Hi Helicopter_Ambulance. I think the function zombie_attack(..) on data.lua is missing the hr_version of the mask layer...

That is very likely. I was learning Blender to make some better masks, but hadnt got back to that as I wanted to add some other features.
My next plans are to finish the masks, and potentially create a different model for the spitters :)

Good to know =)

Also, unfortunatelly the api does not have animations for idle poses, so those sprites files you added are not used, sadly. But could make an amazing hologram animation here https://mods.factorio.com/mod/holographic_signs =]

Yeah I have removed them from the other mod, and will in the next update for this one.

When you integrate this mod, is there anything I should be aware of so I don't break compatibility?
I have added a mod option to have biters and zombies on the same force, and I will likely only be finishing the graphics.

I have already added them as a extra challenge to populate some plantes on my mod, warp drive machine. Even added some zombi bosses. The only things I've changed are: 1-force the option to do not replace vanilla, and the separated force, so I've made then normal "enemy" and removed autoplace, so I spawn then by script on that specific planet on my mod. A mod option on your side to be the same enemy force is very welcome