[gdal-dev] Troubleshooting palette management. Potential bug in palette for overviews?

Thu Nov 24 08:22:50 PST 2016

On Thu, Nov 24, 2016 at 5:01 PM, Even Rouault <even.rouault at spatialys.com>
wrote:

> On jeudi 24 novembre 2016 16:51:59 CET Daniele Romagnoli wrote:
>
> > Doh!
>
> > Thanks for the feedback, Even.
>

> (Indeed it seemed strange to me that nobody had ever noticed before)
>
Sorry, what I meant with that was that I didn't see that ticket before
sending my email so it seemed strange to me that nobody noticed the palette
math weirdness :)

>
>
> Not that much regarding software using GDAL since if you don't explicitly
> request the overview bands/datasets but rather use the subsampling
> RasterIO() calls, you'll never work directly with the color table of the
> overviews but still use the one of the main dataset (or even if you request
> the overviews directly I guess in most cases applications don't bother
> checking if the color table of the overview).
>

As far as I can see, the TIFFImageReader parses the metadata information
based on imageIndex (so overview number). I will double check if things
were working differently in the colormap topic before our
changes/optimizations to this class.

> In GDAL, overviews conceptually share the same palette than the main
> image. Otherwise subsampling RasterIO() will return values of the overview
> palette that will be interpreted with the palette of the main image. You
> can imagine the fun visual effects if the palette are not the same... So
> this is assumed, but not checked.
>
>
>
> Could be worth fixing that if that cause issues (apparently in your use
> case at least), but I'd be interested by information of what is the
> standard/recommanded/best-practice-in-the-industry/etc... formulas to use
> for the mapping between the [0,255] and [0,65535] ranges.
>
>
>

