Understanding DPI

Posted on by Larry

[ This article was first published in the February, 2011, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]

 

One of the confusing concepts with using still images for video is understanding the DPI just doesn’t matter. It is the total pixels across by the total pixels down that is important.

However, this still confuses a lot of people. So, when Donald Smith sent in the following explanation, I thought this would be something you might want to read.

Donald Smith writes:

In your article – Still Images, Resolution, and DPI – you talked about DPI and your advice was to ignore DPI for the purposes of video.

 

I questioned what you said in the link above but took you at your word that you were correct, but I have to confess that I really didn’t understand the explanation. That’s because I really didn’t know what DPI was in the first place. Now I do and it validates what you said.

 

Here’s the link to which I’ll be referring: http://www.scantips.com/no72dpi.html

 

This fellow buries the lead but when he said (in so many words) that DPI is simply an instruction to a printer as to how far to space the pixels (dots) on the paper, it suddenly all made sense. Yes, a picture with a high DPI and a copy of it with a low DPI are exactly the same file size, and, if you leave off resampling when you change the DPI in Photoshop, the frame size remains the same. Thus, zooming in the same on both gets the same results.

 

Resampling is Photoshop’s way of changing the frame size to protect the printing output despite a change in DPI. You point out that PS thinks in terms of printing and this is an example of that.

 

I think many of us thought of DPI as sort of extra ‘hidden’ pixels that could be exposed by drilling down (zooming) into a picture. I mean, doesn’t sound like it makes sense that a given dimension of a picture with a high DPI has more pixels packed into that same dimension then a copy with a low DPI? However, it’s like inventing a perpetual motion machine with an electric motor and an electric generator on a common drive shaft where the generator keeps the motor going which keeps the generator going. It sounds logical that once you get it spinning it’ll go on forever. But it won’t.

 

As proof, the author presents the same picture with a high DPI and a copy with a DPI of 7 (yes, seven) but both are the same frame size. On video, they both look and zoom the same because, as you tried to explain, you only have the pixels across by down to display. That’s it. Video ignores the printer instruction (DPI) because it’s not placing dots on paper. Want to zoom in on a picture? Simply scan or create it with a frame size that’s larger than your video format.

 

So, thank you for heroically trying to explain it, but I wanted to let you know that the simple phrase that DPI is just a printer instruction on how to space the dots on paper reveals the man behind the curtain for me.

UPDATE – MAR. 3, 2011

Loren Miller wants to offer a correction.

This tiff (pun intended) will go on for a while, I suppose, but this from you is incorrect:

 

“One of the confusing concepts with using still images for video is understanding the DPI just doesn’t matter. It is the total pixels across by the total pixels down that is important.”

 

It’s more than semantics. Dots Per Inch is indeed part of the analog world of print, but you gloss by the crucial connection to video: DPI is what producers and editors must deal with in getting images into their systems. Without a DPI setting offered by your scanning software, and an effective way to leverage it for specific image formats, you simply can’t scan flat art effectively to support zooms which remain sharp at the close end, or, for that matter, images which are sharp at full frame. No scanner plugin that I know of offers “PPI” as a setting. Scanners deal with DPI, and so must we. That matters very much.

 

DPI is the analog equivalent to video PPI, pixels per inch, a close representation of the analog scan when related to a specific frame size and its pixel requirements. To say that DPI is irrelevant to video in this context is just plain inaccurate.

 

Although your pixel wisdom is of course correct, your approach doesn’t address the real world connection, and needs of producers with tons of analog photos and documents to scan. I developed the ScanGuide Pro system to employ the connection between DPI and PPI for myself, now in use by hundreds of folks in post who are very satisfied with its results. So maybe it’s time to make the proper connection between the two terms rather than thowing the baby out with the bathwater?

Larry replies:

Loren, you are correct. It is wrong to say that DPI doesn’t matter. However, my point, which I will clarify later in this issue, is that regardless of the DPI you set for the file, video looks at the total number of pixels in the image, rather than the DPI number, to determine image size and shape. As you know, further muddying the waters is that computers assume that pixels are square, while video treats them as a never-ending variety of rectangles.

Still, your comments – and your ScanGuide Pro system – are vey useful. And for editors that haven’t discovered ScanGuide, they should check out your website at www.neotrondesign.com.

 


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