I have been using the CImg library, and have been pleased with how easy it is to integrate and use. However, I now want to draw thick lines (i.e., more than one pixel thick). It is not clear from the API documentation of the draw_line function (here) how this can be done. A second version of the function (just below the first in the documentation) even takes a texture as input, but again no width. It seems strange that such a comprehensive library would not have this feature. Perhaps it's supposed to be done using some kind of transformation? I know I could do it using a polygon (i.e., a rectangle where I would compute the corners of the polygon using a normal to the line), but I fear that would be significantly slower.
Apparently, it is not possible 'out-of-the-box', but creating your own routine that calls multiple times the 'draw_line()' routine of CImg, with one or two pixels shifts should give you the result you want, without much work.