A couple of points with using Vio3.
In the three times I've used it, I made the first frame reference to fit the 720P (1280px x 720px). I have not tried using a reference with a different aspect ratio, so it might work that way.
If your end goal is a looping GIF, that white background makes a lot of difference to file size. The max size for image uploads on this forum is 10mb, and I was initially seeing about 40mb and having to downsize and reduce colours in the Save-for-Web panel. This one was under 10mb first try.
The workflow is simple. Open the video in Photoshop with the Motion workspace. Crop to size, and go File > Export > Save for Web (Alt Shift Ctrl S) same as you would with a frame animation. The results are always way better if you start with a video, and it is way easier making the animation in a video timeline. When I think of all the times I have manually added a second moving object in a frame animation (you have to select each from, and turn on the relevant layer) it makes me want to cry!
I've tried knocking out the background so I can place the video over an exising video timeline by using Blend If. To completely eliminate the white fringe it was also knocking out parts of the shirt and head. There's not much movement of the body, so I copined the video group and used a mask instead of Blend If. That worked, but only because the subject was not moving. If you don't mind using Prem Pro or AE you could make the first frame image green or blue. I just looked and found YouTube videos on keying out the BG with Photoshop. I've never tried them because it is easier with Prem Pro. I wonder if it is possible to create the video with transparency? Maybe using a PNG for the first frame reference?
When first heard about Firefly video there was mention of credits used for 1080p, 720p and 540p (960px x 540px). I'm only seeing 720p as an option right now, but if and when 540p becomes available, I was wondering about using it in a matte painting. So, in the image below, Rey and the robot would have been shot against a green screen, and the entire background is a painting.
In this case the live video is the central area, and nearly all of the Storm Troopers are painted on a matte.
I'm thinking this would give you maximum impact for minimal credits use. A useful thing if you do not have a full subscription, limited credits. It would also be an interesting and fun project, and one that I will defitely be having a go at.
https://helpx.adobe.com/nz/firefly/work-with-audio-and-video/work-with-video/use-video-as-composition-reference.html
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