Talk:Pixels per inch
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Can anyone explain what this "Pixels per inch" has been used for?
It could also be mentioned, that Pixels per inch has spawned the myth that graphic for the screen should be in 72 dpi, since this is screen's resolution. More about it here: http://www.scantips.com/no72dpi.html Kasper Hviid 12:30, 10 Nov 2004 (UTC)
I'm not sure it's a myth, so much as a misunderstanding of purpose.
The author of the article you linked to appears to have a great deal of misunderstanding about DPI, as well. The article states "There is no concept of dpi on the video screen," which is, strictly speaking, true - DPI in the strictest sense is a metric for printer ink/toner dot size, and nothing more - but the remainder of the article treats DPI as synonymous with pixels per inch. The author states "Dpi means 'pixels per inch'", and then goes on to say "the concept of dpi simply does not exist in the video system", which is a curious contradiction. If pixels-per-inch can't exist in a video system (and pixels certainly don't make any sense outside of a video system), where does that leave us?
Some computer displays actually do display 72 pixels per inch--if you have a monitor that is, say, about 14.2 inches wide (like my 19-inch-diagonal monitor is), and run at a screen resolution of 1024x768, you get about 1024/14.2=72 pixels in a horizontal inch.
I don't know the actual historical reasons for the preponderance of the 72 DPI number. I suspect it was at least partly a guideline intended for graphic designers who were most familiar with the concept of output resolution as a DPI measurement; a designer working for print may have followed the notion "300 DPI looks good on paper," and carried that into the digital realm, where "72 DPI looks good on the computer screen" (again, DPI being synonymous with PPI). It does suffer from a misunderstanding of how DPI works, but let's use a simple example:
Say a photographer has a bunch of photos to publish; he wants to publish them in a book, and on his website. For print, it will be necessary to have a fairly high resolution in order for the images to look good. The photographer says, "I will make sure I use 300 DPI images for the printed photos." This ensures that, say, if the photos need to be printed at 8x8 inches, the image would need to be 2400x2400 pixels. His digital camera would need to be about 5 mexapixels, for optimal quality.
Now, the photographer wants to publish these photos on the web. Being new to this internet thing, he says "OK, I'll just publish these 2400x2400 images on my website." But after uploading them to his website, he realizes that's not a very good idea. "What I need," says the photographer, "is a target DPI for these images so I know when they will look good on the computer screen, instead of being way too big." The photographer gets out a ruler and measures 8 inches across his screen, and finds out that somewhere around 72 pixels for each inch would do the trick, and get the images to fit in an 8x8-inch area on the screen. So he scales them to what he perceives as "72 DPI" by going into Photoshop's "scale image" feature, choosing an 8x8-inch size, and "72 DPI". Photoshop scales the iamge to 576x576 pixels, and voila! The image looks good on the computer screen, without scrolling way off the edge.
Again, I've no idea if this is the way it happened. Probably not; it surely had something to do with the "logical inches" that the author talks about. I don't think the 72 DPI concept is totally useless, but it's definitely no substitute for a proper understanding of the way video display works. While video displays today commonly have more pixels per inch, I don't think it's so wrong to conceive of them as being in the ballpark of 72 DPI.
-- Wapcaplet 22:25, 10 Nov 2004 (UTC)
You are right, the author uses pdi in a wrong way. But his message is hard enough to swallow already! He might lose his audience if he said: "Images for the web does not need to be 72 dpi, and the resolution of an image is not called dpi either!"
About the purpose for ppi ... we could say: "The ppi-value is usefull, when the graphic on the monitor needs to be shown at the exact same size as the printout." That is, if this is the intended use of ppi. It could also be a way for monitor manufacturers to boast about their monitor's horse power.
As I understand it, the reason behind the 72-idea is that some old Mac's had a standard screen, which was 72 ppi. A point is 1/72 of an inch. So a screen with 72 ppi would have 1 point per pixel. This means that a 12 pixel font would be printed at 12 Points.
--Kasper Hviid 18:13, 11 Nov 2004 (UTC)
[edit] Article Issues
1. Mention typical confusion with DPI -- even used commonly in manufacturer descriptions. In the DPI article, disambiguate: DPI refers to printing but often used incorrectly for monitors - for monitors see PPI.
2. Create "pixels per centimeter"/"PPCM" stubs that refers here for more info, have an inches-centimeters conversion chart for 72, 96, 106 PPI, whatever.
3. This article needs to be the place where the whole issue of how big something is on screen -- vs. resolution -- is clarified, with illustrations.
4. Complex issue of font sizing. The original Apple graphic design standard of 72 PPI, which gave 72 point text on screen the actual physical size on screen of one inch (72 point = 1 inch in computer typography) -- "what you saw was what you got." Then the rise of Windows in graphic design, and the shift to the 96 PPI standard. How Web browsers dealt with this.
5. The huge consumer problem of newer monitors with PPI's higher than 96 PPI, and how they think they're getting a "bigger" image -- but in fact there getting an image too small to read. The notorious 1280x1024 on 17" LCD problem, leading people to use non-native lower resolutions, often with distorted aspect ratios, etc.
6. Coordinate issues for cross-reference with the Resolution (computers) or whatever article(s).
