Doing a simple ‘Divide’ in After Effects

[Filip Vandueren]

I’m not sure if “compound arithmetic” or “aritmetic” could help you.
Not sure what the final result looks like

[Erik]

In one sense screen is the opposite of multiply, in that multiply is:

imageX * imageY

and screen is:

1 – (1-imageX) * (1-imageY)
in other the words the invert of two inverted images – multiplied.

What I’m looking for is a super simple:

imageX / imageY

The grand purpose here, if anyone is interested, is to recreate existing tranfer modes (such as color dodge, etc) with simple math ops so that can be used under eLin. Without that basic building block of a divide function this is hard to do.

And thanks for pointing me towards calculations and arithmetic – but unfortunately, neither offer divide.

Erik

[Filip Vandueren]

Right,

since x / y = x * (1 / y)

you can multiply two images, if you ‘invert’ the second.
Now invert does not mean just “channel -> invert” (255 – brightness)
but 255 / brightness

You can do this using a Curve
I suggest using Color Finesse, it has a better interface for finetuning and a more logical interpolation between curve-points

Just make a curve with these values:

input / output:

1 – 255
2 – 128
4 – 64
8 – 32
16 – 16
32 – 8
64 – 4
86 – 3
128 – 2
255 – 1

Is that what you mean ?

I’ve tried it and can’t seem to find a use for this: it returns a Lot of dark values

[Ben Rollason]

Hi There….

Only about 4 years too late, but if you’re listening, here’s a couple of things that approximate Shake’s mDiv function… you should be able to work backwards from there with a bit of channel juggling, I think.

The method that ought to work, but doesn’t really seem to is using Channel Combiner…
set ‘From’ to Straight to Premultiplied, check ‘invert’ and ‘Solid Alpha’.

This is a little unsatisfactory, in that when you then set the track matte to the alpha of an original copy of the image, it doesn’t look quite like the original copy… i.e. it’s not the opposite math of the premultiply that the track matte is doing…

This version is simpler and seems to work better. Put a Channels-> Set Channels effect on and set the ‘Set Alpha To Source’ to ‘Full’. This appears to perform a matte divide and when I perform the above check, it looks exactly as it should.

Hope that helps.

Ben.

http://www.benrollason.com