samplerBack and layers

I'm writing a shader, and I was wondering if there is anyway to only check a specific layer when sampling the background.

For example when I use:

lowp vec4 back = texture2D(samplerBack, mix(destStart, destEnd, vTex));

if (back.a > 0.5)

is there anyway to exclude or specify a layer, or is it always the sum of every layer other than the current layer ? So in this instance back.a would only refer to the alpha of the layer under the current layer.

It's either the entire layout drawn so far (by default), or the current layer drawn so far only (if using its own texture). We can't force a separate texture per layer, it will murder the fill rate on low-end systems.

That makes sense, thank you Ashley