That's it! Man, I was overthinking things and was trying to crete that manually. I see now that the Shapes transition was used in the examples, some probably with the smoothing thickness set to a high value. Using an off centre circle and 200% smoothing thickness I can have the desired effects that look like a variation of the standard dissolve transition, but with an off centre starting point.
Thanks a lot for pointing me to it.
Thanks, that helps me to create some truly custom masks.
I do wonder if PTE would allow me to manually build that preset transition though It's probably possible.
/edit1: The example project and files from JEvans in topic The Third Image on the Beckham Digital Forums are more what I had in mind. They help me a great deal on figuring out how to make my own.
/edit2: All right, I was finally able to incorporate this on my test project. All of my photos make a slight 'Ken Burns' movement across the screen. That makes it a bit hard to keep things right. But, it works
Here's a little demo video.
When using a mask (combined with the standard dissolve effect) to sort of peak through earlier as you can see in the demo (from 1st to 2nd image), the most important thing for it to maintain it's fludity is to make sure the photo beneath the mask is lined up exactly the same as the image on the next slide that it will transition to. That means doing some simple math and using keyframes.
The difficult thing I'm facing now is that my custom Ken Burns effect needs to be animated 'Linear'. When using the much prefered Smooth, or Accelerate or Slow down, the small part visible on slide 1 needs to have the exact amount of animation type applied so the whole will be either Smooth, Accelerate or Slow down.
So with my 7.25s Ken Burns animated photos, when animated as Smooth, the first and last 20% are the accelerate and decelerate portions (both 1.45s), while the remaining 60% (4.35s) is the linear part. A part of the first 20% is visible in the masking bit on slide 1. So far, I think it's impossible to configure part of that accelerated 20% bit at the end of slide 1. The Speed option windows just don't give me the control.
I'll need to continue testing, but I haven't been able to so far. Playing around with the cusom animation options where you can cut and glue them based on keyframes. I think this is impossible and limited by PTE, but I'm not 100% sure yet. I'm sure I'll get burned by this again, but let's hope so
I realise this has now become a different question than I initially asked in this topic. I'll gladly make a new topic if needed.