Span Length
When running SFGEN in LCD mode, the user must select a value for the parameter called Span Length. What is this parameter and what is the optimum value for span length? We'll try to answer this below.
First, imagine we have an image area to process of 2.5 x 2.2 meters (or since we generally think in terms of UM when working with semiconductor masks, it is 2.5E6 um x 2.2E6 um)
The goal of SFGEN is to identify repeating geometries in the input data that the rasterizer can process just once -- then all additional instances of the repeating geometry can be output by using the bitmap of the first instance and copying the bitmap it wherever it occurs again.
The span length defines the side of a region that will be the basis for finding repeating geometries.
These regions and the repeating geometries they contain (which cover the image area) will then be fed to the rasterizer.
So, how to choose a good value?
a) the span length should be large enough to encompass a geometry that repeats. If it is too small, the program won't be able to capture the repetition and you won't get maximum advantage of the bitmap copy later on in the rasterizer.
b) the span length should not be too large. If it is set too large, SFGEN will take a very long time to process the data contained inside the region since it will have to make a very large number of computations to identify what is repeating. The density of polygons will help you decide. If polygon density is low, then the span length can be larger without slowing down SFGEN exponentially.