Re: how to convert metric to pixels

Eric Sosman <esosman@ieee-dot-org.invalid>
Tue, 30 Oct 2007 22:05:43 -0400
RussellT wrote:

"Eric Sosman" <esosman@ieee-dot-org.invalid> wrote in message

Joshua Cranmer wrote:

Patricia Shanahan wrote:

I'm typing this on a laptop with an external display connected, in
extended desktop mode, and the composition window placed so that it lies
across the boundary between the displays. The two displays have
different characteristics. What is the pixel size for this window?

On most Windows computers, the answer will be 96 dpi: the value is
controlled by a setting in Display Properties->Settings->Advanced, and
very few people actually change it.

    The setting has little to do with physical reality. I've
just measured, and on the plain-vanilla monitor I'm using right
this minute, the resolutions are 76.2+ and 71.4+ dpi, horizontally
and vertically (30 dots/cm and 28.1+ dots/cm). Windows calls it
"96 dpi" -- high by one-third -- and ignores orientation.

I tried your method, a little bit rougher, but still only 0.6% error, on my

     The main point to take away from all this is that the O.P.'s
question is pretty much unanswerable. The computer feeds the
video card, the video card pumps signals over a cable, and then
the display device determines the physical size. Sometimes the
display unit can communicate some information about that size
back to the computer (if the computer is paying attention), but
other times it cannot. For example, consider displaying through
a projector onto a reflective screen: the projector-to-screen
distance influences the physical size of the displayed image.
Slide the projector farther from or closer to the screen to
enlarge or shrink the picture -- how is the computer supposed
to keep track of that sort of thing?

Eric Sosman

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