Transparent and considered alpha value of the alpha value
original regarded as transparent (0..255 transparency of, 0 is transparent) to specify. Increase the value if you want to erase and almost transparent garbage. Usually, you do not have a problem with 0.
Update the polygon (Ctrl + R)
When you click the button to re-calculate the polygon. If you run into when you make modifications to the texture in the dust removal tool may polygon is beautiful.