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72.009 DPI and screenshots using PNG format

Oct 6, 2009 11:54 AM

Hi all,

 

This may not be a specific PS question, but has anyone else experienced screenshots which end up as 72.009 DPI when you use the PNG format?  I'll take a screenshot using cmd-shift-3 and my Mac will save it as a PNG on my desktop.  When I open it up in PS and inspect the image size, it's 72.009 DPI.  But when I save screenshots using any other format like jpg, gif, etc. they show as 72 DPI.

 

Thanks!

 
Replies
  • Currently Being Moderated
    Oct 6, 2009 12:06 PM   in reply to Vee S

    This is a long time issue. Google for '72.009' to see that you are not alone.

     

    DPI (or PPI) is irrelevant when it comes to screen capture so there is no reason to worry about it. All you really deal with is the number of pixels high by the number of pixels wide.

     
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  • Currently Being Moderated
    Oct 6, 2009 12:17 PM   in reply to Vee S

    Its not DPI its PPI

     

    Please read this so you know the difference.

     

    http://forums.adobe.com/thread/370714?tstart=0

     
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  • Currently Being Moderated
    Oct 6, 2009 3:06 PM   in reply to Vee S

    Nothing is flawed. when posting to the web resolution is meaningless.

     

    image size is the actual pixel dimensions. Also chances are good that your monitor is higher than 72PPI, mine for instance is 100PPI. but when I make a screen shot it still opens at 72PPI or a variation there of.

     

    photoshop is defaulting to 72 because there is on resolution data in the file. If you wish to change this number do it and save the file. the next time you open the file it will be the same as what you changed it to.

     
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  • Currently Being Moderated
    Oct 6, 2009 4:30 PM   in reply to Vee S

    Vee S wrote:

     

    Ok, changing the PPI shouldn't have any affect on its display.

     

    But say I change the PPI to 72 then copy and paste from the PNG into a blank PS file (also at 72 PPI).  I notice that the image changes slightly.  I'll use the eye dropper on the exact same point on both and come up with different values.  Is this the expected behavior?

    Only if the color space of the file you are pasting is different than the color space of the file you are pasting into. screen shots are usually the monitor profile so you need to convert the file to your working space. Like AdobeRGB or sRGB for instance.

     
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  • Currently Being Moderated
    Oct 7, 2009 5:56 AM   in reply to Vee S

    Vee S wrote:

     

     

    But say I change the PPI to 72

    Let's say you don't because it does not matter.   By altering the ppi in Photoshop you either resample (damage) the image (if that option is enabled in the image size dialog) or you do nothing to affect the image by altering number of pixels wide by high (if resampling is not checked in the image size dialog). Neither is beneficial to you in any way.

     

    You do not need to make screen captures as PNG. The OS X capture function can be changed to TIFF, which does not create images at 72.001 ppi.

     
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  • Currently Being Moderated
    Oct 7, 2009 4:53 PM   in reply to Vee S

    It's the DPI value in the PNG file - and Photoshop keeps what is in the file.

     

    But for screenshots, it doesn't really matter at all.

     
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  • Currently Being Moderated
    Nov 15, 2009 2:47 PM   in reply to Chris Cox

    I'm having difficulty with that statement.

     

    If I open a 32x32pixel, 72 PPI PNG image in CS4 and save it out to another folder, close the file, and reopen it, the resolution is now altered to 72.009 ! It's not set at the orginal exact value of 72.

     

    When I attempt to reset the resolution using Image Size -> Document Size -> 100%, and Resolution = 72, it still comes back as 72.009 ! And Document Size = 99.99% Not at the 100 % it was initially.

     

    The reason this is a problem for me is that I am attempting to track an application display problem using a PNG file in a ribbon button.

     

    I've become somewhat convinced that the application is rescaling the image, and clipping or not displaying it completely based on some additional information in the PNG file. Even though it's still set at 32x32 pixels.

     
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  • Currently Being Moderated
    Nov 16, 2009 7:00 AM   in reply to plasticAnimal

    RICHARD HENLEY wrote:

     

    ...Even though it's still set at 32x32 pixels.

