Background shows through where separately rendered objects meet. How to go about this?

[Alex_M]

I want to render two objects separately and then combine them back cleanly. The problem is that after combining the objects the composite alpha where the objects meet is not solid white and an outline appears through which the background can be seen. Please check the attached image.

[dlparisi]

What program are you combining them in? I'm just thinking it might be a premulipty problem. Can you post the individual files?

[Alex_M]

I just tried rendering them unmpremultiplied and it looks better (no black fringe at the edges) but a faint gap can still be seen. I tried with both Affinity Photo and Photoshop and the result is exactly the same. The images are intended for a website product configurator which is being developed by another team so I'm not sure what's the best way to deliver them. I guess unpremultiplied against black background with an alpha channel as a separate image? Please see attached the files where I've rendered the images unpremultiplied/straight.

[dlparisi]

I'm not sure about Affinity but photoshop really sucks at layering up multiple images with transparency (I got similar results to yours). As such, I loaded them up in fusion and was able to get a clean solid alpha on the handle with no gaps in it using a standard additive merge but with the burn-in set to 1. The burn in of 1 is necessary to avoid the foreground alpha from cutting a hole in the background alpha which occurs right at the overlap parts. Additionally, in either Photoshop or fusion I can simply add the two alphas together and get a nice clean solid alpha so it's most likely something going on with the way the alphas being combined.

I'm not sure how this will work once it gets handed off to the website team though.

I'm sure someone with more compositing experience might have a better handle on what's going on and how to fix it. Anyone?

[Alex_M]

I've just heard from the website guys. They told me that the way I saved them with straight/unpremultiplied alphas work good for them so I think this is what I'll go with.