https://bugs.kde.org/show_bug.cgi?id=445564
Alvin Wong <al...@alvinhc.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|REPORTED |CONFIRMED
--- Comment #1 from Alvin Wong <al...@alvinhc.com> ---
Can reproduce on a1f3b48. No, I don't think it is strictly limited to high-DPI
scaling as I can also get the canvas to shift when using 100% scaling. Seems to
me that it shifts more the slower you move the cursor while in the rotate
action (which produces smaller angle deltas for each move events).
`snapToDevicePixel` in `KisCoordinatesConverter::correctOffsetToTransformation`
is there to ensure that the top-left corner of the image area always aligns
with the corner of a screen pixel. From my understanding, this is necessary for
the QPainter (non-OpenGL) canvas to draw the pre-scaled projection correctly.
For both QPainter and OpenGL canvas, this also makes sure that when the canvas
is at a right angle and with a canvas scaling of power-of-two (e.g. 50%, 100%,
200%) the image pixels align with screen pixels to produce a crisp display.
The problem is that rotation is handled by delta which causes the rounding
difference to accumulate. This is not even specific to rotation as just panning
the canvas with the touch gesture can also produce an offset. To really
eliminate such offsets, I think the canvas movements need to be handled by
storing the transformation at the start of a gesture, then have the overall
change caused by a gesture applied on top of that. Might need to ask Dmitry
about that.
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