[gdal-dev] GDAL 1.11 vs 2.1 and JPEG2000?

[Even Rouault]

Le mercredi 06 janvier 2016 20:32:04, Jim Lacy a écrit :
> Thanks Even.  I was actually more curious about the quality differences of
> jpeg2000, GDAL 1.11 vs 2.1.  File size is important too, of course.

Quality and size are highly correlated. The JPEG2000 encoder honours quite 
strictly the compression ratio you ask for (QUALITY=12 means 1/12 of the 
uncompressed size. It is not an objective measure of the "quality" in the 
traditional meaning). So if naturally YCC=ON leads to better compression rate, 
when you ask for the same compression target, the quality will be better when 
YCC=ON. The naming of the "QUALITY" setting is a bit arguable indeed (I think 
I took inspiration from the same parameter name of the JP2KAK driver)

> 
> I did some further tests.  For example, in GDAL 1.11.2:
> gdal_translate -of JP2OpenJPEG -b 1 -b 2 -b 3 -co "QUALITY=12" Kenosha.tif
> Kenosha_gdal1_11.jp2
> 
> compared to GDAL 2.1:
> gdal_translate -of JP2OpenJPEG -b 1 -b 2 -b 3 -co "QUALITY=12" -co
> "YCC=OFF" Kenosha.tif Kenosha_gdal2_1_ycc_off.jp2
> 
> .... produces results that are visually identical.
> 
> However, simply by turning YCC=ON (the default) in GDAL 2.1 produces
> noticeably "better" images at higher zoom levels. gdal_translate -of
> JP2OpenJPEG -b 1 -b 2 -b 3 -co "QUALITY=12" -co "YCC=ON"  Kenosha.tif
> Kenosha_gdal2_1.jp2
> 
> It's interesting to me that the simple addition of YCC=OFF can noticeably
> affect the visually quality of the output image.  I certainly don't claim
> to understand the nuances of jpeg2000!
> 
> If you have time/interest, my test files are here:
> https://uwmadison.box.com/s/xfc2f0ehwe6x6nu7chxu3ykvmk46mw85
> 
> Thanks again.
> 
> 
> -----Original Message-----
> From: Even Rouault [mailto:even.rouault at spatialys.com]
> Sent: Wednesday, January 06, 2016 4:25 AM
> To: gdal-dev at lists.osgeo.org
> Cc: Jim Lacy
> Subject: Re: [gdal-dev] GDAL 1.11 vs 2.1 and JPEG2000?
> 
> Le mardi 05 janvier 2016 18:51:03, Jim Lacy a écrit :
> > Can anyone with knowledge of the JP2OpenJPEG driver confirm or deny
> > the following observation?
> > 
> > I've been generating jpeg2000's in both GDAL 1.11.2 and GDAL 2.1.
> > With all settings the same, I've noticed the visual quality of the
> > images generated in GDAL 2.1 are noticeably "better." (a subjective
> > term, yes)
> > 
> > To my eyes, in 1.11.2,  -co "QUALITY=20" yields results visually
> > comparable to roughly -co "QUALITY=12" in GDAL 2.1.  Win32 platform,
> > binaries from gisinternals.com.
> > 
> > Are there significant improvements in the 2.1 driver that would
> > explain my observation?
> 
> Jim,
> 
> Checking the NEWS, I believe this might be related to this improvement I
> did in GDAL 2.0
> 
>  * add YCC creation option to do RGB->YCC MCT, and turn it ON by default
> (#5634)
> 
> And a quick test confirms it with a BMNG 21600x21600 3 bands :
> 
> $ gdal_tanslate in.tif out1.jp2 -of jp2openjpeg $ gdal_tanslate in.tif
> out2.jp2 -of jp2openjpeg -co YCC=OFF $ ll out1.jp2 out2.jp2 -rw-r--r-- 1
> even even 111977681 2016-01-06 11:07 out1.jp2
> -rw-r--r-- 1 even even 151981743 2016-01-06 11:08 out2.jp2
> 
> RGB->YCC MCT conversion is a step that occurs before other processings,
> RGB->which
> tends to decrease the size of the resulting file, all other things equal.
> This transform is not completely lossless when using the default lossy
> compression (there's a lossless version that is used when using -co
> QUALITY=100 -co REVERSIBLE=YES), but the differences in pixel values tend
> to be very small between YCC=ON/OFF.
> 
> Even
> 
> --
> Spatialys - Geospatial professional services http://www.spatialys.com

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