Thick Lines

Normally we are only concerned with thick lines up to 6 pixels thick when drawing shapes, otherwise use polygons.

Before looking at anti-aliased thick lines, it is as well to check out simple thick lines. One might use multiple single lines, this could work for two lines used as a 2 pixel wide line, but when there is a 3 pixel wide line, there are spaces in between the lines. The shape of the PIL thick line is the same as a PIL polygon. When the polygon is used first check whether it fits the Bresenham algorithm.

When drawing horizontal or vertical thick lines there are no surprises. The lines are centralised when the width has an odd number, even widths require an extra line below the horizontal line or to the right of a vertical line.

For 45° lines the width does not correspond to the diagonal distance. The line can be centralised for both even and odd widths. The actual width of standard PIL lines do not change beween 3 and 4 pixels wide nor between 4 and 5 pixels wide, so a 6 pixel wide line is shown only as 4 pixels wide. As we are only trying to see what is judged to be acceptable before trying to make thick antialiased lines, these anomolies are taken as potential problem areas.

3 pixel wide line, polygon and line outline of the thick line.

PIL line dark blue 3 wide with red line starting and finishing at the same points, the pale blue polygon imitates the shape of 3 wide line. Skeletal shape is 3 parallel lines set at the same distance apart as the PIL thick line.

Using a line that has 10 pixels along the x-axis and 8 pixels along the y-axis, draw a line 3 pixels wide in blue, superimpose a red line 1 pixel wide. Compare the shape to a polygon in light blue, and lines drawn around the outline also in light blue. The thick line corresponds to a polygon whereas the skeletal lines do not match either of the other two, which were polygon based. A dark blue central line has been used for the skeletal lines, note that even though there are 3 lines present to give 3 pixels wide the coverage is incomplete.

PIL pale blue line 2 pixels wide, two grey lines drawn in parallel

Similar to the 3 pixel wide drawings. It can be seen that thick line drawing is not without its problems. The 2 wide line is centralised over the single line. Using the same strategy for a 3 wide line results in a line that could be mistaken for a 5 wide line, but the lines do progressively change at each width.