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Tom19a
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document sizes and jpeg compression / quality

Sep 20, 2012 11:44 PM

Tags: #jpeg #document #sizes #compression

In relation to Photoshop, in the status bar at the bottom it displays what it calls “document sizes”.  Would someone be able to clarify can this be used to determin the quality of a jpeg file ?

 

For example if I open up a jpeg with no compression (file size on disk is 4.57mb) it displays Doc:34.5M/34.5M however if I open the same file with compression set at 5 (file size on disk is 748kb) and ‘document size’ doesn’t change.  How does the document size relate to jpeg compression etc...?

 

Thank you

453 Views   9 Replies   Latest reply: c.pfaffenbichler, Sep 21, 2012 12:59 AM
 
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  • c.pfaffenbichler
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    1. c.pfaffenbichler,
    Sep 20, 2012 11:51 PM   in reply to Tom19a
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    How does the document size relate to jpeg compression etc...?

    jpg is a destructive compression so the relation of uncompressed and compressed size is difficult to assess save to simply apply it and let it do its stuff.

     

    If you want to determine the settings jpgs have been saved with maybe the Script Paul Riggott provided at

    http://forums.adobe.com/message/2986855

    might help.

     

    And as so often the Documentation or Help might have provided clues … quote:

    Document Sizes Displays information on the amount of data in the image. The number on the left represents the printing size

    of the image—approximately the size of the saved, flattened file in Adobe Photoshop format. The number on the right

    indicates the file’s approximate size including layers and channels.

     
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  • Tom19a
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    2. Tom19a,
    Sep 21, 2012 12:01 AM   in reply to c.pfaffenbichler
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    thanks for your reply, so just to confirm you are saying that there is no real relation between the document size and quality of the image?

     

    the document sizes are not the 'uncompressed jpeg' size?

     
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  • c.pfaffenbichler
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    3. c.pfaffenbichler,
    Sep 21, 2012 12:05 AM   in reply to Tom19a
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    you are saying that there is no real relation between the document size and quality of the image?

    I am not saying that.

    Naturally there is a relation but the resulting size is also dependent on the image content, so different images of the same pixel dimensions can produce very different compressed sizes with the same jpg-settings.

     

    the document sizes are not the 'uncompressed jpeg' size?

    Once again:

    The number on the left represents the printing size of the image—approximately the size of the saved, flattened file in Adobe Photoshop format.

     
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  • Tom19a
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    4. Tom19a,
    Sep 21, 2012 12:32 AM   in reply to c.pfaffenbichler
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    I'm using photoshop 5.1.  I want to confirm the steps I am doing, perhaps it would be interesting to know what other people see?

     

    • I have opened up a jpeg file with no compression and saved it with jpeg compression of 1

    • I then reopend up the uncompressed file and saved it with a jpeg compression of 5

    • When I look at all three files seperatly, the doc at the bottom says Doc: 34.5M / 34.5M.

     

    I havn't resized the image or changed anything else and when I look at the actual file sizes on the disk they are different (as expected).  In addition as expected when I zoom you can see the quality is different on each file due to the distructive nature of jpeg compression.

     

    From your previous post however you are suggesting then that there is a relation between the Doc size and the quality, why then does the Doc size not change for each file then?  - just to confirm I'm not talking about the file size on the disk, I'm refering to the "Document Size" reported by Photoshop in the status bar.

     
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  • c.pfaffenbichler
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    5. c.pfaffenbichler,
    Sep 21, 2012 12:36 AM   in reply to Tom19a
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    From your previous post however you are suggesting then that there is a relation between the Doc size and the quality, why then does the Doc size not change for each file then?

    The (left) Document Size in the Statur Bar refers to uncompressed data which is determined by pixel dimensions, color mode, bit depth, not the compression used to mangle the image.

    Once again:

    The number on the left represents the printing size of the image—approximately the size of the saved, flattened file in Adobe Photoshop format.

     
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  • Tom19a
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    6. Tom19a,
    Sep 21, 2012 12:44 AM   in reply to c.pfaffenbichler
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    The (left) Document Size in the Statur Bar refers to uncompressed data which is determined by pixel dimensions, color mode, bit depth, not the compression used to mangle the image.

    Once again:

    Thank you, I understand now - that makes sense why it is not changing.  That does mean when comparing images that are the same where only a different jpeg compression setting is used then there is no relation to the document size reported (in the status bar) and the quality.

     

    Do you know what the formula is used to calculate this size?

     
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  • c.pfaffenbichler
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    7. c.pfaffenbichler,
    Sep 21, 2012 12:48 AM   in reply to Tom19a
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    The effect of compression can vary significantly for images of different content (this also includes non-destructive compression).

    So the »formula« you seem to be looking for might not make a lot of sense.

     
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  • gener7
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    8. gener7,
    Sep 21, 2012 1:09 AM   in reply to Tom19a
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    Open a file, say a tiff or psd, zoom in close to the image, about 400% on a recognizable detail. Use the Save As... command, select JPEG, and click Save.

     

    When  you get to the dialog box, run the Quality slider to 0 and observe what happens to the pixels. It's like they are clumped together in large blocks. 

     

    That's how it saves on disk space when it is written back to the file.

     

    477k/477k is the uncompressed size/size in ram and 31.5k is the saved to file compressed size caused by clumping all those pixels together. Of course you trash the file that way,but that's where compression saves space. Not by reducing the pixel count,but by consolidating them.

     

    jpeg compression.png

     

    My point is is that you adjust that slider by eye and from there decide what optimal quality number is worth the space saved.

    With broadband connections and terabyte drives, I would not see any point to less than 12 quality compression these days.

     
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  • c.pfaffenbichler
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    9. c.pfaffenbichler,
    Sep 21, 2012 12:59 AM   in reply to Tom19a
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    To get a handle on compression effects I would recommend a simple test:

    Create an image that is white with one or a couple of black square/s and save a tif with LZW compression.

    Then duplicate the image, and apply the Filter Add Noise and save that as a tif with LZW compression, too.

    One of the files on disc will likely be much smaller than the other, because its content compresses »better«.

     
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