ClutterActor is a particularly heavy user of matrices, and
switching to graphene_matrix_* APIs means we had to change
the order of operations due to left-hand vs right-hand
differences.
When applying the actor transform, there are 2 main branches
that can be followed: the default transforms, and when a
custom transform is set.
To facilitate review, here's the table that I've made to
guide myself:
+--------------- Case 1: Default Transforms --------------+
| CoglMatrix | graphene_matrix_t |
+----------------------------+----------------------------+
| multiply (child transform) | translate (-pivot) |
| translate (allocation)¹ | rotate_x (angle) |
| translate (pivot)¹ | rotate_y (angle) |
| translate (translation)¹ | rotate_z (angle) |
| scale (sx, sy, sz) | scale (sx, sy, sz) |
| rotate_z (angle) | translate (translation)¹ |
| rotate_y (angle) | translate (pivot)¹ |
| rotate_x (angle) | translate (allocation)¹ |
| translate (-pivot) | multiply (child transform) |
+----------------------------+----------------------------+
¹ - these 3 translations are simplified as a single call
to translate(allocation + pivot + translation)
+---------------- Case 2: Custom Transform ---------------+
| CoglMatrix | graphene_matrix_t |
+----------------------------+----------------------------+
| multiply (child transform) | translate (-pivot) |
| translate (allocation)² | multiply (transform) |
| translate (pivot)² | translate (pivot)² |
| multiply (transform) | translate (allocation)² |
| translate (-pivot) | multiply (child transform) |
+----------------------------+----------------------------+
² - likewise, these 2 translations are simplified as a
single call to translate(allocation + pivot)
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1439
2406f225a9