I would like to create a sharp transition effect between pixels in my fragment shader, but I’m not sure how I could do this.
In my vertex shader I have a varying float x; and in my fragment shader I use this value to set the opacity of the color. I quantize the current value to produce a layering effect. What I’d like to do is at a very minimal level of the effect to produce a distinct border (a different color entirely). For example, if x>0.1 and for any neighboring pixel x<0.1 then the resulting color should be black.
It don’t see any way in GLSL to gain access to neighbouring pixels (I could be wrong). How could I achieve such an effect. I’m limited to OpenGL-ES2.0 (though if not possible at all on this version, then any solution would be helpful).
You are correct that you cannot access neighboring pixels, this is due to the fact that there is no guarantee which order the pixels are written, they are all drawn in parallel. If you could access neighboring pixels in the framebuffer you would get inconsistent results.
However you can do this in a post-process if you want. Draw your whole scene into a framebuffer texture, and then draw that texture to the screen with a filtering shader.
When drawing from a texture in your shader you can sample neighboring texels all you want, so you could easily compare the delta between two neighboring texels.