In order to despeckle an image, GeoExpress must determine the mask or "outline" of the valid image data. This outline is created as a polygon of connected points, similar to a "connect the dots" drawing. Image data that fall outside the outline are treated as invalid and subject to despeckling.
Whether image pixels are incorporated into the mask or abandoned as "islands" of data is largely a function of the point spacing value, which determines the distance between the points that make up the outline. The less distance there is between points in the outline, the more points it takes to create the entire outline.
Therefore, decreasing the point spacing value raises the number of points, which is generally desirable and is particularly useful when the outline is complex, such as that of a coastline. However, higher numbers of points may slow performance or cause the job to fail. Conversely, increasing the point spacing value reduces the total number of points, which makes for better performance. When the outline is simple, such as a rectangle, the point spacing value can safely be increased.
For any given dataset, some trial and adjustment may be required to arrive at the best balance between a more accurate mask (lower point spacing value) and better encode performance (higher point spacing value).
Generally, even values between 6 and 12 work best. By default, the point spacing value is 6.