Blacks end up being see through to various degree

[myanonapps]

I'm very new to rembg and image manipulation. I created a UI around rembg and I have tested various models. What I have found is that foreground blacks become see through to various degrees depending on the model. I've mostly used birefnet-general, birefnet-profile, birefnet-massive as they do the best job of removing background in my cases. But as I said, foreground blacks become see through to degrees.

I know this isn't a rembg bug. Is there a configuration or some process that can help?

[myanonapps]

As a workaround I programmatically merged a black layer under the non-transparent portions of the results of rembg. Not very AI but seems to work ok but not great in my test cases. Works also manually to do it in GIMP

I suspect there are settings in rembg that would help me not require that workaround.