bug#46556: 27.1; transparent images are displayed incorrectly if rotated

[Alan Third]

On Wed, Feb 17, 2021 at 07:05:38PM +0200, Eli Zaretskii wrote:
> > Date: Tue, 16 Feb 2021 21:24:24 +0000
> > From: Alan Third <alan@idiocy.org>
> > Cc: ynyaaa@gmail.com, 46556@debbugs.gnu.org
> > 
> > > Would it be possible for you to test with Emacs 28 and see whether the
> > > problem you report (on Windows) is still present there?
> > 
> > It does sound like something must be going wrong on Windows.
> > Unfortunately I don't know what that could be because, as I said on
> > the other bug report, NS and Windows use the same rotation logic, and
> > NS is fine, so it must be when it comes to actually drawing that the
> > problem manifests.
> 
> I've now stepped through the code which implements rotation, and I see
> nothing wrong with the results.  The pixel coordinates of the rotated
> square are exact and accurate, without any roundoff that I could spot.
> Each square starts exactly 50+8 = 58 pixels after the previous one (8
> pixels are taken by the SPC character between the squares), and ends
> exactly 50 pixels after it starts.
> 
> So I have no idea why the one-pixel shift happens.  Of course, I don't
> really understand what that code does (although I hacked it quite
> extensively), so maybe someone who really understands that stuff could
> take a look and tell what's wrong there.

Can either you or the OP provide a screenshot? It's not entirely clear
to me what's happening. It sounds like some of the behaviour of this
bug would be explained by the mask not being rotated with the image,
but other bits of the description don't seem to match that.

The other bug with the single pixel white line sounds more like an
off-by-one in SVG production, but you'd see that in every image, so
it's probably not that.
-- 
Alan Third