I searched the net and found 161 short matte (black and white) MP4 video clips which can be used to create interesting transitions in SC using the Mask:From File filter.
I’ve collected them together in to a folder and categorised them into 7 subfolders -
Brush and Paint Strokes (24 designs)
Circles and Clocks (28 designs)
Lines and Rectangles (56 designs)
Other Shapes and designs (Honeycombs etc) (16 designs)
Splat and ink transitions (13 designs)
Squares, Diamonds and Grids (20 designs)
Triangles (4 designs).
Here’s a short demo video I made showcasing some (not all) of the transitions:
I’ve made a tutorial PDF with information about what matte video clips are and how to apply them in Shotcut:
[Matte transition video files explanation and instructions.pdf (968.6 KB)
I’ve uploaded the whole collection of MP4s for anyone to download on my Google Drive here:
Please note: as far as I am aware they are all copyright-free for anyone to use. Should this not be the case, and I have included some copyrighted designs in error, please let me know and I will immediately remove them from the collection.