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This is used to set the compositing operator for all cairo drawing operations.
The default operator is
The operators marked as unbounded modify their destination even outside of the mask layer (that is, their effect is not bound by the mask layer). However, their effect can still be limited by way of clipping.
To keep things simple, the operator descriptions here document the behavior for when both source and destination are either fully transparent or fully opaque. The actual implementation works for translucent layers too. For a more detailed explanation of the effects of each operator, including the mathematical definitions, see http://cairographics.org/operators/.
Clear destination layer (bounded)
Replace destination layer (bounded)
Draw source layer on top of destination layer (bounded)
Draw source where there was destination content (unbounded)
Draw source where there was no destination content (unbounded)
Draw source on top of destination content and only there
Ignore the source
Draw destination on top of source
Leave destination only where there was source content (unbounded)
Leave destination only where there was no source content
Leave destination on top of source content and only there (unbounded)
Source and destination are shown where there is only one of them
Source and destination layers are accumulated
CairoOperator::OVER, but assuming source and dest are disjoint geometries