Currently, Clutter does picking by drawing with Cogl and reading
the pixel that's beneath the given point. Since Cogl has a journal
that records drawing operations, and has optimizations to read a
single pixel from a list of rectangle, it would be expected that
we would hit this fast path and not flush the journal while picking.
However, that's not the case: dithering, clipping with scissors, etc,
can all flush the journal, issuing commands to the GPU and making
picking slow. On NVidia-based systems, this glReadPixels() call is
extremely costly.
Introduce geometric picking, and avoid using the Cogl journal entirely.
Do this by introducing a stack of actors in ClutterStage. This stack
is cached, but for now, don't use the cache as much as possible.
The picking routines are still tied to painting.
When projecting the actor vertexes, do it manually and take the modelview
matrix of the framebuffer into account as well.
CPU usage on an Intel i7-7700, tested with two different GPUs/drivers:
| | Intel | Nvidia |
| ------: | --------: | -----: |
| Moving the mouse: |
| Before | 10% | 10% |
| After | 6% | 6% |
| Moving a window: |
| Before | 23% | 81% |
| After | 19% | 40% |
Closes: https://gitlab.gnome.org/GNOME/mutter/issues/154,
https://gitlab.gnome.org/GNOME/mutter/issues/691
Helps significantly with: https://gitlab.gnome.org/GNOME/mutter/issues/283,
https://gitlab.gnome.org/GNOME/mutter/issues/590,
https://gitlab.gnome.org/GNOME/mutter/issues/700
v2: Fix code style issues
Simplify quadrilateral checks
Remove the 0.5f hack
Differentiate axis-aligned rectangles
https://gitlab.gnome.org/GNOME/mutter/merge_requests/189
14c706e51b