[Gimp-developer] Some questions about tiff

[Adalbert Hanßen <Adalbert Hanssen gmx de>]

XSane produced a color scan from a document with 600 dpi and fill color, 
1.1MB file size.
According to tiffinfo (under Xubuntu) the properties of the 
Sane-generatd tiff file is
TIFF Directory at offset 0x11af5a (1159002)
  Image Width: 4976 Image Length: 3190
  Resolution: 600, 600 pixels/inch
  Bits/Sample: 8
  Compression Scheme: JPEG
  Photometric Interpretation: YCbCr
  Orientation: row 0 top, col 0 lhs
  Samples/Pixel: 3
  Rows/Strip: 16
  Planar Configuration: single image plane
  Reference Black/White:
     0:     0   255
     1:   128   255
     2:   128   255
  Software: xsane
  DateTime: 2022:09:18 18:15:59
  JPEG Tables: (574 bytes)

When I load this file into Gimp, I get an error message about an 
incompatible TIFF format (additiona channels without the field 
ExtraSamples). It gives me choice to let the additional channel worlk as
* non pre-multiplied alpha
* pre-multiplied alpha
* channel

I see no difference whatever choice I select. But this might be due to 
my sample.
However: When I try to adapt colors by the contrast curve, I see no 
Histogram under it.
** Is this due to the error message when loading the file? **

I see histograms under the contrast curve for other tiff images not 
resulting from XSane.
Regards

Adalbert Hanßen

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