I know rotoscoping can be done with AE, and any number of other apps. But was also under the impression that this can kind of simple mask *is* possible within PPro, using the Titler(?) It certainly ought to be -- every other NLE does it, if only as a preview, without having to leave the app.
And which difficulties exactly are you experiencing with a shape drawn in Titler and the Track Matte Key? A screenshot may help.
I'm not at the machine at present, but track 1 is the skyline shot, track 2 is the title clip with a mask drawn around the sky of that same shot, and track 3 is a new clip with the sky material which is supposed to replace the content inside the mask.
The idea is to make the area enclosed with the mask transparent, and the area outside it, opaque. So I'd get the clouds from clip 3, but everything outside that mask would come from track 1.
Every combination of track matte filter I've tried (putting it on either track 1 or 2, and selecting the track above) seems to have no effect. The clip on track 3 is the only thing which appears.
The main issue you're facing might be in the fact that Titler doesn't create a shape with alpha channel. Hence, your should set your Track Matte Key to 'Matte Luma' instead.
Here is briefly how to create procedural matte in PrPro:
1. Duplicate your footage in the sequence, place it above the previous one and apply Threshold effect onto it. Adjust Threshold Level to taste so as to get more or less acceptable edges with the sky keyed out. Apply Fast Blur to soften edges a bit.
2. Open up Titler and draw a shape for garbage matte. Set Graphic Type to Filled Bezier and Color to Black.
3. Drop your newly created garbage matte above your footage with Threshold effect and nest them.
4. Apply Track Matte Key onto your lower clip and set Composite Using to Matte Lume.
This worked perfectly. Thanks!