My game is at a rather large scale, maybe you could scale it up a lot and move away to make sure mipmapping is taking place. I ended up using the green channel instead of opacity, but the opacity map doesn’t seem to be related to the issue and it indeed being due to mips.
There is no need to disable mips or find and alternate way to achieve the effect. Just calculate the mips using an unaltered texture coordinate like I mentioned and it should work fine.
To be more precise, the shader guesses which mip it needs by comparing two adjacent coordinates in screen space. So if you skip a bunch of pixels in your UVs, it will look like the texture is really far away to the shader when it sees the coords change fast. This will cause it to choose the lowest quality mip for that spot, which creates a seam.
So you simply need a second texture coordinate that isn’t altered to calculate DDX and DDY from. DDX and DDY are a measure of rate of change in X and Y. If you want smooth mips, you need a smooth rate of change in your coordinates. It’s basically always necessary to do this extra step when messing with the UVs non linearly.
I think I get the idea, but it’s currently far past my skill level, but I’ll see what I can figure out with some google-fu ing about setting DDY and DDX