add versus screen

[radii]

Does anybody know the difference between "add" and "screen" in compositing apps such as Shake or Fusion ( not Photoshop ) ?

Also, the normals, materialID, objectID and UV passes put out by 3ds max are always non-antialiased, which seem to make them useless in the compositing stage. Does anybody have a workaround for this ?

 

Thanks for any help

[William Alexander]

In shake...as with other comp apps add is a literal adding of pixel values of inputs into a node on top of each other to modify the output. 0+1=1

[William Alexander]

Scratch the previous screen definition.....

 

1-((1-imageA)x(1-imageB)) is a true screen operation

 

Had to look it up... sorry

 

WDA

[radii]

thanks William

 

this would mean that with a screen function, you would never really get beyond 1 ( pure white ), whereas an add function would throw you over the top after the first computation. Mmhhhh ...

 

CG light seems to be additive ( I've read somewhere ), but using the add function ( at 100 % ) always seems to whiten everything. Screen always looked OK, but somehow felt "wrong".

[William Alexander]

Anything past one would be considered clipped to pure white. 0-1 being equivelent to 0%-100% white of any color space.

 

There are many variations on how to use each of the functions. Mostly just because they brighten darken a certian way artisitically for compositing passes and such. Add is a very heavy handed way to increase pixel values. Think about it in terms of 50% grey added to 50% grey = 100% white. Most images used in passes have a greater "value" than 50%. Where as the screen has a very different application of pixel values....do the math yourself....IT HURTS MY BRAIN tonight. The hard core mathematics are very good to know and understand, essential when pulling mattes for compositing live footage and stuff, though there are times you swear you see numbers flying down your screen when looking at pixels...it's evil I tell you.

 

As far as light passes and light being "additive" in CG applications they are two different animals, imho. Lighting passes sometimes work using "add" functions, screen is good for specualr pass, sometimes even using 'mulitply' with the lighting pass over the raw RGB diffuse works well after a levels call....LOL now we dive into more math, histograms and dynamic range within a color space.

 

Well the hard core math is really about how to pull mattes/masks from the impossible images to work with. LOL If it was easy everyone would do it

 

WDA

[radii]

Have a look at this one:

 

http://www.tahlniran.com/tuts06/Compositing_Ambient_Occlusion.htm

 

I was trying to recreate the tutorial in Fusion, as I don't have Shake. In Fusion however, there is no "iAdd" node. Add is available within another node, but only at 100%, which always whitens everything. Merge nodes set to screen, do have a level/percentage slide though, which helps adjust each light.

 

The end result, using the screen function and after some tweaking, is virtually identical to the Shake comp.

 

I was just wondering if I was missing a vital component of a pass-comp-flow :-)

 

Now, if only I could get this normals, UV etc. pass thing worked out

[William Alexander]

Have a look at this one:

 

http://www.tahlniran.com/tuts06/Compositing_Ambient_Occlusion.htm

 

I was trying to recreate the tutorial in Fusion, as I don't have Shake. In Fusion however, there is no "iAdd" node. Add is available within another node, but only at 100%, which always whitens everything. Merge nodes set to screen, do have a level/percentage slide though, which helps adjust each light.

 

The end result, using the screen function and after some tweaking, is virtually identical to the Shake comp.

 

I was just wondering if I was missing a vital component of a pass-comp-flow :-)

 

Now, if only I could get this normals, UV etc. pass thing worked out

 

I have to head out to studio....I have an associate in who uses fusion, I'll ask him to verify.

If you have a mask/alpha for that node in Fusion you might be able to apply a grayscale mask to control the 'amount-depth' of Add. Modifying the greyscale of the alpha using levels-HSV-any function that will uniformly brighten-darken a 50% grey to act as a mask. LOL The beauty of learning compositing in shake...."we don't need no stinkin' pre build nodes", sometimes you have to build them yourself.

 

Cheers

WDA

[Limbus]

Also, the normals, materialID, objectID and UV passes put out by 3ds max are always non-antialiased, which seem to make them useless in the compositing stage. Does anybody have a workaround for this ?

 

They have to be non-antialiased because if you would AA them, you would get color shift on the AAed edges and the ID passes would not work anymore.

 

Theoretically you could AA them and then use a materialID coverage pass as a mask. I think max can do this for the z buffer.

 

Florian