[Image-SIG] BUG: PIL DPI trouble - fix for TIFF files (PIL 1.1.6)

[Markus Kemmerling]

I would really like to see these fixes included in a next PIL  
release, if there will be one. (I also would like to see an egg  
release, but that's a different story.)

Note, that I posted a similar fix (combining Florian's and your's) to  
the list a long time ago (july 2006). There is a small difference  
though, see below.

Maybe a test for these fixes should be added, but I can't see any  
TIFF image tests at all in the PIL distribution.

Am 10.10.2007 um 16:18 schrieb Gary Bloom:

> Here's a quick fix for the writing of the file, as well...
>
> Before making the fix below, go to line 721 and change the '1' to a  
> '2' !
>
>     721:        ifd[RESOLUTION_UNIT] = 1
>
>         becomes
>
>     721:        ifd[RESOLUTION_UNIT] = 2
>
> Then make the changes listed below.    This is consistent with  
> those changes.  This is a quick and dirty fix that I have not  
> tested extensively, but it quickly allows me to read and write TIFF  
> images with both the PIL and Photoshop, and each can see the DPI as  
> set by the other.  It fixes the problem I've had with PIL reading/ 
> writing TIFFs.  Thanks, Florian!
>
> Regards,
>
> Gary Bloom
> Sales Engineer
> Sefas Innovation
> www.sefas.com
> ============
> [Image-SIG] BUG: PIL DPI trouble - fix for TIFF files (PIL 1.1.6)
>
> Florian Höch lists+Image_SIG at hoech.org
> Fri Oct 5 00:05:16 CEST 2007
>
> Previous message: [Image-SIG] fromstring image question
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> PIL only adds a dpi entry in the info dictionary if no resolution unit
> is specified in the TIFF file. This seems to be because of a
> misinterpretation of resolution unit values: they have a different
> meaning than in a JPEG file. JPEG: 0 = None; 1 = inches; 2 = cm.  
> TIFF: 1
> = None; 2 = inches; 3 = cm (see
> http://www.awaresystems.be/imaging/tiff/tifftags/ 
> resolutionunit.html and
> http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html)
>
> Here's a quick fix:
>
> Open TiffImagePlugin.py and go to line 577. Change the code:
>
> xres = getscalar(X_RESOLUTION, (1, 1))
> yres = getscalar(Y_RESOLUTION, (1, 1))
>
> if xres and yres:
>      xres = xres[0] / (xres[1] or 1)
>      yres = yres[0] / (yres[1] or 1)
>      resunit = getscalar(RESOLUTION_UNIT, 1)
>      if resunit == 2: # Inches
>          self.info["dpi"] = xres, yres
>      elif resunit == 3: # Centimeters
>          self.info["dpi"] = xres * 2.54, yres * 2.54
When converting from inches to centimeters, shouldn't you rather  
divide by 2.54 instead of multiplying?
>
>      else: # No absolute unit of measurement.
>          self.info["resolution"] = xres, yres
>
>
> Regards,
>
> Florian Hoech
> _______________________________________________
> Image-SIG maillist  -  Image-SIG at python.org
> http://mail.python.org/mailman/listinfo/image-sig

Regards,
Markus Kemmerling

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