On a whim, I decided to play with the End Credits Crawl effect, specifically to see if I could create a look I've seen occasionally where the top and bottom of the credits are masked (guessing) to fade in and out gradually as the credits scroll. I thought it would be fairly easy, but it turned out to require more steps than I expected. I'm doing this in HitFilm 4 Express version 4.0.0146 (Update 8) on a Mac Mini running OS X 10.10.5.
My test was rather simple. In a new comp, I made two colored planes, both black, and dropped the End Credits Crawl onto the top one. After playing with the effect settings to get several lines of text crawling up the screen, I then zoomed out a little and made a rectangular mask on the top plane that was wider than the comp dimensions, but shorter vertically, so that there was a clear gap between the top and bottom of the mask and the edges of the comp frame. I planned to feather the mask next, but I already noticed the problem: the crawl text wasn't being masked. Here's a screenshot showing the problem (with the lower plane hidden):
The crawl text should cut off where the plane is masked, but for some reason it continues beyond the mask.
Long story short, I was able to get what I wanted by taking advantage of this bug (I'm assuming it's a bug) and doing a workaround. I took this plane with the crawl and made it into its own comp. In that new comp, I shrunk and moved the mask so that it was completely out of frame, leaving the crawl over a transparent background. Back in my original comp, I masked that embedded comp as I had originally planned, and got what I wanted:
I'm guessing this is a bug because my assumption is that any effect applied to a plane layer should be affected by masks added to that layer, so I was surprised when this wasn't. If this is the way that the crawl effect is supposed to work with masks, could someone please explain why?