void glLineWidth( GLfloat width)
If line antialiasing is disabled, the actual width is determined by rounding
the supplied width to the nearest integer. (If the rounding results in the
value 0, it is as if the line width were 1.) If
If antialiasing is enabled, line rasterization produces a fragment for each pixel square that intersects the region lying within the rectangle having width equal to the current line width, length equal to the actual length of the line, and centered on the mathematical line segment. The coverage value for each fragment is the window coordinate area of the intersection of the rectangular region with the corresponding pixel square. This value is saved and used in the final rasterization step.
Not all widths can be supported when line antialiasing is enabled. If an unsupported width is requested, the nearest supported width is used. Only width 1 is guaranteed to be supported; others depend on the implementation. To query the range of supported widths and the size difference between supported widths within the range, call glGet with arguments GL_LINE_WIDTH_RANGE and GL_LINE_WIDTH_GRANULARITY.
Nonantialiased line width may be clamped to an implementation-dependent maximum. Although this maximum cannot be queried, it must be no less than the maximum value for antialiased lines, rounded to the nearest integer value.