Trying to animate texture using fragment shader and two FBO, problem with lack of precision

So I need to apply some effects on a texture and I’m doing this by ping ponging the texture between two FBO’s. I have the original image on one FBO and I use a shader to draw this FBO to the second one. Next frame I do it the other way.

It’s working but I’m having an issue with the texture degrading every time I do this. It happens even without any custom shaders and I’ve made it a bit better by using a high precision sampler in the shader.

What happens is all the alpha’d edges of the image start contracting and going weird, it’s very noticeable.

I assume I somehow need to make these FBOs more precise but I don’t know how, I’ve tried changing the internal format of the FBO but I can’t find one that works on iOS.

Any ideas?

Many thanks