ABC's of DPI on TV

Many video people still believe that DPI has anything to do with video. When creating content for the screen, you should only ever be concerned with the PIXEL size of an image, not its DPI. After all, how many DPI is your logo when you see it on the seven-foot wide image on the wall from your video projector? Or how about on the portable 3" LCD WatchMan TV? Both have the same image size (CCIR-601 defines NTSC video as 720x486 PIXELS), but they have drastically different "DPI"s (in this example, about 8.5 PPI vs. 240 PPI)*

When you scan a logo to create a 1/4 screen video title, you need’nt worry about what DPI the scanner is set to, as long as the pixel size gives you an image that fits within 360 pixels wide and 240 high. If the logo is on the client's business card, that roughly translates to a scan at 500 DPI or more (for example, given a 3/4" logo in the corner of the card, 360 pixels / .75" = 480 DPI). If you scan the same logo off the 8x10 glossy from your client’s press kit, the DPI you need will be very different, perhaps as low as 50 DPI ( 360 pixels / 8" = 45 DPI). The resultant image is still the 360 pixel wide logo for the 1/4 screen title that you wanted all along.

Incidentally, that quarter-screen logo scan is only 253KB uncompressed (360 pixels x 240 pixels x 3 bytes per pixel / 1024 bytes per K = 253KB), whether 480 DPI or 45 DPI. Sometimes graphics designers accustomed to print media will specify the file size of an image instead of it’s DPI or pixel size. As file formats and compression ratios vary, this too can be a little misleading. Uncompressed file size however can give you a slightly better feel for an image’s size and quality than a raw DPI number.

Video, our final output medium, has no relevant real-world measure. I.E., there is no such thing as an inch on screen, since you never know the size of screen on which your video will appear. Without a real-world inch measure, how many “Dots Per…” do you make a graphic? What does a 300 DPI image vs. a 72 DPI image mean, if you don’t know how many inches it is? If you do the math, you see that a 300DPI image that is 1 inch across is exactly the same as a 72DPI image that is 4.16 inches across. And they're both small files - 300 DPI doesn't automatically mean big files. So you see, from just stating that one is 300 and the other is 72 DPI - without any other information - you really can’t know anything.

DPI is a secondary unit of measure, and can only be derived from a primary one: inches. On its own, it means nothing. Print materials are created at a known real-world size. No matter what printer or press or publisher, an inch is an inch, so you always know how big your work will be, and DPI becomes a valid measure of an image's potential detail. Video, however, has no fixed inch. Thus, DPI is meaningless. Instead, talk in terms of pixels and everyone around the world immediately knows exactly how big - on ANY screen and relative to that screen - your graphic will look.

*Note, the "PPI" above is not a typo either. It stands for Pixels Per Inch. Dots and pixels are not truly interchangeable terms. A 'dot' requires there to be whitespace around it to be distinguishable from surrounding dots. Also, in printing dots are typically the individual color droplets of ink or spots of toner. Many drops of several different colors are required to mix together to form a single pixel of an image. Similarly, a 'line', the standard for measuring video image quality, must have space to set it off from the next line over. Otherwise you don't have two lines; you have one thick line, giving the appearance of lower quality and detail. A pixel, on the other hand, is a single defined region of a given color, with a grid system to differentiate it from adjacent pixels. No whitespace is needed there. Video, IF inches were relevant, whould be more properly measured in PPI, not DPI.

DPI is a term brought over from the print industry, where it has served them quite well. For those of us creating imagery that will be used in both print and video (or web), it is imperative that we know the difference between the mediums, and how to scan, create and scale material in order to get the desired results on both. But the application of the term DPI to video is erroneous and confusing.

I have met countless students, beginning and advanced video folks and graphic designers who have been misled by the term DPI. They've mistakenly believed that everything created at 300 DPI is too big, that it automatically means files are too large, and that 72 DPI is the magic answer to life’s graphical mysteries. They create material that is ill suited to their purpose, and wonder why sometimes images look blocky and other times the computer bogs down loading them. Hopefully this text can help clear up this problem in our communication, so we can all speak, write and create a little more clearly and accurately and get the quality results we truly want in our profession.

Chris Rakoczy