Re: OS X Preview -- wrong size
Re: OS X Preview -- wrong size
- Subject: Re: OS X Preview -- wrong size
- From: Robert Clair <email@hidden>
- Date: Thu, 24 Feb 2005 15:48:17 -0500
This is by way of explaining why things may appear to be the
wrong size on the screen.
Quartz uses default units of 1/72 of an inch. They also implicitly
assume the display is 72 dpi. To be precise: if the developer doesn't
scale a view
and asks Quartz to draw a 72 x 72 square, it will draw a 72 pixel by 72
pixel
square. This will have an actual (measured with a physical ruler) size
of about
3/4 inch on many modern displays. The easiest way to see this is to take
Photoshop), set the view to Print Size, turn on the screen rulers and
compare
them with a physical ruler.
Therefore - if you ask for a pixel match ("Actual Pixels") you will get
actual
pixels, if you set the view scaling to "Print Size" you will most
likely get something
on screen that is smaller than the physical print size.
This can be fixed when writing the app by determining the monitor DPI
and
scaling the view internally. Most apps don't do this. (Mine does :-) ).
The DPI can be determined by asking the user to figure it out and enter
it
as a preference or by using the Quartz API:
CGDisplayErr GetDisplayDPI(
CFDictionaryRef displayModeDict,
CGDirectDisplayID displayID,
double *horizontalDPI, double *verticalDPI )
This was broken on 10.2 and idiotically returned 72 no matter what, but
it
appears to have been fixed on 10.3. (This assumes you have a monitor
smart enough to report this - I'm don't know when such monitors became
reasonably common.)
......Bob Clair
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