Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 124

32. Make sure you are viewing the output of the last Merge node, which should be
called Merge5 ( FIGURE 3.36 ).
Figure 3.36. The bottom Merge node's Operation property is set to Minus.
Shadow is a subtractive pass. It is light that needs to be taken away, so it needs to
be subtracted from the rest of the tree. Using the Minus operation sounds like the
rightthingtodo—however,judgingbytheresultingimage,somethingisn'tright.
It's all black.
ThewaytheMergenodeworksisAthenB.FortheOveroperation,normallythis
means A over B. For the Plus operation, this doesn't matter as 1+3 is the same as
3+1. For the Minus operation this means A minus B, which will work on some
occasions, but in this case you need to subtract the shadow from the composite,
not the other way around. You need B minus A. One way to do this is to switch
inputs (the hot key Shift-X will take care of that and is worth remembering), but
there'sanalternative.TheoperationFromdoesexactlywhatMinusdoes,onlythe
other way around. It subtracts A from B.
33. Change the same Merge operation from Minus to From.
Using these passes, you completed the basic building of the lemming's beauty
pass. Your tree should look like FIGURE 3.37 .
Figure 3.37. The tree at the end of the beauty pass build.