Quote:
Originally Posted by jswinden
But even for LCDs, the dpi seems to be a calculation based on the Pythagorean theorem which takes into account the number of vert. and horz. pixels (dots, points, whatever) in relation to the diagonal screen size to give us a single dpi amount. Is it highly accurate? Probably not. Does it make a lot of sense? No. Does much in marketing make a lot of sense? No. Is marketing filled with lots of BS? Yes!
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It SEEMS that way, because the way you're doing it makes it appear to work, but it doesn't:
800^2 + 600^2 = 1000^2 is meaningful if 800, 600, and 1000 are some kind of length units. mm, inches, whatever, because there is 1 inch per inch in any direction you care to measure. There are EXACTLY the same number of millimeters per inch measured vertically as there are measured at 45 degrees.
But that doesn't work when you're talking about pixels with a fixed position in 2D space. Even if we assume pixels that are the same size h & v (square or circle, etc, which I think is probably correct in most cases) there are a DIFFERENT number per inch vertically than there are diagonally,
by at least a factor of SQRT(2) assuming h=v.
Think of this way: regardless of any overlap or anti-aliasing technology or fuzzy pixels, or whatever eink may have rather than nice clean dots, I think we all agree that dot density is consistent all over the display. There are just as many dots per capital W at the upper left corner as in the middle, or anywhere else on the page, yes?
For the DPI figure to be right H, V and diagonally, which theorem would claim it is, there would need to be MORE dots filling that diagonal space than the H and V space, resulting in some complex Moire pattern of variable dot density. Why would anyone design such a thing?
The simplest explanation is that the addressable spaces are nominally the same size vertically as horizontally, therefore one dpi value is nominally correct for both directions, and makes a useful single figure to compare densities.
Diagonal measurements are the same, or they were when every TV and monitor in the country had a 4:3 aspect ratio, in that you could report one diagonal size number, and buyers could compare that one value across sets. It's less straightforward now with more widely varied aspect ratios available on modern portable devices, but, as you say, it's marketing.
Thanks, now I'm going to be up all night with my K3, a microscope and a ruler.
ApK