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  1. #1
    Super Moderator jscher2000's Avatar
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    Flat panel pixels-per-inch

    I can't seem to come up with a formula to easily calculate pixels-per-inch across a TFT monitor. I view this as important to determining how readable the monitor will be at its native resolution (they gets fuzzy at other resolutions). Here's the background:

    I currently use a 14.1" laptop screen which has a native resolution of 1024x768. Assuming that the pixels are square (or round) rather than rectangular (or elliptical), the screen panel dimensions should match the 4:3 ratio of the displayed image. So I know from the pythagorean theorem that the squares of the width and height of the screen add up to the square of 14.1 inches, and that the height is 0.75 times with width. But Excel doesn't like this because it involves a circular reference; I once used some kind of "iterate" formula with another spreadsheet, but I have no idea whether Excel does that.

    Using the "human approximation" method of trying various numbers with smaller and smaller differences, I can calculate that on my screen, there are about 90.1 pixels per inch across.

    What I wanted to know was, if I buy an 18.1" flat panel that has a native resolution of 1280x1024, will this be more or less pixels per inch across, to get a rough idea of its readability. Using the same human approximation method, I got about 88.5 pixels per inch across, which is a tiny improvement, probably not noticeable, but at least not worse than what I have now. I'd expect a 17" monitor at 1280x1024 to be considerably harder to read, and a 19" at that resolution to be considerably easier.

    And now the question: Can anyone think of a way (or know of a tool) to generate a table of this data so I can narrow down all the different options out there?

  2. #2
    New Lounger
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    Re: Flat panel pixels-per-inch

    I think you are trying to make this more difficult that it actually is.... or I could be missing something. <img src=/S/smile.gif border=0 alt=smile width=15 height=15>

    I can think of at least 3 ways of do this:
    1) Get a tape and measure it. Possibly the only way to confirm that your assumptions regarding the 4:3 ratio is correct.
    2) Algebraically using pythagoras. Excel sheet attached. (It can deal with ratios not exactly 4:3 if you can measure them)
    3) Trigonometrically, i.e. using tan, sin, cos.... I can't remember which, math classes were way too long ago. The assumptions regarding the ratio of width to height are critical to the accuracy here.

    Don't forget that the actual size of the pixels could still be different if the gaps between them differ in size.

    My <img src=/S/2cents.gif border=0 alt=2cents width=15 height=15>
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  3. #3
    Plutonium Lounger Leif's Avatar
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    Re: Flat panel pixels-per-inch

    Interesting.
    I have a 17" monitor with a spec of 0.264mm - which I take to be the pitch of the pixels.
    Measuring the screen (in metric, I'm afraid) I get a width of around 340mm and a height of 270mm. When divided by this 'pitch', I arrive at a figure of 1288 x 1023.
    The maximum resulotion I can set the monitor to is (surprise, surprise) 1280 x 1024, which makes a lot of sense.
    (I actually just tried setting it to that and was pleasantly surprised at just how crystal clear the display was. Unfortunately it is just a tad small for my old eyes.)

    What I think you want to do, is to find either the dpi of the screen or the pixel size and multiply it by your preferred resolution. With my current setup, 1024 x 768, every 4 horizontal pixels are being displayed across 5 dots (1280/1024)and every 3 vertical pixels are being displayed across 4 dots. Or horizontally, only every 5th dot is an accurate representation of what it should be!

    In other words, I may be just as well off with a physically smaller display with a 1:1 resolution simply because it should be a much crisper display!!!

    Does that make sense? I don't think the 'gap' between pixels, or the actual shape of them, matters as much as the actual ratio between resolution and total number of dots. Thus, in an ideal world, if you wanted a resolution of 1024 x 768, and had a pixel size of 0.264mm, the ideal(?) width of the monitor would be 270mm (1024 x 0.264) (13.5" diagonal) or 540mm (1024 x 0.264 x 2) (27" diagonal).

    This, of course, could all be a load of cobblers, but it may be food for thought.

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    WS Lounge VIP rory's Avatar
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    Re: Flat panel pixels-per-inch

    Speaking of a load of cobblers, this is some important information regarding pixels...
    Regards,
    Rory

    Microsoft MVP - Excel

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    Super Moderator jscher2000's Avatar
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    Re: Flat panel pixels-per-inch

    Wow, clearly you were one of those who stayed awake in math class. <img src=/S/clever.gif border=0 alt=clever width=15 height=15>

    I know that the results are approximate, but if I get a metal object near an LCD screen, I'm almost certainly going to scratch it and have to buy it, so this is a much safer way to guesstimate. <img src=/S/grin.gif border=0 alt=grin width=15 height=15> (This reminds me of a story of carpenters of a certain ethnic group who measure wood with strings - the kind made of cotton, not some kind of advanced theoretical physics materials - but I digress.)

    Anyway, I borrowed your formulae and extrapolate several additional columns to cover all of Dell's current offerings, as well as my laptop, for reference. I had to depart from the named ranges to do this, but they were really helpful for me "getting" the concept, so I left Column B that way.

    Thanks for your help.
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    Super Moderator jscher2000's Avatar
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    Re: Flat panel pixels-per-inch

    Leif, I cross-compared the pixel pitch method, and it's very close to the method I was using originally, as refined by John. It's certainly close enough for in-store use (with the calculator in the cell phone). All I have to remember is that there are 25.4 mm per inch!

    The problem you have with not using the native resolution is the one I'm trying to avoid. It seems, perversely, that 17" screens are the most painful to use: they should have left the native resolution at 1024x768 and they would be a joy. Maybe there's a special market for that: screens you can read. Coming soon to a retailer near you (we can only hope).

  7. #7
    5 Star Lounger
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    Re: Flat panel pixels-per-inch

    <img src=/S/rofl.gif border=0 alt=rofl width=15 height=15> ...and some important information regarding other things such as...

    99. The average human eats eight spiders in their lifetime at night.

  8. #8
    Plutonium Lounger
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    Re: Flat panel pixels-per-inch

    I bet it's a lot less than 8 if you exclude the Australians <img src=/S/smile.gif border=0 alt=smile width=15 height=15>

    StuartR

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