Comment 4 for bug 1044989

I explain what I'd expected

for left value:
 max(0,0.5) = 0.5 (ok)
 min(0,0.5) = 0 (ok)
 k2* max + k3 * min = 1 * max + (-1) * min = max - min = 0.5 - 0 = 0.5 = grey

for middle value:
 max(0.5 , 0.5) = 0.5 (ok)
 min(0.5 , 0.5) = 0.5 (ok)
 k2* max + k3 * min = 1 * max + (-1) * min = max - min = 0.5 - 0.5 = 0 = black

for right value:
 max(1 , 0.5) = 1 (ok)
 min(1 , 0.5) = 0.5 (ok)
 k2* max + k3 * min = 1 * max + (-1) * min = max - min = 1 - 0.5 = 0.5 = grey

If I didn't miss something about premul or clamping, I think this is right