>DPI in Video: Totally Useless

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DPI = dots per inch is only relevant when you output digital images to an output device, such as a printer (or a display,) since DPI is a measure of how many of pixels the output device will show on a line that is one inch long.

That means that the DPI measurement is totally useless as long as you aren’t printing. Video and things like DVD menus have a fixed size in pixels (called resolution) and DPI has no relevance. As long as you create the document with the correct size in pixels, you can set the DPI to 1 or 3,000, it doesn’t make any difference.

To demonstrate this, open Photoshop with any picture and choose the command “Image Size…” from the “Image” menu and uncheck “Resample Image”. You can then easily see that if you change the “Resolution” in pixels/inch, the actual pixel dimensions aren’t altered, only the printed width and height are changed!

If you want to measure the DPI of a DVD menu, you must play the DVD and measure with a ruler on the TV set itself. You’ll get a much higher DPI resolution on a small portable player than on a big hunkin’ 60-inch plasma display, since you you are pushing the same amount of pixels to displays of different sizes.

The pixel resolution in square pixels (which is the preferred format when working in for example Photoshop) for NTSC Widescreen is 864×486 pixels, for NTSC 4:3 it’s 720×540, for PAL Widescreen 1,024×576 and for PAL 4:3 it’s 768×576. Don’t forget the action-safe and title-safe either!

PS: The correct term is actually PPI (pixels/inch) for displays. The term “dots” comes from the raster dots that are traditionally used in printing presses.

PS. The only time the DPI/PPI is relevant is when importing/exporting graphics, like this Flash bug.

- Jonas

5 Responses to >DPI in Video: Totally Useless

  1. Anonymous

    >Im going to risk getting this totally wrong, but I think you are partially incorrect, or, at least, not telling the whole story wrt importing media.

    For example, an 720×486 image created at 300 DPI (not re-sized, generated as such) is much much larger than an 720×486 image created at the nominal screen resolution of 72 DPI which is typically standard.

    Your computer has to process a bit more than 3 times the number of pixels in the 300 DPI file than the 72 DPI file. This makes a huge difference in apps like FCP and AE for rendering and working, as they are processing more pixels.

    While you are correct that, wrt to CRT screens/Plasma/LCD displays physical sizes spread all (for NTSC) 720×486 pixels across various dimensions (the ‘DPI’ for a 60″ plasma is lower than a 20″ old CRT since they display the same number of physical pixels) The thing is, the pixels are actually quite a bit larger, which is why the ‘best viewing distance’ for larger sets is further away from the tube.

    Suffice it to say, 72DPI is the standard screen resolution for importing graphic assets into video pipelines (you can see this in Photoshop CS3 if you create a file based on a film/video preset).

    If you think working at 3000 DPI wont effect anything, er, well, Id have to disagree, no pipelines/workflow I know works like that.

  2. Anonymous

    >er. Nevermind all that. Im an asshole! I got it wrong :)

    The 300 DPI image would print ~3 times smaller, but still CONTAINS 720×486 PIXELS

    Oh well :) Thanks. Doh!

  3. Jonas Hummelstrand

    >Dear Anonymous,

    No problem! :-)

    I’m shortly going to post an illustration that clarifies the DPI/PPI confusion.

  4. Anonymous

    >It’s funny, I’ve had a client ask for stills from a sequence in 300dpi. I just exported the image from AFX and gave it to them. They called back and told me it’s wrong, they need it 300dpi for print and 72dpi just won’t do. So I converted the image to 300dpi in Photoshop, without resampling, and sent it again. The client was happy with that. I don’t get it. It’s exactly the same image!

  5. Jonas Hummelstrand

    >If you are sure that you unchecked the “Resample Image” option in Photoshop’s “Image Size” dialog box, then I guess they are using a page layout program that respects the DPI information in the file when it places the image on the page, and that they are unaware that they can accomplish the same thing by scaling the image inside the layout program.

    I really need to post that illustration I’ve been working on, I hope it explains the confusion.