   uvec2 ij = uvec2(pos.xy);
 
   gl_FragDepth = 1.0;
-  position = vec4(-2, -2, 1, -2);
+  position = vec4(-2, -2, 1, POSITION_NOT_SET);
 
   uint maxAge = u_Counts[Saddr(ij)];
 

    // sample accumulation
    fragmentColor = fragmentColor + sampleColor * (1.0f - fragmentColor.w);

    if (fragmentColor.w > alphaThresholdForDepth && position.w == -2)
    {
      setPositionAndDepth(vec4(startPos_eye, 1));
    }

    int activeObjectCount = bitCount(activeObjects);

    if (objectId == 0 && fragmentColor.w > alphaThresholdForDepth &&
        position.w == POSITION_NOT_SET)
    {
      setPositionAndDepth(currentFragment.eyePos);
    }
