Rotating Canvas

Being able to rotate the canvas while working on a drawing is a highly desirable feature. Unfortunately, most of Inkscape's code assumes a non-rotating canvas.

Basic Strategy

The Inkscape SPDesktop class's Geom::Affine _w2d variable defines the Window to Desktop coordinate transformation (and _d2w defines the inverse). In 0.92.x _w2d is limited to scaling. Rotating the canvas can be added to this variable. It might also be possible to add translation (to handle scrolling) as well.

Steps

• Adapt current zooming code to work if _w2d includes a rotation.
  • New function: SPDesktop::set_display_area() that takes as input:
    • The current transformation (_w2d).
    • A point on the drawing and a point on the screen which are to be aligned.
  • All basic zooming functions can be handled as special cases of this new function.
  • Update/replace all instances of set_display_area() to use new version.
  • Replace all instances of get_display_area() to handle rotated frame.
• Add rotation functions that follows the zoom functions pattern.
• Add GUI to set rotation.
  • Menu items with keyboard shortcuts.
  • On-screen editing tools using a class derived from SPCanvasItem.
  • Fix problems (see below).

Problems

• The tools (Rectangle, Path, etc.) seem to already work when _w2d includes a rotation.
• On screen elements need to be transformed.
  • Guides (already shown at correct angles but ends are in wrong places.
  • Grids
  • Bounding boxes
  • Page outline
• Scrolling needs to be adjusted (it should work using the center of the drawing).