Crop tooltip displays incorrect center coordinates when rotating

[leejuyuu]

Describe the bug

When the crop rectangle is rotated with the drag handle, the tooltip displays the center x and y coordinates. The displayed values change when rotating the rectangle, which is incorrect as the rectangle rotates around the center.

To Reproduce

1. Open the crop tool and drag any rectangle.
2. Drag to rotate the rectangle, and a tooltip with "center x: 123, y: 456..." will be displayed. The values will change as the rectangle is rotated.
3. Drag to translate the rotated rectangle. The displayed coordinates values are correct now.

Expected behavior

The center coordinates should be fixed when rotating the rectangle.

Screenshots

nomacs_crop_rotate.mp4

Desktop:

• OS: Arch Linux
• Nomacs version: 3.17.2295 (94e9ada)

[leejuyuu]

@haferburg It's probably unrelated, but would you mind confirming this on your system?

[haferburg]

Yes, it's also buggy on Ubuntu. The angle starts at +-90 degrees, which is odd. It should start at 0 IMO.
When you release the mouse button, the rectangle sometimes snaps, probably to 0 or 90 degrees. But the angle value in the toolbar isn't updated from the snapping.

It was my suspicion that #944 was caused by adding the rotate functionality.

It's a bit unclear to me how rotate is supposed to work, from a design perspective. It's probably: First the (axis-aligned) rectangle is defined, then the rotation is applied. So the (x,y,w,h) coordinates of the rectangle don't change when the rotation changes.
Otherwise you'd have to define the rectangle by the center c_x, c_y, plus a width and height and an angle. But this seems awkward and unintuitive for the basic use case without any rotation.

[leejuyuu]

@haferburg Thanks for confirming! The angle offset is indeed odd. I did not observe any snapping to 0 or 90 degrees.

The implementation seems quite similar to the first case you described, but the rotation changes are somehow stored not as an angle, but as a transformation matrix mixed with the center offsetting translations.