Get index of each pixel?

Layers can be any size.  Only a Layers pixel over the document canvas are visible if the layer visibility is on.  Any of the visible pixels can be address with a color sampler and you can access its color.  You can set up to 10 Color samplers.  I do not know how you can address a layer pixels that are clipped by the Document canvas size.  Color samplers are relaive to canvas size and seem to be confind to be within the canvas Area. If you add or remove camvas a color sampler position remains relative to canvas and is adjusted by Photoshop during the canvas size change. Which may mean removing samplers that would no longer be within canvas the document canvas. The Sampler Color is for the Image composit pixel so set layer visibility to what you want to sample.

function cS(v1, v2) {
     with(preferences) {
          s = (parseFloat(version) > 14.1 ? 10 : 4)
          if ((len = (cs = (aD = activeDocument)
          .colorSamplers).length) < s) {
               pixels = Units.PIXELS; function rU(v) {
                    if (v) cU = rulerUnits, rulerUnits = pixels
               }
               if (u = rulerUnits != pixels) rU(u)
               cs.add([v1,v2]); if (u) rulerUnits = cU
               hexColor = cs[len].color.rgb.hexValue
               return cs[len].remove(), hexColor
          }
          else aD.colorSamplers.removeAll(), cS(v1, v2)
     }
}
cS(225, 275)