Regarding the recording of transparent background sequence pictures:
2021.3.15 LTS test, the built-in rendering pipeline does not require special settings, just set the camera’s Clear Flags to Solid Color, and the alpha channel of the Backgound color to 0. The specific recording settings are as shown in the figure below, and many configurations can be changed by yourself.
But the particles cannot be recorded, and no solution has been found yet.
Unity2019.4LTS, the Recorder version is 2.5.7, under the URP rendering pipeline, the background is transparent only after turning off the camera’s post-processing and HDR. But the particles cannot be recorded, and no solution has been found yet.
Unity2021.3LTS, the Recorder version is 3.0.3, under the URP rendering pipeline, there is no include alpha option, and pictures with transparent backgrounds cannot be recorded. You can use the 2.x version for testing.
Regarding the reason why the particle effect cannot be recorded under a transparent background, it is guessed that it may be a problem with the particle shader, which can be recorded with a normal shader (transparent or opaque).