How can the Levels compositor node be used? I was looking at the compositor output nodes, and I saw the Levels node. The wiki says:


  The Levels Node takes an image as an input, and can output a 1D value based on the levels of an image. It can read the input's Combined RGB, Red, Green, Blue, or Luminance channels.
  It can output a Mean value, or average of values, or a Standard deviation, which measures the diversity of values.


However, the wiki says nothing on how to use such a node. So, how can I visualize the data the Levels node outputs?
This node will output a single float value based on input image.




Mean is the average value of all image pixels in specified channel (combined, red, green, blue, luminance). It tells you how dark or bright the image is and can be used as such for setups that depend on how is input "bright" or "dark".
The Std Dev is how much those pixel values differ from the mean. A low standard deviation indicates that the pixel values tend to be very close to the mean. A high standard deviation indicates that the values are spread out over a large range of values.


The visualization of such data is just a grey rectangle.