making big GIFs and little GIFs

Discussion in 'Digital Photography' started by Franklin, Feb 7, 2005.

  1. Franklin

    Franklin Guest

    How do I scan a map to a GIF so that when I view the GIF file at its
    "normal" size it is a maginification of the original.

    In other words, what took one inch in the original document now takes
    1.5 inches in the GIF at its normal size.


    OTHER INFO ... I have tried to do this with my Epson Twain settings
    but I seem to end up with a lower resolution (even though the
    resoultion setting is kwpt the same).

    When I have a resizing utilities (like Pic2Pic) on a GIF file I seem
    to get a quite noticeable drop in quality. Is this inherent in
    resizing a GIF? Is it better to use a different image format such as
    TIFF, PNG, JPG in order to enlarge the image after scanning?
    Franklin, Feb 7, 2005
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  2. Franklin

    Jim Townsend Guest

    You have to know the resolution in dots per inch of your monitor.

    To find this resolution, grab a ruler and measure the width of the
    image on your monitor. (Be careful not to scratch it).

    Next find what resolution your video card is set to.

    In Windows, right click on the desktop -> choose 'Properties'
    When the dialog box pops up, click on the 'Settings' tab.

    Here, you'll see your screen resolution listed in pixels. There
    will be a bar you can slide to the left or right to change it.

    From the physical width and resolution in pixels, you can calculate
    how many pixels per inch, PPI (or dots per inch DPI) your monitor

    For example:

    - My laptop screen measures 12 inches across.

    - The screen resolution is 1024 x 768 pixels.

    To calculate the horizontal DPI I divide the physical size by
    the resolution. So... 1024 pixels / 12 inches = 85.33 DPI
    or if we round off, my laptop LCD panel displays 85 DPI.

    This means an image 85 pixels wide will be exactly one inch
    wide on my monitor. An image 85x2 pixels wide (170 pixels)
    will be exactly 2 inches wide.

    You can follow this up to an image 1024 pixels wide.. This will
    fill the full 12 inches of the screen completely. Images of
    over 1024 pixels will scroll off the edge.

    You don't have to scan at this DPI.. Just scale the image
    after the fact. Usually it's better to scan at higher
    resolutions then scale down to fit.

    In my case (with my 85 DPI monitor).. If I scan a 6 inch wide
    document, and I want to see it take up 6 inches of my screen, I
    just have to make sure it's 6" x 85 DPI = 510 pixels across.

    Note that there are so many monitor sizes and video resolutions
    in use, there is no way you can size an image to fit all monitors.

    For example, if someone has a 12 inch wide monitor set to 800 x 600
    pixels rather than 1024 x 768 pixels , then his/her horizontal DPI
    will be 800 pixels / 12 inches = 66 DPI.

    The 510 pixel image that was 6 inches on an 85 DPI screen will
    appear larger.. (510 pixels / 66 DPI) = 7.7 inches on a 66 DPI screen.
    Jim Townsend, Feb 8, 2005
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  3. Franklin

    Wayne Fulton Guest

    You said View, so I am assuming you want to view it on the video
    screen, instead of printing it. The best way (highest quality) to
    enlarge it on the video screen is to scan it again at 50% greater
    resolution. That will create 50% more pixels in each dimension, and so
    it will view on the screen at 50% larger size.

    Or you could resample an existing image to be 50% larger, but yes, that
    will loose quality. If you can scan it, then scan it.

    For the video screen, GIF is NOT the problem, GIF is desiged for the
    video screen. Certainly GIF is not the SIZE problem on the screen.
    GIF is limited to 256 colors, a problem for photographs, but this is
    surely no problem for a map. Probably you could even reduce it to 16
    colors for a smaller file without losing anything (but there are always
    some ifs and buts, I dont know what you have). Or you could use TIF or
    PNG, also very fine if you can use them. I would try hard NOT to use
    JPG for a map, quality suffers.

    The size problem might be because there are no inches in the video
    system, meaning that you cannot work in inches for the video screen.
    Your video screen is instead dimensioned in pixels, for example perhaps
    1024x768 pixels is a common size.

    I dont know the size of your map, so I am making up numbers, but if for
    example, you scan a 6 inch paper map dimension at say 100 dpi, you will
    create 6x100 = 600 pixels of image dimension. This will fill 600
    pixels of your 1024 inch screen dimension. Regardless, more resolution
    is a larger image, less is smaller (on the video screen - printing
    works rather differently).

    Or per your question, if you instead scan this example at 150 dpi (50%
    more) it will create 900 pixels which will fill 900 pixels of your
    screen dimension, and it will appear 50% larger on the screen than the
    100 dpi image.
    Wayne Fulton, Feb 8, 2005
  4. Franklin

    Franklin Guest

    Jim, thank you for a detailed reply. My need is also to **print**
    the GIF (which is of a text document) at normal size as well as to
    view it at normal size on the PC screen.

    Let me explain some more .... My experience is that if I shrink or
    enlarge a GIF then even if the change in size is relatively small
    then the new GIF can be seen on the screen to be of noticeably lower

    In your reply you have explained more about this and told me how to
    predict dimensions and to accommodate dpi settings. But what about

    If I were then to print the GIF image I view on the screen then I
    reckon it would probably print just as badly if not even worse than
    what can be seen.

    So I figure that I should print the GIF at its "normal" size in order
    to avoid any noticeable reduction in quality. As my original post
    here mentions, I have tried to do this by (1) changing Twain settings
    as well as by (2) using resizing utilities. But I have had poor
    results. Maybe I am not using these applications correctly?

