LMS Processing

Quite often when processing images, extreme stretches result in washed out highlights and bloated stars. Fortunately any layer and mask  capable image processor offers a solution in the form of layered and masked stretch (LMS) processing. This is a synthetic HDR process that allows the dim areas to be brightened without blowing out the brighter areas. The technique is equally usable on daytime images, but more on that later.


Take the M8 image to the right. If a simple stretch is applied, the bright central area will saturate before the outer areas are brightened sufficiently. By using layers we can correct the situation. The idea is to use different stretches on different layers then use a mask to combine the layers depending on brightness.

The steps are shown below.

  1. Duplicate the image on another layer. Name the bottom layer bright and apply an agressive stretch using curves or any other technique. The idea here is to reveil faint detail at the expense of the bright areas on this layer.
  2.  Now name the top layer dim and reduce the brightness of the brightest areas or in the case of the this image just leave it unstretched.
  3. Place a mask made from the bottom layer on top of the dim layer and blur it with a large radius gaussian blur.
  4. Adjust the brightness and contrast of the mask to reveil the faint detail while preventing the brighter areas from washing out.
  5. The layer stack is shown below and the resultant image to the right of the layer display.


Final Image