Cheers,
Daniele

> >
>
> > Best Regards,
>
> > Daniele
>
> >
>
> > On Thu, Nov 24, 2016 at 1:35 PM, Even Rouault <
> even.rouault at spatialys.com>
>
> >
>
> > wrote:
>
> > > Daniele,
>
> > >
>
> > >
>
> > >
>
> > > This is indeed an inconsistency I spotted a long time ago :
>
> > >
>
> > > https://trac.osgeo.org/gdal/ticket/2213
>
> > >
>
> > >
>
> > >
>
> > > Nobody was apparently sufficiently annoyed by this to fix it. Actually
> I
>
> > > see I proposed a patch. Was probably waiting for feedback. I'm not
> sure if
>
> > > there's a standardized way of converting between the [0,255] and
> [0,65535]
>
> > > ranges.
>
> > >
>
> > >
>
> > >
>
> > > Even
>
> > >
>
> > > > Hi List,
>
> > > >
>
> > > > I'm investigating on a issue I have with a paletted image with
>
> > > > overviews,
>
> > > >
>
> > > > using java ImageIO tiff reader (wait... I know on this list we
> discuss
>
> > > >
>
> > > > about GDAL topics :) ).
>
> > > >
>
> > > > I'm trying to better understand palette interpretation and palette
>
> > >
>
> > > storing
>
> > >
>
> > > > on tags and how they are handled by GDAL.
>
> > > >
>
> > > >
>
> > > >
>
> > > > From TIFF specifications, colormap entries are stored as short
> values.
>
> > > >
>
> > > > I have a sample file with this color tab (as reported by gdalinfo).
>
> > > >
>
> > > >
>
> > > >
>
> > > > Color Table (RGB with 256 entries)
>
> > > >
>
> > > > 0: *164*,164,164,255
>
> > > >
>
> > > > 1: *255*,255,255,255
>
> > > >
>
> > > > 2: 0,0,0,255
>
> > > >
>
> > > > 3: 0,0,0,255
>
> > > >
>
> > > > .... repeat 0,0,0,255 to the end.
>
> > > >
>
> > > >
>
> > > >
>
> > > > Using an Hex editor or AsTiffTagViewer, I can see the byte values of
> the
>
> > > >
>
> > > > colormap as stored on the file.
>
> > > >
>
> > > > For the first 4 mappings of the colortable the Red components are:
>
> > > >
>
> > > > *A4 A4* *FF FF* 00 00 00 00
>
> > > >
>
> > > > (8 bytes = 4 shorts)
>
> > > >
>
> > > >
>
> > > >
>
> > > > As far as I have understood, this is related to that part of the
>
> > > >
>
> > > > specification (quoting it, section 5):
>
> > > >
>
> > > >
>
> > > >
>
> > > >
>
> > > >
>
> > > > *In a TIFF ColorMap, all the Red values come first, followed by the
>
> > > > Green
>
> > > >
>
> > > > values, then the Blue values. In the ColorMap, black is represented
> by
>
> > > >
>
> > > > 0,0,0 and white is represented by 65535, 65535, 65535*
>
> > > >
>
> > > >
>
> > > >
>
> > > > So the second entry of the map, being white (255, 255, 255) has its
> red
>
> > > >
>
> > > > value stored as 65535 = *FF FF *whilst the first color (164,164,164)
> has
>
> > > >
>
> > > > its red value stored as 42148 = *A4 A4*
>
> > > >
>
> > > >
>
> > > >
>
> > > > I see this behavior is confirmed by the way the geotiff.cpp create
> the
>
> > > >
>
> > > > colortable:
>
> > > >
>
> > > > https://github.com/OSGeo/gdal/blob/trunk/gdal/frmts/gtiff/
>
> > >
>
> > > geotiff.cpp#L5043
>
> > >
>
> > > > panTRed[iColor] = static_cast<unsigned short>(257 * sRGB.c1);
>
> > > >
>
> > > > panTGreen[iColor] = static_cast<unsigned short>(257 * sRGB.c2);
>
> > > >
>
> > > > panTBlue[iColor] = static_cast<unsigned short>(257 * sRGB.c3)
>
> > > >
>
> > > >
>
> > > >
>
> > > > Applying a color*257 multiplication allows to have its 8 bits color
>
> > > >
>
> > > > representation stored into a short where intensity goes from 0 to
> 65535.
>
> > > >
>
> > > >
>
> > > >
>
> > > > The issue I have is that when using gdaladdo, the overviews are
>
> > > > generated
>
> > > >
>
> > > > with a different color table.
>
> > > >
>
> > > > So that the first 8 bytes of the red component stored in the
> colormap,as
>
> > > >
>
> > > > reported by AsTiffTagViewer are:
>
> > > >
>
> > > > *A4* 00 *FF* 00 00 00 00 00
>
> > > >
>
> > > >
>
> > > >
>
> > > > When converting it back to a 0-255 color component with the formula
>
> > > >
>
> > > > R*255/65535 it become *163* instead of 164.
>
> > > >
>
> > > >
>
> > > >
>
> > > > I see in the GDAL code that both the IBuildOverviews and
>
> > > >
>
> > > > CreateOverviewsFromSrcOverviews call the CreateTIFFColorTable
> function
>
> > > >
>
> > > > which multiplies the colorMapEntry by 256 instead of 257.
>
> > > >
>
> > > >
>
> > > >
>
> > > > https://github.com/OSGeo/gdal/blob/trunk/gdal/frmts/gtiff/
>
> > >
>
> > > geotiff.cpp#L9137
>
> > >
>
> > > > This may explain the *A4* 00 *FF* 00 entries in the overview with
>
> > > > respect
>
> > > >
>
> > > > to the *A4 A4 FF FF *entries in the main level.
>
> > > >
>
> > > >
>
> > > >
>
> > > > Do you have any feedback on this?
>
> > > >
>
> > > > Please, let me know what do you think about it.
>
> > > >
>
> > > >
>
> > > >
>
> > > > Best Regards,
>
> > > >
>
> > > > Daniele
>
> > >
>
> > > --
>
> > >
>
> > > Spatialys - Geospatial professional services
>
> > >
>
> > > http://www.spatialys.com
>
> > >
>
> > > _______________________________________________
>
> > > gdal-dev mailing list
>
> > > gdal-dev at lists.osgeo.org
>
> > > http://lists.osgeo.org/mailman/listinfo/gdal-dev
>
>
>
>
>
> --
>
> Spatialys - Geospatial professional services
>
> http://www.spatialys.com
>
> _______________________________________________
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