Line Drawing - Snapping

While AGG supports subpixel accuray via anti-aliasing, it can have some issues for an image containing a lot of vertical / horizontal lines or texts. These lines can be drawn with thicker than intended line width and lighter than desired color.

The following example illustrates the problem.

Example

tutorial_linedrawing_4.cpp generates a bunch of horizontal lines of 0.5 in width. Notice some of the lines occupy two pixels in height despite the fact the desired linewidth is 0.5.

Solution

For thin lines (usually line width thinner than 5) where such effect is quite prounced, shift x / y position slightly so that after subtracting (linewidth / 2), x / y is an integer.