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This is used to set the compositing operator for all cairo drawing operations.
The default operator is CairoOperator::OVER
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/.
CairoOperator::CLEARClear destination layer (bounded)
CairoOperator::SOURCEReplace destination layer (bounded)
CairoOperator::OVERDraw source layer on top of destination layer (bounded)
CairoOperator::INDraw source where there was destination content (unbounded)
CairoOperator::OUTDraw source where there was no destination content (unbounded)
CairoOperator::ATOPDraw source on top of destination content and only there
CairoOperator::DESTIgnore the source
CairoOperator::DEST_OVERDraw destination on top of source
CairoOperator::DEST_INLeave destination only where there was source content (unbounded)
CairoOperator::DEST_OUTLeave destination only where there was no source content
CairoOperator::DEST_ATOPLeave destination on top of source content and only there (unbounded)
CairoOperator::XORSource and destination are shown where there is only one of them
CairoOperator::ADDSource and destination layers are accumulated
CairoOperator::SATURATELike CairoOperator::OVER, but assuming source and dest are disjoint geometries