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      Using the original image and this mask together enables changing the contrast 
      and density in the water/cloud area without affecting the surrounding areas: 
       
       
        
       
       
      Above is shown the application of a Brightness Curve transformation to the 
      image through the mask just made. Note the mask image is selected in the 
      little box to the right of the Amount sliders. When an amount mask is chosen, 
      the slider scale shows two “knobs”, one black, one white. The 
      setting of the black slider will be applied to the image “under” 
      the black part of the mask, and the setting of the white slider applied 
      to the image “under” the white part of the mask. The resulting 
      image can be seen above in the preview window. The settings can be changed 
      and the results observed immediately if the “Auto” preview box 
      in the upper left corner is checked.   
      Note that a mask may be a continuous tone image, rather than just black 
      and white. If the mask is a continuous tone image, the image under the gray 
      levels between black and white will be transformed at amounts between the 
      settings of the black and white sliders. The enables some powerful effects 
      to be applied. 
      
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