Loss of quality when scaling image down.

Discussion in 'Photoshop' started by Jason, Mar 15, 2006.

  1. Jason

    Jason Guest

    Hi,

    We had some photos taken for our web site, unfortunately the dimensions are
    way to big for the web and when the imagea are scaled down the loss of
    quality & clarity is very evident. Is there a technique to preserve as much
    as possible the quality of an image when scaling it down to a much smaller
    size.

    Thanks, Jason.
     
    Jason, Mar 15, 2006
    #1
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  2. Hi Jason,

    Welcome to the wonderous land of JPEG (or so I presume).
    JPEG is reknowned for its artifacting habit.

    When you take pictures, or even save freshly created creations in
    Photoshop to a heavy compressed JPEG format image, there's bound to be
    ugly artifacting in the picture. That's why it is called a lossy
    compression. It throws away detail to get it compressed to smaller
    files.

    If you have, however, shot the pictures in a less compressed manner
    (see the manual of your camera for the settings) you have higher
    quality pictures with less artifacting. This will make big files for
    your photographs though. It's a weighing decision, have more pictures
    with less quality on your cam/memory card, or less photographs but with
    a higher quality.

    But to get to your problem at hand, those pictures you scaled down...
    Are they of bad quality within Photoshop already before you ever saved
    them ? And this is with the assumption you do use Photoshop to resize
    them. S-Spline Pro is even better for resizing photographs but that's a
    commercial add-on on top of Photoshop, so it gets more expensive :)

    Mind you, when resizing within Photoshop, notice the title bar of the
    photographs. There's a percentage there. Usually when you open a
    larger-than-your-screen-resolution image in photoshop, PS resizes it
    realtime to a percentage so that the whole image fits within the
    interface. That could have you looking at a preview resizing to say
    66%. That looks VERY rough and bad. When you choose the menu
    View->Actual size/Pixels then it won't fit fully into the window, but
    you are in fact looking at the actual pixels of the picture. When you
    have resized, or before resizing choose this, then you're sure you're
    looking at the real thing instead of some rough preview-quality
    realtime resize of PS.

    Hope you have some information to go on. If it's not enough, please reply.

    With kind regards,

    The PS user.
     
    Photoshop user, Mar 15, 2006
    #2
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  3. Jason

    2 Guest

    It is also a bit more of a problem than JPEG behavior; downsampling a great
    deal can destructive regardless of the final format.

    Like the OP, I look forward to some answers, too. I've a number of MF
    (6x6cm) and LF(4x5") negative scans to downsample to a good web presentation
    and I am very unhappy with the way it has gone.

    There might be a math trick - possibly downsampling to a certain frequency
    that fits the typical monitor (which I take to be 96 or 120ppi). I've tried
    the 10% iteration technique without adequate results.

    It seems (and I hope I am wrong) that the negatives have to be rescanned all
    over at a modest frequency/resolution.
     
    2, Mar 15, 2006
    #3
  4. Jason

    Bill Hilton Guest

    Jason writes ...
    Are you talking about high rez film scans or digital capture images?
    Should be no problem with digital photos from most 6 - 11 Mpixel
    digital cameras I've used ... here are some jpegs of shots my wife took
    a few weeks ago in Tanzania with an 8 Mpix camera, we just run USM once
    at 300/0.3/0 (per Canon's recommendation), do any tonal corrections
    required and then do Image - Image Size with bicubic sharper ...
    http://members.aol.com/bhilton665/tanzania_2006/carol_lions_2.htm

    With really large film scans sometimes we have reduce in steps but
    usually big steps, say 50%, but this is for 6x7 cm scans at 4,000 dpi.

    Bill
     
    Bill Hilton, Mar 15, 2006
    #4
  5. Jason

    Bill Hilton Guest

    2 writes ...
    I took an advanced Photoshop and LightJet printing class from Bill
    Atkinson a few years back and he was showing us how he takes 11,000 x
    11,000 pixel files (Tango drum scans at 5,000 dpi of 6x6 cm Velvia) and
    reducing them to small thumbs, I think 96x96 pixels or so ... he
    basically downsampled in 50% steps using bicubic, running a very light
    USM step on the 2nd or 3rd iteration on some images (depended on the
    data structure). His thumbs looked fine to me, you can find his web
    page and see for yourself. This was his method before Photoshop came
    out with 'bicubic smoother', which works better at downsampling than
    bicubic, so I dunno what he's doing now (though he's switched to
    digital so no doubt has a different flow than with film).

