toMaskAlpha

Turns the pixels of a drawn mask group into mask values, in place, for a backend that gates by pixels because it has no table colour filter. Each pixel of argb, straight ARGB, becomes black with the alpha of its mask value: its luminosity 0.30 R + 0.59 G

• 0.11 B with luminosity, else its alpha, through the table (ISO 32000-1, 11.5.3 and 11.6.5.2). A luminosity group is drawn over opaque black, so its pixels are opaque.