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This implements the update cache described in T95401.
The cache is currently only used for drawing strokes and
sculpting (using the push brush).
**Note: Making use of the cache throughout grease pencil will
have to be done incrementally in other patches. **
The update cache stores what elements have changed in the
original data-block since the last time the eval object
was updated. Additionally, the update cache can store multiple
updates to the data and minimizes the number of elements
that need to be copied.
Elements can be tagged using `BKE_gpencil_tag_full_update` and
`BKE_gpencil_tag_light_update`. A full update means that the element
itself will be copied but also all of the content inside. E.g. when a
layer is tagged for a full update, the layer, all the frames inside the
layer and all the strokes inside the frames will be copied.
A light update means that only the properties of the element are copied
without any of the content. E.g. if a layer is tagged with a light
update, it will copy the layer name, opacity, transform, etc.
When the update cache is in use (e.g. elements have been tagged) then
the depsgraph will not trigger a copy-on-write, but an update-on-write.
This means that the update cache will be used to determine what elements
have changed and then only those elements will be copied over to the
eval object.
If the update cache is empty or the data block was tagged with a full
update, we always fall back to a copy-on-write.
Currently, the update cache is only used by the active depsgraph. This
is because we need to free the update cache after an update-on-write so
it's reset and we need to make sure it is not freed or read by other
depsgraphs.
Co-authored-by: @yann-lty
This patch was contributed by The SPA Studios.
Reviewed By: sergey, antoniov, #dependency_graph, pepeland, mendio
Maniphest Tasks: T95401
Differential Revision: https://developer.blender.org/D13984
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