    Here's an interesting page showing problems with aliasing artifacts
    from downsampling by Bart van der Wolf ...
    http://www.xs4all.nl/~bvdwolf/main/foto/down_sample/down_sample.htm ...
    I downloaded his test pattern and if you downsample it in Photoshop
    with one step it does alias but I think (it's been a couple of years)
    if you downsample 50% and then downsample a second time to the target
    value using 'bicubic sharper' then it looked very good. At any rate
    his test pattern is a boundary condition so if you can get it to
    downsample smoothly you should be OK with your large format film scans.

    Bill
     
    Bill Hilton, Mar 15, 2006
    #5
  6. Hi there '2',

    You should check out, just to make sure, this following website :
    http://www.s-spline.com/
    They use splines to resize images, which is a hefty mathematical
    procedure which imo is pretty darn good. Check out the demo and if you
    want to, reply here when you've tested it what you think about the
    results it creates ?

    With kind regards,

    PS user
     
    Photoshop user, Mar 15, 2006
    #6
  7. Jason

    reboot Guest

    Very helpful! I've got his sampling image, and will do some new trials.

    Thanks, Bill (and Atkinson)!
     
    reboot, Mar 15, 2006
    #7
  8. Jason

    reboot Guest

    :( I tried it on one of the problematic images: greyscale TIFF, 10301x6250
    pixels and the program bombed out, crashed, kaput.
     
    reboot, Mar 15, 2006
    #8
  9. Jason

    Bill Hilton Guest

    when the imagea are scaled down the loss
    In my posts I was presuming the problems are caused by downsampling,
    not jpeg compression, but I could be wrong. To the original poster,
    use 'save for web' and choose the '2 - up' tab to see your original vs
    the compressed jpeg version ... I typically try to get 40 - 60 KByte
    file sizes (so download times are reasonable for dial-up viewers) with
    50% 'quality' setting unless the image has been sharpened too much.
    You can see it on the screen so if you have to go with a higher setting
    I'd do that rather than settle for poor image quality in the jpegs.
    Typically I don't have problems with artifacts or over-compression, I
    feel. If the image has been aggressively sharpened then the file sizes
    are often much larger.

    Bill
     
    Bill Hilton, Mar 15, 2006
    #9
  10. Jason

    Bill Hilton Guest

    I wrote ..
    My mistake, it's 'bicubic sharper' that works better at downsampling
    .... 'smoother' is better at upsampling ...
    At least I got it right here :)

    Bill
     
    Bill Hilton, Mar 15, 2006
    #10
  11. LOL, no, that's not what you want from a commercial tool :)
     
    Photoshop user, Mar 15, 2006
    #11
  12. Jason

    Jason Guest

    Yes, I did notice it was ' bicubic sharper', I'll try it tomorrow when I
    can get my hands on a Mac. I am hopeful.
     
    Jason, Mar 15, 2006
    #12
  13. IrfanView will give you great results.
    HTH Richard
     
    Richard Oliver, Mar 15, 2006
    #13
  14. SNIP
    You just beat me to posting it ;-)

    The risk of potential down-sampling artifacts is usually
    underestimated, especially if one needs to sharpen a bit afterwards.
    The trick is to remove all detail in the original that exceeds the
    resolution potential of the output size.

    For those who think the above page is too theoretical, I've also
    applied some of the methods to a filmscan, and it confirms the
    predictions:
    <http://www.xs4all.nl/~bvdwolf/main/foto/down_sample/example1.htm>
    Only high quality down-sampling will avoid potential 'disasters',
    like grain-aliasing and regular pattern aliasing, and edge
    stair-stepping, and...

    Bart
     
    Bart van der Wolf, Mar 15, 2006
    #14
  15. Jason

    DP_Pro Guest

    The unfortunate truth is that when you downsample, as well as upsample,
    you will lose detail. This has nothing to do with file format, but has
    everything to do with how many pixels are in your image, and how you
    arrived at that many pixels.

