Fill index 0 corresponds to no polygon filling, and is the default value established at CORE initialization time. The other built-in fill indices 1-84 are constructed by repeating the twelve basic patterns in the following table seven times, each time halving the line spacing parameter, d, of the previous group. The value of d for the first group of twelve is 0.032, so that the closest lines in fill indices 79-84 (which have spacing d/64, d/128) are 0.00025 NDC unit apart. On typical raster CRT displays, with resolutions of 512 to 4096 scan lines, this is equal to or smaller than one pixel in width, giving solid filling. Because coordinate roundoff to pixel boundaries can result in blank raster lines or missed pixels in a tight shading pattern, it is desirable to choose a line spacing which is about half the raster spacing in order to get solid filling.
Fill Index Spacing Angle Hatch 1 d 0 No 2 d 45 No 3 d 90 No 4 d 135 No 5 d 0 Yes 6 d 45 Yes 7 d,d/2 0 No 8 d,d/2 45 No 9 d,d/2 90 No 0 d,d/2 135 No 1 d,d/2 0 Yes 2 d,d/2 45 Yes
For fill index values greater than 84, the line spacings fo the pattern are specified by the digits of the index. The digits taken from left to right represent the spacing between adjacent lines of the pattern, measured in steps of 0.002 unit in normalized device coordinate (NDC) space. When the digits are exhausted, the pattern is repeated cyclically. Thus, the digits in the fill index 631 represent the parallel lines with NDC spacings 0.012, 0.006, 0.002, 0.012, 0.006, 0.002, etc. Zero digits in the fill index represent zero spacing, and are therefore ignored. They are nevertheless useful for defining patterns whose indices would otherwise conflict with the built-in fill indices.
Center for Scientific Computing
South Physics Building
University of Utah
Salt Lake City, UT 84112
Tel: (801) 581-5254
(Manual page by R. P. C. Rodgers, Computer Applications in Laboratory Medicine Project, UCSF, San Francisco, CA 94143).