mirror of
https://github.com/brl/mutter.git
synced 2024-11-23 08:30:42 -05:00
40305f53f0
This one was overlooked when we migrated the wiki to the repo, so let's add it before we delete the wiki. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3485>
46 lines
2.3 KiB
Markdown
46 lines
2.3 KiB
Markdown
# Clutter Rendering Model
|
|
|
|
Clutter renders the stage and actors in a 3D space. Most of the time, all the scene is composed of only 2D actors. Remember that common operations, like rotating on the Z axis, and scaling and translating on the X or Y axis, do not make actors 3D, which allows Clutter to optimize rendering for 2D.
|
|
|
|
## Camera
|
|
|
|
ClutterStage builds the view matrix (i.e. the matrix that transforms world coordinates into camera coordinates) assuming that the camera is placed at (0, 0, 0) with a normal (0, 0, -1). That means the camera is pointing *down*:
|
|
|
|
|
|
|
|
The camera is implicit, and as of now, hardcoded.
|
|
|
|
## Perspective
|
|
|
|
When setting up the projection, ClutterStage uses a traditional perspective projection, with the addition of a "2D plane". The 2D plane (`z-2d`) is a plane in the Z axis that all the stage will be rendered into. The perspective projection is build with the following parameters:
|
|
|
|
* **field of view Y**: 60º (hardcoded)
|
|
* **aspect**: width / height (depends on monitor configuration)
|
|
* **z-near**: 1.0 (hardcoded)
|
|
* **z-2d**: `z-near + z-near * 49,36` ( = 50,36)
|
|
* **z-far**: `z-2d + z-2d * tan(30°) * 20` ( = 631,97)
|
|
|
|
|
|
|
|
## Culling
|
|
|
|
Figuring out what **not** to draw is an important optimization of the rendering process. Clutter relies on clip frusta to detect which actors it can skip drawing.
|
|
|
|
### Depth Culling
|
|
|
|
The z-near and z-far values above are used to build the clip frusta, which culls out actors based on their position. If they're below than z-far, or above z-near, they are not rendered:
|
|
|
|
|
|
|
|
### Clip Regions
|
|
|
|
Clutter supports defining which regions of the 2D screen changed. Suppose you hover a button; only the rectangle that that button cover is redrawn, instead of the entire screen. For example:
|
|
|
|
|
|
*GNOME Shell with 3 clip regions (green)*
|
|
|
|
This is translated to the 3D scene by using multiple clip frusta. Each frustum is a slice of the view cone, and only those actors and geometry and intersects it is rendered. If an actor doesn't touch any of the frusta, it is skipped when drawing.
|
|
|
|
|
|
*View cone with 3 clip frusta (green)*
|