How to get neigbor pixel coordinates in Unity HLSL shader

I am writing Mandelbrot set shader for Unity.

I wrote an Image Effect shader and it works.

Unfortunately, each point “shimmers” on translate scale. This is because, for each shader call I compute value for only one mathematical point, which is infinitesimal. On even small translation, the value can change drastically.

So, I would like to compute math value not for one point, but for several. But for this I need to know, which aother points belong to my pixel, and for this I need to know the distance to neighbor pixel. Unfortunately, my incoming values are float and I don’t know, how to step to next pixel.

How to accomplish?