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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
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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.
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## Camera
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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*:
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The camera is implicit, and as of now, hardcoded.
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## Perspective
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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:
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* **field of view Y**: 60º (hardcoded)
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* **aspect**: width / height (depends on monitor configuration)
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* **z-near**: 1.0 (hardcoded)
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* **z-2d**: `z-near + z-near * 49,36` ( = 50,36)
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* **z-far**: `z-2d + z-2d * tan(30°) * 20` ( = 631,97)
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## Culling
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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.
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### Depth Culling
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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:
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### Clip Regions
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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:
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*GNOME Shell with 3 clip regions (green)*
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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.
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*View cone with 3 clip frusta (green)*
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