The image (within the currently selected region) is scanned for bad pixels. When one is found then six good pixels (this excludes already patched ones) are looked for in each of the x and y directions, preferably 3 on either side of the bad bit. A linear fit is performed and an interpolated value derived separately in the x and y directions. The value assigned to the bad pixel is the simple mean of these.
The std.dev. of the good points about each fitted line is used as the interpolated error, given by sig**2 = sum(res**2)/(n-2). These are combined to give a variance for the patched pixel using: var = sig**2 = (sigx**2 + sigy**2)/4.
PASTE mode
Using the cursor the user selects a box to copy from. Clicking in an area of bad QUALITY will then cause the box to be copied, centred around the cursor position.