And remember if you need to grade or colour correct an image with alpha, first you must unpremultiply the image, then grade, the premultiply the image, otherwise you will be colour correcting the partially transparent pixels incorrectly.
Mmm not sure to understand...
But, actually rendering on a black BG worked well, thank you !
It's not very convenient (imo) because :
- it adds a setting to get what the alpha is made usually for
- when using a black BG, it's visually disturbing while rendering to have a black sky for example (and much more to use Lightmix as a correct exposure of the sky is imo important for an overall correct exposure)
- if a client want finally to change your sky but you rendered without a black BG, you're trapped.