     

    Even if Chris' statement is off, neither 72.009, 72, nor 72000 ppi measure matters. You still have a 32x32 image.

     

    Screen display graphics should not be meddling with ppi. If you have an application that is being confused by this, fix the application.

     
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  • Currently Being Moderated
    Nov 16, 2009 7:43 AM   in reply to Marian Driscoll

    I think you missed my point.

     

    Additionally I don't have control of the application nor it's interpretation of the PNG file. Possibly it's incorrect in it's behavior, but since it's actually using Windows .NET API's to do the display of the PNG file I tend to think something else may be involved. It would be useful to know what that is.

     

    However my point is that Photoshop is changing some parameters in the re-saved PNG file so the PNG is not interpreted correctly by the application.

     

    If for instance I use on the Windows platform, MS Paint to resave the Photoshop saved PNG, then the file is set back to the 72, and it displays as expected.

     

    As far as I'm concerned Photoshop is altering some parameters in the resaved PNG file, which alters how that image is interpreted by other applications. Who is correct in the display is still at question, but that Photoshop is altering the saved PNG file is quite clear to me.

     
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  • Currently Being Moderated
    Nov 16, 2009 8:16 AM   in reply to plasticAnimal

    It appears that you have not been saving through the 'Save for Web' function under the file menu. Try that and see if your other application is happy.

     

    The 'save for web' function removes resolution (ppi) data from the image. When re-opened in Photoshop, an image without any resolution data is assumed to be 72 ppi.

     

    The PNG spec seems to indicate that it is based on the metric (not inch/Imperial) system of measurement.

     

    http://www.w3.org/TR/PNG-Chunks.html#C.pHYs

     

    What you are seeing is an approximation/rounding. This is done in all applications. Those few applications that display 72ppi are rounding to the nearest whole number.

     

    If you must maintain resolution data by using the regular 'save as' function (not 'save for web'), consider defining pixels per centimeter instead.

     
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  • Currently Being Moderated
    Nov 16, 2009 8:47 AM   in reply to plasticAnimal

    RICHARD HENLEY wrote:

     

    If for instance I use on the Windows platform, MS Paint to resave the Photoshop saved PNG, then the file is set back to the 72, and it displays as expected.

    The reason why you think the file is set back to 72 ppi is because MS Paint is not including any resolution data in the PNG file when it saves. It is stripping it out like 'Save for Web' does in Photoshop.

     

    Photoshop is opening the PNG file without any resolution and assigning the default value of 72.

     
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  • Currently Being Moderated
    Nov 16, 2009 12:25 PM   in reply to Marian Driscoll

    Which only appears to confirm my point that Photoshop is altering the original PNG file data when it's saved.

     

    The original PNG file when opened, starts out as 72 but after saving via PS becomes 72.009 when re-opened.

     

    The data that's suggested shouldn't have any bearing on my problem, but in fact must.

     
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  • Currently Being Moderated
    Nov 16, 2009 12:29 PM   in reply to plasticAnimal

    I think you missed my point.

     

    You have never defined the PNG file at 72.000 ppi because it is not possible to save the file at that resolution. You may think that you have typed that in the image size dialog but internally, Photoshop, or any other image editor, cannot assign that 72ppi value when the resolution in a PNG file is based on the metric system. There is always going to be a rounding/conversion issue because you are trying to put inch/Imperial measurements on a file format that is based on the metric system.

     
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  • Currently Being Moderated
    Nov 16, 2009 12:35 PM   in reply to plasticAnimal

    plasticAnimal wrote:

     

    The original PNG file when opened, starts out as 72

    Again, when an image opens in Photoshop at 72 ppi, it means one of two things.

     

    1) there was no resolution data in the image so Photoshop is assuming 72 ppi

     

    2) the file was defined at 72 ppi

     

    You are most likely dealing with option #1 if you see a PNG file open into Photoshop at exactly 72.000000 ppi since PNG cannot be saved at exactly 72 ppi.

     

    Try opening your image in Bridge or a tool like XNView to see whether resolution data (ppi) was saved with the file.

