I did not see the parallelogram, but when you Rotate your Clip, you will need to also Zoom in just a bit, to get the edges of the Frame beyond the Frame Size in the Project. I use the Fixed Effect>Motion>Scale, and increase that Scale just a bit, to get the edges of the Clip beyond the rectangular Frame "aperture."
Good luck, and hope that helps,
Thanks for your input. I do not know what is going on: I can see the parrallelogram-shape clearly as of second 15 in the YouTube clip. The right side of the video is lower than the left side. Both the left and right boundary of the image are vertical, but the top and bottom definitively not. I know I have to scale too when rotating: about 3% per degree rotation. Without scaling I can see in the preview window 4 black triangles coming up on each four sides. I think that is what puzzles me most: why are the right and left side exactly vertical, whereas the top and bottom are not horizontal. I hope you or someone else has more suggestions.
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Just a guess on my part, but I would say that the borders that are square, are slightly beyond the Frame "aperture" in the Video, and what you are seeing is actually that Frame aperture, where the two offending sides are slightly inside of the Frame aperture, so you are seeing them.
The Frame Size in a Video is exactly like the aperture in a photo's matting board. The aperture is, say 8 x 10, but the photograph behind it is 8.5 x 11. One only sees the 8 x 10 through that aperture.
To get rid of those "triangles," I would adjust the Fixed Effect>Motion>Scale UP by just a little bit, until that part of your Clip is also behind the "aperture."
If I understand what you are doing correctly, and what you are seeing, the "black triangles" are actually showing nothing, which appears as black in Video.
As a test, Move one of your Rotated Clips up onto Video Track 2. Then from the New Icon on the Project Panel, create a Color Matte, with say Yellow, and place that on Video Track 1, below the Rotated Clip. Do you now see yellow triangles?