Hi Kevan. Thanks for trying to help.
The photos are JPEGS with an RGB profile attached, and you are correct; if I just stick one of these photos down on the timeline all by itself, and put the clip/mask beneath it, put the photo clip into the Composite mode, and then put a matching cross-dissolve on both clips, then they both come into view smoothly and simultaneously as the playhead moves across them, just the way I want.
However, if I put a video clip in front of the photo clip, and make a transition between them, and then put the clip/mask below the photo with a matching transition, that the mask pops into effect suddenly instead of dissolving in gradually. So it has something to do with the fact that the photo is joined to a preceding piece of video that makes the mask come in suddenly.
I thought it might be because the photo is in a composite mode and the video preceding it is not, so I tried putting the video clip in Composite Mode also (Composite Mode > Travel Matte-Luna), but that doesn't help, because instant the mask/clip gets under it (whether the clip/mask has a cross-dissolve on it or not) the mask just suddenly pops over the video. This happens at the very beginning of the dissolve, just as if the dissolve were not there. If I take the video clip out of Composite Mode, then the mask pops into effect the instant the playhead hits the mask/clip underneath the photo clip, even (again) if the playhead is passing over a cross-dissolve on the front end of the mask/clip.
In other words, a cross-dissolve on the front end of the mask/clip has no effect on the mask/clip whenever a video clip is joined onto a photo clip on V2 above it. A cross-dissolve only works on the mask/clip if the photo clip above it is all by itself, unjoined to anything.
I guess I'm too new to Final Cut to figure out why this should be so. Can you think of any reason?
Tom