Sometimes, it is desirable to keep an object or area from being included in an auto-generated mask. It is possible to have the auto-mask generator operate only on designated areas;

1Clear the mask, and select the part of the image you wish to protect with the Flood Fill Lighter or Lasso tool, then click Invert.2In the Auto mask generator, set the parameters you need to generate your mask. Be sure to set 'Old Mask' to 'Add New Where Old Is Set'.3After clicking 'Do'. The auto-generator will generate the desired mask, however excluding the area we specified earlier.