I'm trying to add fire+smoke to a video by rendering out the simulation as a PNG sequence with transparency. It looks fine in the render window and in the compositor but when I drag in the final PNG and view it, it looks half transparent and grainy. I'm only rendering it on 15 samples with NLM de-noising on so that could be it, but otherwise I'm not sure what's going on! here are some screenshots:
Use denoise node in compositing instead of the one you are using. It is better for viewports only.
To use denoise node,go into the compositing tab and SHIFT+A to add denoise node. Now add color,normal and albedo in their respective channel.
To get better results,enable denoising data from the layer settings.
Disable the denoiser you are using and it will give better results