white behind alpha of shadow matte

[ulrichGTAV3]

Hey, is it somhow possible to get the rgb values of pixels that have alpha 0 to be 255? What I want is a rendered image with my object on transparent. But when I remove the alpha channel, I'd like to have a white background.

In redshift I set my domelight alpha to 0 but the background to a custom color (white). That gave me what I was describing above.

In Arnold I tried different settings with a background object etc. But completeley transparent pixels would come out black when I removed the alpha channel of my rendered image.

I know I could use PS to pack a white layer behind my render. But with around 1200 product images (no video sequence) this would be much easier if I could just set my alpha channel to 0 and be done with it.

TYVM!