Ignore this post
having a transparent background is quite possible with CC4
While waiting for RL to fix the problem, I found a workaround
how to render a character with a transparent background :
Preparation
1 - go to template project
2 - load the project : 3. cloth physics
3 - in the Scene / props pane : delete "Backdrop
(to avoid making this manipulation each time, save the project)
Render
1 - Format : image / PNG
2 - if necessary, adjust the other parameters
3 - Export
To render a custom character
replace the character (from project 3. cloth physics) with your own character and render it.
