Preview and "Respect Image DPI for Actual Size"
Preview and "Respect Image DPI for Actual Size"
- Subject: Preview and "Respect Image DPI for Actual Size"
- From: John Stiles <email@hidden>
- Date: Tue, 6 Dec 2005 16:39:21 -0800
Our app is writing out JPEG and PNG files, using QuickTime. We expect
that the user will be viewing these images with the built-in
Preview.app. We would like these images to appear unscaled on the
user's monitor, since the scaling leaves jaggy artifacts which look
bad. To that end, we have been trying to set the image's DPI to match
the screen DPI. Traditionally, Mac OS has been a 72 DPI system; I'm
not sure what Preview is doing, but 72 DPI doesn't seem to be working.
Fortunately, it's easy to get QuickTime to set the DPI on our files
to whatever we want with "GraphicsExportSetResolution." (QuickTime's
default, if you don't set anything yourself, appears to be 72 DPI.)
However, at 72 DPI, Preview scales the image signficantly larger than
it started. Next, we tried explicitly setting 96 DPI; this appears to
be close to correct, but I'm still getting slightly chunky edges. At
this point, I'm stumped.
We also noticed that PNGs were reporting oddball sizes like "72.007
DPI" in Preview's Get Info panel. I don't know why that might be;
we're definitely passing "72 << 16" for our DPI size. JPEGs didn't
have this problem.
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