    In the case of web thumbnails, sharpening is the only solution to end
    up with a better looking thumbnail. I have had some luck making them
    look better by pre-sharpening before I scale them down using unsharp
    mask, a very strong sharpening value, and a pixel factor of 3 to 8.
    The object of which is to force the edges of objects out so that when
    the image is downsampled there is more obvious divisions between
    objects in the image. It also helps to expand the tonal contrast using
    an auto levels or something similar before reducing. The more levels
    you have when you start sharpening the more levels the computer has to
    work with when it is averaging the pixels down. Theoretically this
    could not be a bad thing.

    Keep in mind that when you are resampling, you are increasing or
    decreasing the number of pixels in your image by using interpolation.
    This means that if you reduced an image to 25% of its original size,
    then every 4 pixels will be averaged into 1 pixel. The relative
    brightness of all the color channels in each pixel will be averaged and
    flattened into one color brightness for each color. This shrinks the
    image, and makes it blurry because everything is being averaged. (Of
    course, the same thing happens when you enlarge an image.. 400%
    enlargement takes an average of a pixel, and based on the eight pixels
    that surround it, averages to create 4 pixels from the single pixel you
    started with.

    Thumbnails are representative images, and so my philosophy is to just
    go ahead and sharpen until they give the viewer some idea what the
    thumb represents. Other posts point to different sharpening techniques
    (multiple scales, bilinear, etc.) and these are the accepted norm when
    dealing with this issue. Sometimes it helps to visualize what is going
    on when to scale these images, however.
     
    DP_Pro, Mar 16, 2006
    #15
  16. Jason

    Paul Furman Guest

    I think it should be sharpened after reducing. The only real problem is
    sharpening increases contrast so repeated sharpening gives a washed out
    look with a more gritty black & white feel, less soft colors.
     
    Paul Furman, Mar 16, 2006
    #16
  17. Jason

    Paul Furman Guest

    Irfanview is real handy but does not give the best results. I use it for
    speed and convenience.
     
    Paul Furman, Mar 16, 2006
    #17
  18. Jason

    Paul Furman Guest

    I got a batch action recommended in here that does something similar,
    several steps, light sharpening for each. I think it does better than
    one step and since it's a batch, it's easy to use. I customized it to
    suit my sizes and create 640 pixel image and a 100 pixel thumbnail for
    each also and it puts them in a reserved folder where I go to retrieve them.

    You can download what I've got (will take some editing) here:
    <http://www.edgehill.net/1/?SC=go.php&DIR=Misc/photography/raw-batch/web-batch-action>
    click on: "paul-resize-for-web.atn"

    move up the folders for more explanation & other options...
     
    Paul Furman, Mar 16, 2006
    #18
  19. Jason

    PacMan Guest

    need more info to evaluate:

    1 ) what's the original: size, file format and PPI
    2) what's the final JPG size you want. 3) how much JPG compression is
    there in the original. Open it up in photoshop, Zoom to 500%, Do you
    see lots of squarish patterns? Do you see more noise?


    Without knowing these 3 concerns, everyone is just crap guessing. But
    it could be right. but it's lucky.
    If you can't answer these then you'll never really know the problem.

    I'll take a real wild guess that they are JPG compressed at HIGH setting.
    You'll have to save them as tif's. Go into each RGB channel and filter
    out any JPG patterns on the channel.
    Reduce the size first then the resolution ( in case they are above 72 PPI)

    Apply an unsharp mask only after you have done everything. It's always last.
    Also double up the contast with a curves adjustement or even a second
    "hard light" layer set to 50% or less.


    finally call the client and get better size resolution. if you can get
    300PPI , it will go better. Reduce to 72 PP1 after size reduction.
    you could even keep it at 120 PPi or 100 PP1 if too blurry :)


    Good Luck
     
    PacMan, Mar 19, 2006
    #19
  20. Jason

    z2m Guest

    try to use Photoshop CS2 "bicubic sharper" algorithm of scaling images down.

    Paul Furman napisał(a):
     
    z2m, Mar 19, 2006
    #20
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