     
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  • Currently Being Moderated
    Nov 16, 2009 3:06 PM   in reply to Marian Driscoll

    Let me repeat myself, I'm not concerned especially with what or how Photoshop displays with a PNG file.

     

    I am concerned what it writes into the file, which prevents it from being interpreted correctly by another application. That's my only concern.

     

    What Chris appeared to say was that it didn't do so, when from my experience it will alter a file based on it's own parameters. And that is the only item that's of concern to me here.

     

    The rest of these points while interesting and informative don't directly solve my problem.

     

    And then too this 72 default appears to me to be something new. I've never seen Photoshop make an assumption like that before when opening a file, but then I've not been looking for that when usually this is not of much concern unless I'm printing.

     
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  • Currently Being Moderated
    Nov 16, 2009 3:22 PM   in reply to plasticAnimal

    plasticAnimal wrote:

     

    I am concerned what it writes into the file, which prevents it from being interpreted correctly by another application. That's my only concern.

    Then use the 'Save for Web' function instead of 'Save' or 'Save As'.

     

    plasticAnimal wrote:

     

    this is not of much concern unless I'm printing.

     

    And this is why I'm confused why you are bothered by 72.009. You are apparently not printing.

     
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  • Currently Being Moderated
    Nov 16, 2009 3:34 PM   in reply to plasticAnimal

    If another application is confused by the difference between 72.0000 dpi and 72.0009 dpi - that application has some problems.

     

    Photoshop reads what is in the file, and writes back out what is in the file, to the best of the format's capabilities.

     

    72d pi as a default is far from new, that's been in Photoshop since day 1.

     

    Your problem is with another application that interprets PNG files differently (possibly) based on the resolution tag.  That other application is where you should be directing your efforts.

     
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  • Currently Being Moderated
    Nov 16, 2009 6:19 PM   in reply to Chris Cox

    Actually what you say is a bit of a stretch for me.

     

    All I know is that after Photoshop saves the file, the file becomes clipped when the Ribbon control displays it. Why that is, and what in the file that causes it, I don't know yet.

     

    It's also obvious to me that the PNG file I save out of Photoshop is not the same exact file that was read into it, despite what you suggest.

     

    When I have some time to see what is at the bottom of this problem, I will. But you know as well as I do that until I can show exactly where in the file, and why the PNG file is clipped, there's no chance of having the problem addressed.

     

    Especially when a simple solution is to use an application which doesn't cause the problem to occur in the first place.

     
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  • Currently Being Moderated
    Nov 17, 2009 5:56 AM   in reply to plasticAnimal

    plasticAnimal wrote:

     

    It's also obvious to me that the PNG file I save out of Photoshop is not the same exact file that was read into it

    Here's the recipe you seem to be missing from the past discussion...

     

    1) have any PNG file open in Photoshop

     

    2) assign a ppi of 72

     

    3) use the 'save for web' function to save it as file1.png

     

    4) re-open file1.png and notice that its ppi is still 72 in Photoshop

     

    5) use the 'save for web' function again to save that file as file2.png

     

    6) use a binary comparison tool to see that file1.png and file2.png are identical. Photoshop has not altered the image.

     

    7) re-open file2.png and notice that its ppi is still 72 in Photoshop

     

     

    Sure, there's a magic feather in that recipe but it answers your needs.

     
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  • Currently Being Moderated
    Nov 17, 2009 10:34 AM   in reply to Marian Driscoll

    Actually I haven't missed a thing.

     

    I already noted your suggestion earlier AND when I have time to deal with this situation again I will.

     

    Still there's nothing incorrect in what I stated. It's just that I find other applications quicker and easier to use for this given this PS feature with PNG SAVE/AS.

     
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  • Currently Being Moderated
    Nov 17, 2009 3:44 PM   in reply to plasticAnimal

    All I know is that after Photoshop saves the file, the file becomes clipped when the Ribbon control displays it.

     

    Which just means that whatever is displaying that Ribbon Control is handling PNG resolution information incorrectly.

    Photoshop is not the cause of the problem.

    Photoshop just exposes the problem that already existed in whatever you are using to display the Ribbon Control.

     
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