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      Here we’ve opened up the Brightness Curve transformation while the 
      mask creation window is still open, and we see the little mask window in 
      the Brightness Curve box (just right of the Amount sliders) is mostly black 
      with a little white dot near the center – that dot is where our mask 
      is located. The white slider is at 100% and the black at 0%, so any curve 
      change we apply will affect the center of our mask most, and fade towards 
      the edges of the mask. This is what we want. I have applied a strong curve, 
      pulling the top of the curve to the left three divisions on the histogram 
      scale and bowing the center down. The results can be seen in the preview 
      window, and on clicking OK in the Brightness Curve window, a new image will 
      be created with the change . . . 
      
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