resampling a digital image to make it bigger?

Discussion in 'Digital Cameras' started by Mr.Will, Oct 22, 2003.

  1. Mr.Will

    Mr.Will Guest

    Recently I was asked to print several of my shots at a 10 x 8 size, and to
    crop quite heavily (about a quarter of the original frame). I took this
    image with a Canon d60, and was concerned about noise and pixellation etc.
    The day after that I saw the same photograph on the cover of a UK paper
    blown up even bigger - and without pixellation or any noise! I was surprised
    to say the least.

    I had always thought that all images should be printed at 300dpi - so if one
    wanted a bigger image than the frame, a loss in DPI would be neccessary. Not
    so according to the people at the press, they merely resampled it in
    photoshop.

    I tried this instantly with the picture I had been asked for, and indeed it
    worked out very nicely at 10 x 8. I simply resampled and kept the resolution
    at 300dpi. What I dont understand is, what has the computer and photoshop
    done to actually create more "pixels" for want of a better term?
     
    Mr.Will, Oct 22, 2003
    #1
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  2. Mr.Will

    Bob Hatch Guest

    Yes, and you can print at 250 PPI and you'll never see the difference
    between that and 300. Our pro lab wants all images at size at 250 PPI. I
    print at 250 PPI on my Epson 7600.

    You know by now that your computer did not blow up and by using these extra
    pixels you have not created a warp in the fabric of the universe. :)
     
    Bob Hatch, Oct 22, 2003
    #2
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  3. To create new pixels, PS invents them. But it does so in an intelligent manner.
    Its bicubic resampling algorithm looks at every pixel as well its nearest
    neighbor and their nearest neighbors......etc.
    It then creates new pixels that that blend best with all the other pixels in
    the neighborhood.
    This helps to make the images look smooth rather than pixelated at high
    enlargements.

    The 300 pixels/inch (ppi) criterion is for making prints indistinguishable from
    traditional wet chemical prints.
    High quality Art Magazines may also request 300 dpi ( really ppi).
    I usually try to print at 240 uninterpolated ppi. If I don't have enough native
    pixels in my image to print at 240 ppi, I'll have PS invent them. At other
    times I'll just print at 200 ppi and get very good results
    Newspapers are such low resolution media, that they can get by with 100 ppi, or
    less.
    Bob Williams
     
    Robert E. Williams, Oct 22, 2003
    #3
  4. To me it would seem that doing it many small step would be bad for exactly
    the same reason photocopying photocopies of photcopies is bad.

    Things don't get better as you interpolate on top of interpolation.
     
    Fraser Wright, Oct 22, 2003
    #4
  5. SNIP
    That would intuitively seem what to expect, degradation with each next
    generation. However, it's different with Bi-Cubic interpolation.
    In a way they don't. However, when you interpolate/resize to a larger
    dimension, Photoshop can add a little edge contrast at sharp brightness
    transitions, depending on the surrounding pixels. Typically, edges (which
    are most important for our impression of sharpness) will thus gain some
    contrast. When repeated, this results in (visually) better (but slightly
    exaggerated) edge contrast and less pixellation with extreme enlargements.
    It offers a benefit over one step resampling which averages the edge
    transitions but exaggerates individual pixel edges.

    Bart
     
    Bart van der Wolf, Oct 22, 2003
    #5
  6. AFAIK, Lanczos interpolation is considered to be the best available
    interpolation method by those who are supposed to know about such things.
    Photoshop doesn't do Lanczos, but ImageMagick (which is free) does. I'd be
    interested to see a comparison of the two sometime, and to know whether
    the difference in quality between the algorithms is visible.

    Mike.
     
    Mike Brodbelt, Oct 22, 2003
    #6
  7. Make a Photoshop Action to do it for you!


    Open an image you want res up.

    Open the Actions palette and select "New Set".

    Name it "Creeping Increase" or whatever you like.

    Now select New Action and call it "1.5 Increase"

    Next go to Image/Size and put a check in "Resample" and "Constrain
    Proportions"

    with Bicubic and change the width drop down to "percent" and enter 110.

    Now go to "Image Size" do the same 3 more times.

    Then "Stop Recording"



    Next "New Action" Name it "2.0 Increase"

    Now "Edit/Purge All"

    Next select the "1.5 Increase" in the Actions Palette

    and press the Play button.

    select the "1.5 Increase" again,

    press the Play button again.

    Stop Recording.



    "New Action" Name it 3.0 Increase.

    "Edit/Purge All"

    Select and play the "2.0 Increase"

    Select and play the "1.5 Increase"

    Stop Recording.



    "New Action" Name it 4.0 Increase.

    "Edit/Purge All"

    Select and play the "3.0 Increase"

    Now "Image Size" 110 percent.

    And again "Image Size" 110 percent.

    Stop Recording.



    Save the Action.
     
    /\\BratMan/\\, Oct 22, 2003
    #7
  8. Stanley Krute, Oct 22, 2003
    #8
  9. Hi Bill
    Do you have a link to a free download of Stair Interpolation ?

    Thanks.

    Stan
     
    Stanley Krute, Oct 22, 2003
    #9
  10. Mr.Will

    Tore Lund Guest

    While we're at it, does anyone have strong opinions on what is the best
    program or plug-in for downsampling? Bicubic seems fine, and some other
    algorithms are close contenders. But I wonder if there is a clear
    winner somewhere. And can step-wise sampling help if you make images
    smaller?
     
    Tore Lund, Oct 22, 2003
    #10
  11. Mr.Will

    JPS Guest

    In message <oSolb.12342$>,
    Maybe the math used in Photoshop loses some dynamic range with radical
    size changes. I know the math is non-optimal in other operations; 360
    one-degree rotations will not get you back where you started, for
    instance (you'd expect the image to be blurry, but not rotated at the
    end).
    --
     
    JPS, Oct 22, 2003
    #11
  12. Bart van der Wolf, Oct 22, 2003
    #12
  13. I don't really consider Cibachrome (now called Ilfochrome) prints "traditional".
    You have to really look around for someone who will process Cibachrome, nowadays.
    I don't even see many Cibachromes showing up in competition anymore.
    I used to be impressed by Cibachrome 25 years ago, but now its super shiny,
    in-your-face, plastic look is a bit tiring.
    Of course, that's only my opinion. However, if you like that look, a 300ppi dye
    sublimation print on Hi-Gloss paper will fool any but the most critical viewer.
    See what this Ex-Cibachromer has to say about Cibachrome vs Fuji Crystal Archive
    (traditional silver halide prints).
    http://www.erwinvoogt.com/order.html
    Also, see what this Cibachrome Guru says about Cibachrome vs Inkjet. See paragraph
    entitled, "Inkjet vs Light Jet."
    http://luminous-landscape.com/tutorials/handmade.shtml
    Bob Williams
     
    Robert E. Williams, Oct 23, 2003
    #13
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