bug#46554: 27.1; 180 degree rotated image is displayed in slightly different position

[Eli Zaretskii]

Ouch, I responded to the wrong bug report.  Redirecting.

> Date: Wed, 17 Feb 2021 19:26:38 +0000
> From: Alan Third <alan@idiocy.org>
> Cc: larsi@gnus.org, ynyaaa@gmail.com, 46556@debbugs.gnu.org
> 
> > 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.

I attach a screenshot, note the 3rd square from the left, where the
red square line seems to be 1 pixel off to the right and down of the
black background.

> 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.

That's the bug I was talking about.  I didn't look closely at
bug#46556, as it talks about transparency, something I have no idea
how it works and indeed whether it's at all supported on MS-Windows.

squares.PNG
Description: PNG image