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

[Daniele Romagnoli]

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










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