Re: display-mm-width return value off on Windows

[Jason Rumney <jasonr@gnu.org>]

Lennart Borgman <lennart.borgman.073@student.lu.se> writes:

> Is there any reason to believe the result should be correct? As I
> pointed out in a previous message the information does not seem to be
> available to the display driver.

Built in LCDs in notebooks and LCD panels connected via digital
connectors are able to report details about the monitor back to the
video drivers/OS, though I am not sure whether manufacturers always
report accurate information, or whether Windows uses the information.

Monitors connected via analog connectors are not able to report this
information, and Windows seems to always guess 320 as the width, and
the height is reported as consistent with the width:height ratio
assuming square pixels.

I have tried the following:

1024x768 built in 15.1" notebook LCD reports 320x240
1440x900 built in 14.1" widescreen notebook LCD reports 320x200
1280x1024 analog connected 19" monitor reports 320x256

But using LOGPIXELSX/LOGPIXELSY is not going to be any more accurate
than what monitors report, or Windows makes up, since it is based on a
dpi setting that is user configurable, defaulting to 96dpi, and AFAIK
for screen devices square pixels are assumed (LOGPIXELSX==LOGPIXELSY),
so if your monitor has non-square pixels and reports a size to the
OS, then that size (or the aspect ratio associated with it) is more
likely to be accurate than a calculation involving LOGPIXELSX/LOGPIXELSY.