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

[Gary Bloom]

Hi Markus,

Thanks for your reply and sorry for the long delay.

Hey, in your travels, have you seen any fixes to allow the TIFF module to support writing with compression?  It seems that the PIL always writes TIFF files uncompressed and doesn't seem to support writing with compression.  I would love to find a fix for that, as well.  Any help would be greatly appreciated.

Thanks!

Gary
----- Original Message ----- 
  From: Markus Kemmerling 
  To: Gary Bloom 
  Cc: Image-SIG at python.org ; gbloom at sefas.com 
  Sent: Thursday, October 18, 2007 2:50 AM
  Subject: Re: [Image-SIG] BUG: PIL DPI trouble - fix for TIFF files (PIL 1.1.6)


  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


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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 * 2.54When 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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