Re: A question about resolution (screen and DPI)

From: Hactar (ebenONE_at_tampabay.ARE-ARE.com.unmunge)
Date: 07/13/03 

Date: Sun, 13 Jul 2003 18:21:26 GMT

In article <pan.2003.07.13.15.37.14.823859@peoplepc.com>,
Gunter Wambaugh <gunterwambaugh@peoplepc.com> wrote:
> Can someone explain to me (in laymen's terms) the connection between screen
> resolution and DPI.

Screen resolution (I am not sure "resolution" is the correct term), e.g.
1024x768, is the number of pixels (screen dots, "picture elements") visible
at any given time. It is given as <screen width>x<screen height>. Pixels
are generally square; IOW, something that's 10 pixels by 10 pixels will
generally appear to be square.

If you switch to a higher screen resolution (say 1152x864) then a feature
which was 128 pixels square is still 128 pixels square. But instead of
being 1/8 screen square, it is now 1/9 screen square (because you now have
more dots on the screen). As a side effect, instead of being
128*12.4/1024=1.55" wide, it's now 128*13/1152=1.38" wide (using my screen
width of about 12.4" as an example). So that's why at higher resolutions,
some features appear smaller.

DPI (dots per inch) is calculated as the number of pixels (dots) in a certain
distance, divided by the length of that distance (in inches). For my screen
at 1024x768, that would be 1024 dots / 12.4 inches = 82.6 dpi. Since for most
resolutions, pixels are approximately square, the vertical resolution is
about the same (768/8.5=90.3 dpi). Maybe I mis-measured at least one of
them. I was using a dial caliper calibrated in cm, help against the glass,
so it's possible.

Notice the horizontal:vertical ratio of most resolutions (640x480, 800x600,
1024x768, etc.) is 4:3 (as is non-widescreen TV), and 1280x1024 is 5:4. It
is the only commonly-used screen resolution that I know of that isn't 4:3.
That's why your dpi changed from being X and Y the same (75x75) to different
(90x96). Your imaging area is still the same, but you have to squeeze the
vertical more to fit 1024 lines in. Features that were square under 1024x768
won't be square under 1280x1024 (they'll look squashed vertically), and vice
versa. A 4:3 resolution is 1280x960 .

> I know that I can force X to a certain DPI, but is that a good thing?

The only way X can _really_ change your DPI is to throw more dots on the
screen, or to cram the same amount of dots into a smaller area. I believe
what it actually does with the DPI either you supply or it calculates is to
supply it to programs in order for them to e.g. calculate how big (in
pixels) an 18-point font should be. ICBW; IANAXGuru.

> If so how do I add that to my XF86Config file.

from XF86Config(5x)

DisplaySize width height
       This optional entry gives the width and height, in millimetres, of
       the picture area of the monitor. If given this is used to calculate
       the horizontal and vertical pitch (DPI) of the screen.

It depends on what you're starting with. mm=in*25.4 so
dots per mm = DPI/25.4 . In a hope (in vain?) that some things would not be
so blasted small, I have this in mine:

        DisplaySize 390 292 # mm chosen to give 100dpi at 1536x1152
                            # (it's really about 120 dpi)

I still have Opera at 180% default zoom.

Higher resolutions are good if you increase the font sizes -- this trn has
39 lines of a 25-pixel font. This gives smoother curves than I'd get at a
lower resolution. 

-- 
-eben    ebQenW1@EtaRmpTabYayU.rIr.OcoPm    home.tampabay.rr.com/hactar
              
      He who will not reason is a bigot; he who cannot is a fool;
        and he who dares not is a slave.  -Sir William Drummond

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