Quote:
Originally Posted by DuckieTigger
The way you calculate makes a little bit more sense since you base your calculations on length. If you try to "calculate" PPI over the diagonal you are in for a surprise - Pythagoras will fail you. Imagine a square display (10x10 pixel) and you draw a diagonal through it. According to Pythagoras you would get about 14.14 pixel (10 x sqrt(2)), but yet if you do it on grid paper it still is only 10 pixels. It just doesn't make sense to say that there is 14 pixel (my example) or 1000 pixel (dsmid calculation) diagonally. I am pretty sure that dsmid is aware of his flaw in calling it 1000 pixel per diagonal, but it does give correct results trying to estimate horizontal and vertical PPI. The only reason PPI is only one number any more is because nowadays pixel are usually square.
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Your a bit confused. Here is the actual formula used to calculate diagonal ppi:
SQRT(horizontal pixels^2 + vertical pixels^2) / screen diagonal distance in inches = ppi
SQRT(600px^2 + 800px^2) / 6" diagonal distance = 166.66666667 ppi
SQRT(360,000 + 640,000) / 6" diagonal distance = 166.66666667 ppi
SQRT(1,000,000) / 6" diagonal distance = 166.66666667 ppi
1,000 / 6 = 167 ppi
ETA: Pixels are not necessarily square. Look at the specs for various devices and you see that the horizontal pixel dimension is often different from the vertical pixel dimension. This part of the equation SQRT(600px^2 + 800px^2) calculates how many total pixels could theoretically stretch across the entire diagonal length of the screen. Since we want to determine ppi (pixels per inch) and since the screen is 6" across the diagonal measurement, we divide SQRT(600px^2 + 800px^2) by 6 to get 167 ppi.