ada5e67f7e
Turns out inverting a matrix was the largest chunk of the CoglMatrix code. By switching to Graphene, a lot of it can go away. The inverse is still cached in the CoglMatrix struct itself, to preserve the optimization. However, switching to graphene_matrix_t to calculate the inverse has a challenge: float precision. We had to work around it here, and it needs an explanation. The way to detect whether a matrix is invertible or not (i.e. whether it's not a "singular" matrix, or not) is by checking if the determinant equals 0. So far, so good. Both graphene_matrix_t and CoglMatrix use single-precision floats to store their 4x4 matrices. Graphene uses vectorized operations to optimize determinant calculation, while Cogl tries to keep track of the matrix type and has special-purpose determinant functions for different matrix types (the most common one being a 3D matrix). Cogl, however, has a fundamentally flawed check for whether the matrix is invertible or not. Have a look: ``` float det; … if (det*det < 1e-25) return FALSE; ``` Notice that 1e-25 is *way* smaller than FLT_EPSILON. This check is fundamentally flawed. "In practice, what does it break?", the reader might ask. Well, in this case, the answer is opposite of that: Cogl inverts matrices that should not be invertible. Let's see an example: the model-view-projection of a 4K monitor. It looks like this: ``` | +0,002693 +0,000000 +0,000000 +0,000000 | | +0,000000 -0,002693 +0,000000 +0,000000 | | +0,000000 +0,000000 +0,002693 +0,000000 | | -5,169809 +2,908017 -5,036834 +1,000000 | ``` The determinant of this matrix is -0.000000019530306557. It evidently is smaller than FLT_EPSILON. In this situation, Cogl would happily calculate the inverse matrix, whereas Graphene (correctly) bails out and thinks it's a singular matrix. This commit works around that by exploiting the maths around it. The basis of it is: inverse(scalar * M) = (1/scalar) * M' which can be extrapolated to: inverse(M) = scalar * inverse(scalar * M) = M' In other words, scaling the to-be-inversed matrix, then scaling the inverse matrix by the same factor, gives us the desired inverse. In this commit, the scale is calculated as 1 / (smallest value in the diagonal). I'm sorry for everyone that has to read through this :( https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1439 |
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| .gitlab/issue_templates | ||
| .gitlab-ci | ||
| clutter | ||
| cogl | ||
| data | ||
| doc | ||
| meson | ||
| po | ||
| src | ||
| subprojects | ||
| tools | ||
| .gitignore | ||
| .gitlab-ci.yml | ||
| config.h.meson | ||
| COPYING | ||
| meson_options.txt | ||
| meson.build | ||
| mutter.doap | ||
| NEWS | ||
| README.md | ||
Mutter
Mutter is a Wayland display server and X11 window manager and compositor library.
When used as a Wayland display server, it runs on top of KMS and libinput. It implements the compositor side of the Wayland core protocol as well as various protocol extensions. It also has functionality related to running X11 applications using Xwayland.
When used on top of Xorg it acts as a X11 window manager and compositing manager.
It contains functionality related to, among other things, window management, window compositing, focus tracking, workspace management, keybindings and monitor configuration.
Internally it uses a fork of Cogl, a hardware acceleration abstraction library used to simplify usage of OpenGL pipelines, as well as a fork af Clutter, a scene graph and user interface toolkit.
Mutter is used by, for example, GNOME Shell, the GNOME core user interface, and by Gala, elementary OS's window manager. It can also be run standalone, using the command "mutter", but just running plain mutter is only intended for debugging purposes.
Contributing
To contribute, open merge requests at https://gitlab.gnome.org/GNOME/mutter.
The coding style used is primarily the GNU flavor of the GNOME coding
style
with some minor additions such as preferring stdint.h types over GLib
fundamental types, and a soft 80 character line limit. However, in general,
look at the file you're editing for inspiration.
Commit messages should follow the GNOME commit message guidelines. We require an URL to either an issue or a merge request in each commit.
License
Mutter is distributed under the terms of the GNU General Public License, version 2 or later. See the COPYING file for detalis.