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Thread: Pixel aspect ratio, DPI and Image Size

  1. #1
    Lord&Master of Space&time Steve_Bennett's Avatar
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    Default Pixel aspect ratio, DPI and Image Size

    Alright, It's time I wrapped my brain stem around this. Can someone please explain how best to go about rendering an image to fill a page that is 24x36?

    It would seem that one would need to render an image that is 10800x7200 to acheive 300 DPI when using the Print Size Wizard for a sheet 36" wide x 24" tall. Something tells me this is overkill and that I could get away with a smaller image size. What would affect the output on a printer that I could use to cheat and have a smaller image? Would saved image format affect output on the printer, say tiff vs jpg? Could I get away with an image size of half the 10800x7200 and still have decent printed quality without jagged lines? I suppose the type of printer being used could affect output quality, but how would I make that determination?
    Steve Bennett |CAD Manager
    Moffatt & Nichol | Adventures in BIM

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    Design Visualization Moderator stusic's Avatar
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    Default Re: Pixel aspect ratio, DPI and Image Size

    Quote Originally Posted by Steve_Bennett
    Alright, It's time I wrapped my brain stem around this. Can someone please explain how best to go about rendering an image to fill a page that is 24x36?

    It would seem that one would need to render an image that is 10800x7200 to acheive 300 DPI when using the Print Size Wizard for a sheet 36" wide x 24" tall.
    That is exactly right; at 300dpi (the culprit of your dilemma), you would need a 10800x7200 image. But that is at the best quality; you don't need the absolute best, just very good.

    To get a proper printed image size of 24" x 36" (actually 24.13" x 35.87" by these resolutions; those are the aspect ratios of photos, but the idea is the same. Adjust accordingly), lower your dpi. Below will give you an idea of what you're facing:

    24" x 36" @ 300 DPI:
    7200x10800

    24" x 36" @ 200 DPI:
    4800x7200

    24" x 36" @ 150 DPI (note: pictures will look fuzzy):
    3600x5400

    I don't print very often, but when I do, I do it at 200dpi. If you're right on top of it, studying it, you may notice slight fuzz; but if you're more than 12-18 inches away from it, it'll look just fine.

    Quote Originally Posted by Steve_Bennett
    Something tells me this is overkill and that I could get away with a smaller image size. What would affect the output on a printer that I could use to cheat and have a smaller image?
    I wouldn't advise going with a smaller image size; go with a lower resolution. If you go with a smaller size then stretch it, the resulting image won't be as nearly as good as a correctly-sized, lower dpi image. The algorithms that dictate how the pixels look in the new size (your photo-editing program) aren't as accurate as the way the render program will calculate it.


    Quote Originally Posted by Steve_Bennett
    Would saved image format affect output on the printer, say tiff vs jpg?
    TIF formats have the best quality; JPEG formats have the best compression/compatibility. Use the TIF (or better yet, PDF) for your printing.

    Quote Originally Posted by Steve_Bennett
    Could I get away with an image size of half the 10800x7200 and still have decent printed quality without jagged lines?
    Not that low - that's 150dpi, and you'll get poor quality. Try 200dpi.

    Quote Originally Posted by Steve_Bennett
    I suppose the type of printer being used could affect output quality, but how would I make that determination?
    Go to a reputable, professional printing company.


    Hope this helps!
    Last edited by stusic; 2007-04-12 at 05:35 PM.
    Phillip Bradshaw
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    I could stop if I wanted to de-co1's Avatar
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    Default Re: Pixel aspect ratio, DPI and Image Size

    I agree with all stusic has explained.

    One thing I do in order to speed up rendering, is in fact to go for the 150 DPI option. In some cases, when we render an architectural image to be added to a photograph, then we blur the rendering slightly in order to match the photographic quality, as nothing in a photo is ever perfectly crisp.

    So the loss in quality at the resolution is negligible. Do that often enough, and people will comment on the crispness of an image, and in my case, the architect didn't like that - looked too unnatural. Eventually I started rendering everything at 150 DPI.

    Except of course for print, which had to be 300, or in some cases, even 600 DPI.

    However, doing this doesn't ring true for product design, and I suppose other industries, where quality is paramount.

    Just a tip from an architectural point of view.
    David


    "Artists to my mind are the real architects of change, and not the political legislators who implement change after the fact."

    William S Burroughs

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    I could stop if I wanted to dellis's Avatar
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    Default Re: Pixel aspect ratio, DPI and Image Size

    Its been my experience that the bigger the image gets printed at, the more you can get away with lower DPI. I'm talking about prints larger then a D size sheet. Anything thats going into a magazine render it at 300dpi or higher...better to have the detail and not need it then need it and not have it. There is just no substitute for quality. Anything larger then letter and i would say you can go lower. Personally i would never go less then 200dpi for the print size you're talking about.

    Having said that there are some programs out there that can do some fantastical stuff with image resizing and all that. So you could render lower and then scale it up in post.

    Steve, are you trying to avoid long render times? There are alot of ways to get around the rendering time thing if this is what is detering you from rendering out gigantic images.
    Last edited by dellis; 2007-04-13 at 02:45 PM.

  5. #5
    Design Visualization Moderator stusic's Avatar
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    Default Re: Pixel aspect ratio, DPI and Image Size

    Quote Originally Posted by dellis
    ...are you trying to avoid long render times? There are alot of ways to get around the rendering time thing if this is what is detering you from rendering out gigantic images.
    Aren't we all trying to avoid longer render times?
    Phillip Bradshaw
    Lead Pixel Pusher, Lucid Design Studio
    CAD Administrator, Heatcraft RPD

    A doctor can bury his mistakes but an architect can only advise his clients to plant vines. - Frank Lloyd Wright

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    I could stop if I wanted to dellis's Avatar
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    Default Re: Pixel aspect ratio, DPI and Image Size

    Yeah...i used to dread the long render times but distributed rendering has changed that though. Now i just have avoid my co-workers while i'm hijacking there workstations for rendering.

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    Lord&Master of Space&time Steve_Bennett's Avatar
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    Default Re: Pixel aspect ratio, DPI and Image Size

    Thanks for the additional input guys! Try to cut render times down is only part of what I was after. Having someone try to render a 10kx7k image with the default scanline render engine on a Pentium D chip isn't the greatest idea either. I have no problem letting a scene render for 12 hours - provided I really need that quality and can justify that. Having a better understanding of DPI and print outputs is what I was really after and I definitely have that now!

    Steve Bennett |CAD Manager
    Moffatt & Nichol | Adventures in BIM

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