Blend

You would take those three source inputs and use percentages to add the values together. Say you had R1, G1, B1, A1; R2, G2, B2, A2; R3, G3, B3, A3. To blend the Red channels in the proportion 30-20-50, would be:

R = R1*.30 + R2*.20 + R3*.50

Thanks ThreeDee.

So, ... if I let just "r = r1 + r2 ; r3", it should blend with 100% opacity.
And, because no particular blending mode was added, the blend should be normal.
Or not?

I did this : ( 5 inputs )

function prepare()
end;

function get_sample(x, y)
local r1, g1, b1, a1 = get_sample_map(x, y, INPUT01)
local r2, g2, b2, a2 = get_sample_map(x, y, INPUT02)
local r3, g3, b3, a3 = get_sample_map(x, y, INPUT03)
local r4, g4, b4, a4 = get_sample_map(x, y, INPUT04)
local r5, g5, b5, a5 = get_sample_map(x, y, INPUT05)
r = r1 + r2 + r3 + r4 + r5
g = g1 + g2 + g3 + g4 + g5
b = b1 + b2 + b3 + b4 + b5
a = a1 + a2 + a3 + a4 + a5
return r, g, b, a
end;

The result should be the same as a multiblend with 100 opacity. Instead... it is not. Why?

Yeah, there is a difference between the average or sum of multiple sources and blending (with an opacity factor). A real blending formula is not that simple really - It has to take into account opacity, alpha from foreground layer and alpha from background layer.

Even with multiple layers, you can still use the FF blending routines - you just have to "nest" things or think in intermediate results: start from the back and blend layer two onto layer 3. Consider that result your new "background" and blend layer one onto that.

With more layers it is really just the same.

Now the annoying thing about the FF routines / Lua is that the blends can't be called nested due to the multiple results... you can't pass the r,g,b,a result of the first blend onto the next directly, as the blend routine call is considered one parameter and not four by lua.

Btw if you don't want to use the build in blending, here is a lua version:

Code

function get_sample(x, y) -- fetch inputs local O = get_sample_grayscale(x, y, OPACITY) local Ra, Ga, Ba, Aa = get_sample_map(x, y, FOREGROUND) local Rb, Gb, Bb, Ab = get_sample_map(x, y, BACKGROUND) -- blendmode callback Ro = Ra + (get_sample_grayscale(Ra, Rb, BLEND_MODE) - Ra) * Ab Go = Ga + (get_sample_grayscale(Ga, Gb, BLEND_MODE) - Ga) * Ab Bo = Ba + (get_sample_grayscale(Ba, Bb, BLEND_MODE) - Ba) * Ab -- combine foreground alpha and master opacity Aa = O * Aa -- blend individual channels    local Ao = Aa + Ab * (1 - Aa) if Ao > 0 then local rAo = 1 / Ao Rb = Rb * Ab Gb = Gb * Ab Bb = Bb * Ab Ro = (Rb + (Ro - Rb) * Aa) * rAo Go = (Gb + (Go - Gb) * Aa) * rAo Bo = (Bb + (Bo - Bb) * Aa) * rAo return Ro, Go, Bo, Ao else --undefined, returning something that makes slightly sense in FF context: return Rb,Gb,Bb,Ab end end;

Oh right.. I forgot about the custom blendmode stuff. Simply comment out that block, i.e. the three lines after -- blendmode callback