Re: Problem with NSImage and .PNG file
Re: Problem with NSImage and .PNG file
- Subject: Re: Problem with NSImage and .PNG file
- From: Rainer Brockerhoff <email@hidden>
- Date: Sat, 16 Aug 2008 08:12:51 -0300
At 03:26 -0700 16/08/08, email@hidden wrote:
>From: Matt <email@hidden>
>Date: Fri, 15 Aug 2008 13:54:32 -0700
>Message-ID: <email@hidden>
>Subject: Problem with NSImage and .PNG file
>Message: 4
>
>I have an image that is 7200x32 pixels, which is used to store a series of
>32x32 tiles. When drawing from this NSImage, the further towards the end of
>the image (right edge) the subrect goes, the blurrier it gets drawn.
>...
>I found that the .PNG NSImage was returning a width of 7199.099 pixels,
>instead of 7200. Manually setting the NSImage's width to 7200 before drawing
>fixed this issue and resulted in the correct, non-blurry subrectangles being
>drawn.
>
>Has anyone encountered this issue before? Is this a bug within NSImage's
>handling of PNG's or could this be a problem with my image file itself, a
>DPI issue perhaps?
It's a bug in the image itself, or rather, in the generating software. I have a bug filed (still open, rdar://5532687) which says, in part:
>When saving .png files, Preview 4.0(467) generates incorrect values for the file's DPI.
>...
>See http://www.libpng.org/pub/png/spec/1.1/PNG-Chunks.html#C.pHYs
>The 'pHYs' chunk can specify either "unknown unit" (which makes the file default to 72 dpi when opened) or "meter". Preview now seems to generate the latter specification. As 72 dpi don't fit exactly into metric units, the measurement is rounded up, producing the 72.009 value.
>Solution:
>When saving 72 dpi pictures, specify "unknown unit" in the 'pHYs' chunk.
I haven't checked if that has been fixed in Leopard Preview, though; I suppose not, as the bug's still open.
--
Rainer Brockerhoff <email@hidden>
Belo Horizonte, Brazil
"In the affairs of others even fools are wise
In their own business even sages err."
Weblog: http://www.brockerhoff.net/bb/viewtopic.php
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