The Pixel Margin parameter allows you to determine the margin of the resulting UV map in pixels of the active texture. In order to use this option set the Pixel Margin parameter to a value greater than zero. In such situation the standard Margin option will be ignored and packer will determine the correct pixel margin.
There are two methods of pixel margin determination:
This method determines the proper pixel margin by running an algorithm for a time specified by the Adjustment Time parameter. During this time packer will be determining what margin should be applied to the given UV map before packing, so that the resulting margin after packing is equal to the requested number of pixels. Such adjustment is required because in a general case the UV islands are scaled after packing is done (so they fit the unit UV square), so is the margin applied to the islands before packing. After the adjustment phase is done the actual packing process will begin. This is the recommend method as it gives exact results in a short time in most cases.
A rule of thumb is that the longer the adjustment time the more accurate the resulting pixel margin will be. In practice the adjustment time set to one second should be enough for a usual UV map. Set the adjustment time to greater values only if the resulting pixel margin is not accurate enough for the given UV map.
Note that the pixel margin adjustment phase is not needed if the UV map is packed without scaling (the Fixed Scale option is on). In such situation the Adjustment Time option will be ignored.
This method guarantees to always determine exact pixel margin, but it requires a number of iterations to be performed and usually gives slightly worse coverage than the Adjustment Time method. Use the Iterative method only in rare cases when you cannot get good margin results with the Adjustment Time method, despite setting longer adjustment times. Because the Iterative method gives sightly worse coverage, it is recommended to always combine this option with the Heuristic Search functionality.