RotoPaint - Nuke 101: Professional Compositing and Visual Effects - page 280

For this shot, you have been asked to dirty up the windshield and write “CLEAN
ME”onit,assomepeopledowhentheywalkpastadirtycar.Themagicisgoing
to be that it needs to appear as if it's being written on with an invisible hand.
The first stage will be to create a dirty image to use on the windshield. You will
usetwoNoisenodesfromtheDrawtoolboxtomakethenoiseimageappearmore
interesting.
2. While Read1 is selected, create a Noise node from the Draw toolbox.
I don't want you to create the noise function on top of the car; I just asked you to
connect it to the car so you'll have the same resolution as the car. You now need
to ask the Noise node to replace the car color channels.
3. Check the Replace check box at the top middle of Noise1's Properties panel
( FIGURE 6.53 ) .
Figure 6.53. The Replace option tells Draw functions to replace rather than
blend with the above nodes.
4. Change the X/Y size property to 1.
5. While Read1 is selected, branch out another Noise node.