[Image-SIG] PIL DPI trouble - Solved

Gary Bloom bloominator at hotmail.com
Fri Sep 21 21:28:57 CEST 2007

Hi Luca,

Thanks for shedding some light on this topic.  However, what you've said has 
left me even more curious than before.  There are still some things I don't 

For one, why do Photoshop (PS) and Paint Shop Pro (PSP) play so nicely 
together when the PIL doesn't play along at all for DPI.  If I create an 
image with certain DPI in Photoshop, Paint Shop Pro can read the image and 
see that set density, and vice versa.  But the PIL doesn't recognize the DPI 
of either a PSP or Photoshop image.  And of course neither PS or PSP 
recognize the DPI of a PIL image.  So I'm a bit confused.  You say that PIL 
seems to be "playing by the rules" and adhering to the JFIF standard.  (My 
images are both TIFF and JPEG, but I'm guessing that it's the same problem 
regardless of which image type it is.), yet PSP and PS cannot read the DPI 
of these PIL-processed images.  This doesn't make sense to me.  If, what you 
say below is true and PSP and PS are adding "extra stuff" outside of the 
JFIF standard, is it coincidental that they can understand one another? 
And, even if they are both writing (and reading the other's) non-standard 
DPI info, shouldn't the PIL be foresighted enough to have a "compatibility 
mode"' for that "extra stuff"?  Sometimes just adhering blindly to a 
standard is a good thing, unless the rest of the world is ignoring the 
standard, which is what it seems PSP and PS are doing.  So do you have any 
idea how to make the PIL compatible with these other programs?  At work my 
company has a product which also deals with images, and one day I imported 
an image that I had resized and changed the DPI of it via the PIL, and it 
made our software crash.  This has never happened when using any other 
program to manipulate images.  Any thoughts on this?  Or perhaps can you 
point me to someone who can shed even more light on the matter?

Thanks so much for your reply,

----- Original Message ----- 
From: "Luca De Santis" <lucadex at gmail.com>
To: <Image-SIG at python.org>
Cc: <bloominator at hotmail.com>
Sent: Friday, September 21, 2007 6:12 AM
Subject: PIL DPI trouble - Solved

>I found out that the behaviour of PIL was totally correct and simply
> Photoshop "adds" some more information sometimes when a JPEG file is
> opened.
> I summarize what I discovered here, hoping it might be useful to others.
> The JPEG standard doesn't cover a way to encode the resolution of a
> file but the JFIF extension does. For a quick and useful introduction
> to these concepts you can visit the Wikipedia page
> http://en.wikipedia.org/wiki/JFIF
> Here you can find a very useful table with the byte map of a JFIF file.
> In particular the "Density Units" field is a byte with three possible 
> values:
>    * 0 - No units, aspect ratio only specified
>    * 1 - Pixels per Inch
>    * 2 - Pixels per Centimetre
> With a value of Density Units == 1 or == 2 you can get the resolution
> information in the next two fields (2 bytes long each), that is "X
> Density" and "Y Density". Of course these map exactly "dpi" when
> Density Units == 1.
> For all the images that I opened with PIL and couldn't find the dpi
> attribute I "hex-dumped" them and found out the following values for
> these fields:
>    * Density Unit: 0
>    * X Density: 1
>    * Y Density: 1
> I assume that in these cases Photoshop simply sets the resolution to
> the default for screens (which is 72 dpi at least on the Mac).
> Correctly PIL doesn't set the dpi attribute in the "info" dictionary.
> To cut this long story short... I was wrong and PIL rocks hard! :-)
> Hope my experience could help others.
> Ciao,
> Luca
> Luca De Santis
> Pisa, Italy
> On 9/17/07, Luca De Santis <lucadex at gmail.com> wrote:
>> That's exactly the same problem I incurred into.
>> I tried PIL  version 1.1.6 on a Mac OS X 10.4 and on an old Linux box
>> and can't get the DPI attribute for JPEG and PNG files that I've
>> created with Photoshop.
>> I tested that both on Python 2.5 and Python 2.1.3 with the same result.
>> I tried also to update libjpeg on Linux but nothing changes.
>> Any help on that is greatly appreciated.
>> TIA.

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