    As an alternative approach, perhaps there are image file formats
    which are better suited to resizing operations than GIF is?
    Franklin, Feb 9, 2005
  5. Franklin

    CSM1 Guest

    To add to Wayne's explanation, to print a document at the same size as the
    original size, Print at the same DPI that you scanned it. To retain the DPI
    in the saved image, you need to use TIFF, because GIF does not include the
    DPI that the image was scanned at. JPEG saves the dpi, but does bad things
    to map images.

    If you scan at 300 DPI, Print the image at 300 DPI.
    If you must use GIF, write a text file that tells what DPI you scanned at,
    so that you know what DPI to print.

    Or always scan at a fixed dpi and always print at that same dpi.
    CSM1, Feb 9, 2005
  6. Franklin

    Jim Townsend Guest

    Franklin wrote:

    As CSM1 indicated in another reply.. For printing, just print
    at the same DPI you scanned the image at.

    Scan a one inch wide document at 300 DPI, you get an image
    that's 300 pixels across. Print the 300 pixels at 300 DPI
    and you get a one inch wide image on paper. Print at
    150 DPI and you get a 2 inch image. 600 DPI will give
    you a 1/2 inch image.

    The problem is if you view this 300 pixel image on your monitor
    (that uses 85 pixels to make each inch) it will be 3.5 inches
    wide. (300 pixels / 85 DPI = 3.5 inches).

    Here's where the DPI setting of an image comes into play.
    The DPI of an image has nothing to do with how big it looks
    on a monitor and everything to do with how big it prints
    on paper.

    If you take your 300 pixel image and sample it down to
    85 pixels, then it will appear one inch wide on a monitor
    that uses 85 pixels to make an inch.

    If you set the DPI of this image to 85, then it will print
    at one inch. (Setting the image to 85 DPI is telling the
    printer to spread 85 pixels across each inch of paper.
    Since the image is 85 pixels across, then it will be one inch
    wide. It has to be :)

    So.. Scan at a high resolution. Sample the resulting
    image so the pixels match your monitor's resolution.
    Then, change the DPI of the image file so the image prints
    at the correct width/height.


    By measuring the width of a monitor and dividing the
    inches by the monitor's resolution, you get the
    pixels per inch the monitor displays. Let's
    say it calculates to 80 PPI.

    You have a 6 inch wide document and you want it to be
    6 inches wide on your monitor and you want it to be
    6 inches wide when you print it.

    Scan the document at 300 DPI. This will result in
    a file that's 6 x 300 = 1800 pixels wide.

    Sample the image to fit the screen. Since in this
    example 80 pixels make up an inch of the screen,
    you have to make the image 80 x 6 = 480 pixels

    If you want this 480 pixel image to print on paper
    at 6 inches, it has to be set to 80 DPI. Setting
    it to 80 DPI tells the printer to spread 80 pixels
    across each inch of paper. Since there are 480
    pixels, the resulting print will be 6 inches wide.

    The only downside to doing this is that you will
    have a low resolution print. At 80 DPI, you lose
    a lot of detail.

    Unfortunatley, this is the only way you can have an
    image appear the same size on your monitor AND on
    your printer. If you want to print at high resolution,
    then you'll have to scale the image on your monitor.

    Here are a couple of web sites that explain the
    same thing.. Perhaps a bit better :)
    Twain settings should have nothing to do with how the image looks.
    What software are you using to size the images ?

    Also.. If you don't need transparent backgrounds, there's no real
    advantage to using GIF.. I'd use JPEG. The files use up less
    disk space, and you have a lot more colors (This makes it better for
    displaying photo images).

    GIF can only display a maximum of 256 colors. For photos, this
    results in a 'posterized' effect because color transitions aren't
    smooth. JPEG images give you 16 million colors that result in
    'photo quality' images.
    Jim Townsend, Feb 9, 2005
  7. Franklin

    Bruce Guest


    What you should be aware of and others have sort of mentioned but not
    gone into, is that JPEG and GIF are popular compressed formats designed
    to reduce file size. JPEG compresses pictures by eliminating detail
    that is less important to your eye's impression of a complex continuous
    tone image. It does a wonderful job of reducing file size of photos
    while minimizing visual loss. But it is lossy in that information is
    discarded. GIF is an older format designed for line art and logos. It
    has a limited color pallet but once converted to 256 colors, compression
    is lossless- all image detail is saved. GIF gives best results with
    cartoons, maps, logos, text and the like, (large areas of single color
    compress very efficiently) but does not give much compression where lots
    of fine detail is present (pictures). It also can create posterization
    artifacts in fine shadings due to the limited color depth. JPEG does
    not do as well with line art as loss of fine detail and introduction of
    edge artifacts become apparent as compression increases.

    Thus your map may likely give a sharper, crisper, better image with good
    compression as a GIF, while for a portrait on the web, JPEG would be far
    superior. TIFF is totally lossless (good for archiving), not as
    convenient in browsers, and may or may not be compressed. But
    compression only relates to file size and does not directly have
    anything to do with DPI.

    This is all to explain that GIF vs JPG, TIFF or PNG is an issue of file
    size, compression artifacts and software compatibility, but not the
    answer to your image DPI resizing question.

    I would suggest that you scan the map at 200-300 PPI for normal use and
    maybe up to 600 PPI for enlargements. Then try the options in a good
    (free) utility like IrfanView to resize, print and save as TIF or GIF.
    Resample the image to different pixel dimensions and you will quickly
    see the effect on default screen and print sizes.
    Bruce, Feb 9, 2005
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