Render ONLY alpha channel from PNG sequence

HI dear blenderheads. I have a cloud png images sequence with semitrasparent and full transparent areas. And I wish to render alpha channel from this sequence to mp4 to use it like a mask. (Have a reason to do so)

I tried to do this such way but get incorrect result. Cloud is gray so if use it like mask it gives semitrasperent result for whole cloud

I wish to get a result like on second image that illustrate correct alpha channel, but can’t figure out how to get it.

Use the alpha convert node to premultiply the alpha

Already tried this, and it does not help(

It is a switchable node, did you try it both ways? Also in UV image viewer window press T for tools and look at the value of the alpha on the waveform display. Does it hit 100%?

Alpha is not pre-, post- or simply multiplied by anything, rgb channels are.

My random educated guess is that you have some tone mapping going on, which lowers the alpha value when it is output as rgb. Disable the tonemapper.

Sorry my mistake.

Maybe you could brighten the cloud using an RGB curves node or a brightness/contrast one?