Z-Effects Blog

premult is what you are looking for, you want to “see” your alpha much like dare devil can see sound and we can’t soo essentially what you want to do is create the sound that you here into a visual represention, or in this case alpha

the premult node is located under merge

what this does is tell nuke “hey buddy, remember that time we were out partying and i was like lets draw a mask, and you were like cool, lets do it i don’t need this extra junk on my image, and then you drew your mask made it effect the alpha, but much like that bottle of jack you and nuke had at the bar, it was gone when you were done, and no where to be found but in the garbage (or in the case of your mask the alpha channel) so now your like, hey nuke, My parents are coming home we need to get that jack back in the bottle or something so they know we weren’t drinking!.Nuke is like, yea yea, fill it with water and food dye, and then nuke goes, but i want every one to see my cool alpha you made me man! and then your like well lets go get a premult!

so you go down to that seedy side street known as the merge menu and go and grab a premult for 50 bucks and a bottle of scotch you stole from your friends parent’s cellar, and what that does is say hey alpha, all the stuff the is represented by 0 will be black and 255 as the image (instead of white in the alpha) and everything in between will be some range of transparent (1 being almost impossible to see 254 being almost 100% opaque) then when it’s all done your like “oh man nuke im sorry that alpha was pretty nasty, let me go add some feathering to you edges”

and thats kind of it…

edit in summation

premult will display all things represented by black in your alpha as black in your image and everything inbetween as some range of transparent. if you need to flip you alpha around you can do to things color->math->inverse or in the bezier node go to color and flip that slider to the other side

-zander

This is another part of the same post from VFXTALK, it will help explain how the channel shuffle works and how, when importing alpha or zdept (or any other monotone pass) to get it to work the way you want. The person had asked

“I have a black and white information , so is it possible to extract the rgb , cuz the shuffle node allows to take out only one channel?”

here was my reply

the shuffle will only take 1 channel this is correct but since white is the accumulation of all colors, any channel will do!

think of it this way, if pure white is (in u bit from 0-255) red=255 Blue=255 green =255 than middle grey would be r=128 b=128 g=128 and black would be 0 across the board

there by any sampling with in this range will have equal color range

for the sake of keeping with in nukes prefered pipeline i suggest using the red channel, though as i stated above it doesn’t make any difference.

edit for edits sake

basically what’s going on here is the following: the world of the rainbow is a magical place, and while most people think of this as san Francisco (go bay area) to us artists we are talking about color through light, if you were to take a prism and hold it up to mr sunshine (assuming your not in the Uk, than you have no idea what the sun looks like as it has never not been over cast) and watch the rainbow form, there are essentially all the colors in the visible spectrum broken down into a gradient, that to the human eye, seems to be rougher than a gradient as the TIR and refraction values for each color aren’t the same do to the wavelength. in other wards if you take you and 6 of your friends and arange them by girth (and or with) and shove them through a door at the same time, and push as fast as they can they wil all pop out at different angles based on their size and shape. the fat ones will most likely fall while the anorexic ones will most likely brake something. This is basically light in a more…human way

in nuke we take the basics of the and take only the primary colors RGB and the main reason for this is that they are clearly the prettiest of all the colors, also they are the most common power rangers! now to understand this we must take a ratio of cool power ranges to bad ones, and when we do this the result is we have 255 votes for each rg and b rangers, and oddly 255 votes for the black ranger, so we tossed him in as alpha. and thats where we get the channel shuffle, because as the seasons went one we started to exchange actors to the point where none of the originals are even there any more.

i’d love to go into cmyk but going into the teletuby’s vs barny friends ratio calculations requires a great knowledge of physics, color blending and a good education in children’s television that, as far as i’m concerned would probably take me about a day to type out, and to be honest wouldn’t be as beneficial as the power ranger rgb breakdown as we don’t use cmyk often in vfx 

-zander 

 When you import something into nuke like an alpha or a Zdepth it comes up not the typical black and white ramp. The following is 1 of a few exerts i posted over at VFXTALK

Your zdepth should be a gradient color like red on black or white on black etc, what are they? esentially what you want to do it use a shuffle node after your read/crop/reformat that looks like this:

what nuke is doing is saying “ok so you’re not an forgetful you did get a zdepth pass out, and I know its in that information you gave me stored as chrominance and luminance values, but for the love of god what colors!” and then you pat nuke on the head and say”hey nuke thanks for understanding buddy, you’re always there for me, like when lizzy and I broke up you were there ready to help me comp my project you suck at photoshop style of me blowing up her car. So any ways, that zdepth channel you have, well this image has the zdepth in red, so if you just want to take all the values in the red channel and say…put them in your depth channel, we can set this up to render, and i’ll order you some chinese food”

and thats basically it

you may have to use a color->math->invert btw.

-editors note…i’m a bit nuts..no coffee yet. brb

second edit, at no time have i ever seriously dated a girl named lizzy, nor have i been on the “dating scene” since i started using nuke, and I do not believe nuke has had anything to do with my lack of dating.

-zander