Re: Preview and "Respect Image DPI for Actual Size"
Re: Preview and "Respect Image DPI for Actual Size"
- Subject: Re: Preview and "Respect Image DPI for Actual Size"
- From: John Stiles <email@hidden>
- Date: Wed, 7 Dec 2005 10:17:49 -0800
On Dec 7, 2005, at 3:43 AM, Heinrich Giesen wrote:
On 07.12.2005, at 02:30, John Stiles wrote:
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.
This does not avoid scaling. Assuming the screen resolution is 72 dpi
the image is not scaled if [theRep size.width]==[theRep pixeldWide]
(same with height). If you see jaggies due to scaling then your
imageRep
has not a resolution of 72 dpi. Check it out:
- (float) dpiX { return 72.0*[theRep pixelsWide]/[theRep
size].width; }
- (float) dpiY { return 72.0*[theRep pixelsHigh]/[theRep
size].height; }
OK, using QuickTime exporters, do you know how generate a JPG or PNG
where the "pixelsWide" will equal the "size"? I don't know how to
influence those criteria with the tools at my disposal.
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.
In PNG-images the resolution is measured in dots per meter, which
leads
to "rounding errors": 72 dots per inch = 72/2.54 * 100 ) dots per
meter
(= 2834.646 dots per meter).
Oh, interesting.
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