pnm >without-artifact.jpg
mozcjpeg -quality 50 -dct float block.pnm >without-artifact.jpg
mozcjpeg -quality 50 -dct fast block.pnm >without-artifact.jpg

I instrumented this line to print workspace before/after and found that the different DCT functions return different results for the same input: https://github.com/mozilla/mozjpeg/blob/cfb713852354b139ca25a8c382de4166a67d41a1/jcdctmgr.c#L714

Running it with different DCTs:

% mozcjpeg -quality 50 -dct int block.pnm >/dev/null
Bef: -128 -128 -128 -118 158 158 158 158 -128 -128 -128 -120 158 158 158 158 -128 -128 -128 -122 158 158 158 158 -128 -128 -128 -128 158 158 158 158 -128 -128 -128 -128 120 158 158 158 -128 -128 -128 -127 116 158 158 158 -128 -128 -128 -128 123 158 158 158 -128 -128 -128 -124 158 158 158 158
Aft: 874 8130 113 2799 -86 -1777 47 1450 106 -18 -139 49 106 -74 -58 87 72 -9 -95 27 72 -41 -39 48 -100 26 130 -75 -100 112 54 -133 38 -11 -49 31 38 -46 -21 56 3 -4 -3 10 3 -15 -1 18 18 -1 -23 4 18 -6 -10 7 -30 9 39 -26 -30 38 16 -45
Bef: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Aft: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Bef: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Aft: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

% mozcjpeg -quality 50 -dct fast block.pnm >/dev/null
Bef: -128 -128 -128 -118 158 158 158 158 -128 -128 -128 -120 158 158 158 158 -128 -128 -128 -122 158 158 158 158 -128 -128 -128 -128 158 158 158 158 -128 -128 -128 -128 120 158 158 158 -128 -128 -128 -127 116 158 158 158 -128 -128 -128 -128 123 158 158 158 -128 -128 -128 -124 158 158 158 158 
Aft: 874 -11452 142 3311 -86 -1407 30 396 145 -34 -253 80 145 -83 -45 31 94 -19 -162 44 94 -45 -28 18 -116 43 200 -101 -116 104 39 -40 38 -16 -64 37 38 -37 -12 16 2 -5 -4 9 2 -10 -1 4 10 -1 -16 4 10 -3 -2 2 -7 4 13 -8 -7 9 3 -3 
Bef: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
Aft: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
Bef: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
Aft: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

As you see, the results are very different after the 1st element.

Given that, I think the bug is in https://github.com/mozilla/mozjpeg/blob/master/simd/x86_64/jfdctint-avx2.asm, as it returns different results than the fast or float versions. block.pnm.gz

[veikk0]

I just ran into this problem. Both input and output images are attached below.

• Input: memoirsofsherloc00doylrich_0015.ppm.zip (zipped for uploading)
• Output (-grayscale -quality 20): memoirsofsherloc00doylrich_0015.jpg

For this specific file, the artifacting occurs at quality 57 and below. It only seems to occur for BW text, and it really is severe.

-version output: mozjpeg version 4.0.2 (build 20210218). Built on Linux Mint 20.1 (Ubuntu 20.04 base) using the build instructions in the BUILD file.

[maikmerten]

Just ran into this problem as well. I built mozjpeg from current git.

Input:

Compressing with mozjpeg: cjpeg-mozjpeg -q 50 ./zeichnung.png > zeichnung.jpg

Output: