🔸blend mode
SwiftUI ⟩ view ⟩ drawing ⟩ blend mode
source : pixels being applied (blend modes)
destination: pixels already stored to the context
R : result of blend operation
S : source pixel values
Sa: source alpha
D : destination pixel values
Da: destination alpha
// blend modes:
R = S × Sa + D × (1 – Sa) // normal
R = S × Da // source in
R = S × (1 – Da) // source out
R = S × Da + D × (1 – Sa) // source atop (?)
R = S × (1 – Da) + D // destination over
R = D × Sa // destination in
R = D × (1 – Sa) // destination out
R = S × Da + D × (1 – Sa) // destination atop (?)
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