doc.org

General Blending formula *

[RGB]* – background color, unknown [RGB]i – i-th stroke color Ai – i-th stroke opacity Ei – i-th stroke erase amount, i.e. amount of background color to mix in

draw –> Ai -> [0 .. 1], Ei == 0 erase –> Ai == 0, Ei == [0 .. 1]

[ ( ) ] [ ( [ ( |R*| |R*| ) |R1| ] |R*| ) |R2| ] [ ( [ ( |G*| (1 - E1) + |G*| E1 ) (1 - A1) + |G1| A1 ] (1 - E2) + |G*| E2 ) (1 - A2) + |G2| A2 ] … [ ( [ ( |B*| |B*| ) |B1| ] |B*| ) |B2| ] [ ( ) ]

[ ] [ [ |R*| |R*| |R1| ] |R*| |R2| ] [ [ |G*| (1 - E1)(1 - A1) + |G*| E1(1 - A1) + |G1| A1 ] (1 - E2)(1 - A2) + |G*| E2(1 - A2) + |G2| A2 ] … [ [ |B*| |B*| |B1| ] |B*| |B2| ] [ ] \______________/ \________/ \_____/ SRC1a SRCa SRCc

Stroke batch rendering into intermediate buffer *

DSTrgb – accumulated premultiplied color DSTa – amount of background to add

DSTr 0 DSTr’ DSTr SRCr DSTg = 0 DSTg’ = DSTg * SRC1a + SRCg * 1 DSTb 0 DSTb’ DSTb SRCb

DSTa = 1 DSTa’ = DSTa * SRC1a + SRCa * 1

glBlendFunc(GL_ONE, GL_SRC1_ALPHA)

Save capture *

rgb – take from save buffer and unpremultiply a – 1-amount of background to add

Blit intermediate buffer *

SRCrgb – premultiplied color SRCa – amount of (unknown) background

DSTr’ DSTr SRCr DSTg’ = DSTg * SRCa + SRCg * 1 DSTb’ DSTb SRCb

DSTa’ = DSTa * SRCa + SRCa * 1 <— don’t care what is in FB’s alpha for now

glBlendFunc(GL_ONE, GL_SRC_ALPHA)

Compose intermediate buffers *

SRCrgb – accumulated premultiplied color SRCa – amount of bg

DSTrgb – accumulated premult color DSTa – amount of bg

DSTr’ DSTr SRCr DSTg’ = DSTg * SRCa + SRCg * 1 DSTb’ DSTb SRCb

DSTa’ = DSTa * SRCa + SRCa * 0

glBlendFund(GL_ONE, GL_SRC_ALPHA) // color glBlendFunc(GL_ZERO, GL_SRC_ALPHA) // alpha