cwebp produce visible artifacts even with lossless compression

[GoogleCodeExporter]

Issue is based on message 
https://groups.google.com/a/webmproject.org/forum/#!topic/webp-discuss/hvw3_k6Tn
_8

What steps will reproduce the problem?

wget http://static.kupivip.ru/V0/00/97/87/59/1b.jpg
convert 1b.jpg -filter Lanczos -resize 240x360 -quality 100 resized_q100.jpg
cwebp -m 6 -resize 240 360 -q 90 1b.jpg -o internal_resize_q90.webp
cwebp -m 6 -q 90 resized_q100.jpg -o external_resize_q90.webp
cwebp -lossless resized_q100.jpg -o external_resize_lossless.webp

What is the expected output? What do you see instead?

There are two problems:
1. external_resize_q90.webp and even external_resize_lossless.webp differ from 
original image. Both images has user visible artifacts. Artifacts appear as 
white lines around man.
2. If internal resize is used than there is no artifacts, but quality is not so 
good (seems like Lanczos resize algorithm is better for this image than one 
used in cwebp).

What version of the product are you using? On what operating system?

I use official build libwebp-0.3.1-linux-x86-64. Operating system is Ubuntu 
13.04.

Please provide any additional information below.

skal has found the reason of problem:
"""
Seems there's a mismatch between the IDCT used by 'convert' and the one used by 
'cwebp'.
The problem disappears when commenting out line 235 of examples/jpegdec.c 
("dinfo.dct_method = JDCT_IFAST;").
There also no problem when using PNG format instead of JPEG (resized_q100.jpg 
-> resized_q100.png).

Not sure about the speed impact when not using JDCT_IFAST. Maybe that could be 
a cwebp option...
"""

and created a patch to fix an issue:
> I already have the patch ready here: 
https://gerrit.chromium.org/gerrit/#/c/67084

Please write if I need to create a separate issue to make possible to use 
different resize algorithms in cwebp.

Original issue reported on code.google.com by M.Mikhu...@gmail.com on 2 Sep 2013 at 9:23

[GoogleCodeExporter]

 https://gerrit.chromium.org/gerrit/#/c/67084/ patch submitted.

Original comment by pascal.m...@gmail.com on 3 Sep 2013 at 8:00

[GoogleCodeExporter]

Original comment by pascal.m...@gmail.com on 11 Sep 2013 at 7:32

• Changed state: Fixed