This module corresponds to section 3.4 (Line Segments) of the OpenGL 2.1 specs.
lineWidth contains the rasterized width of both aliased and antialiased
lines. The initial value is 1. Using a line width other than 1 has different
effects, depending on whether line antialiasing is enabled (see
lineSmooth). Line antialiasing is initially disabled.
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 delta x >= delta y, i
pixels are filled in each column that is rasterized, where i is the
rounded value of
lineWidth. Otherwise, i pixels are filled in each row
that is rasterized.
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.
Likewise, there is a range for aliased line widths as well. To query the
range of supported widths of antialiased lines and the size difference
between supported widths within the range, query
smoothLineWidthGranularity, respectively. For aliased lines, query the
supported range with
The line width specified when
lineWidth is set is always returned when it
is queried. Clamping and rounding for aliased and antialiased lines have no
effect on the specified value.
A non-antialiased line width may be clamped to an implementation-dependent
aliasedLineWidthRange to determine the maximum width.
Line stippling masks out certain fragments produced by rasterization; those
fragments will not be drawn. The masking is achieved by using three
parameters: the repeat count (1st element of the
lineStipple pair, clamped
to the range [ 1 .. 256 ]), the 16-bit line stipple pattern (2nd element),
and an integer stipple counter s.
The counter s is reset to 0 at before the first action during
renderPrimitive is called and before
each line segment during
renderPrimitive is generated. It is
incremented after each fragment of a unit width aliased line segment is
generated or after each i fragments of an i width line segment are
generated. The i fragments associated with count s are masked out if
testBit pattern (( s / factor ) mod 16)
otherwise these fragments are sent to the frame buffer. Bit zero of the
pattern is the least significant bit, i.e. it is used first.
Antialiased lines are treated as a sequence of rectangles of height 1 for purposes of stippling. Whether rectangle s is rasterized or not depends on the fragment rule described for aliased lines, counting rectangles rather than groups of fragments.
Controls whether line antialiasing is enabled. The initial state is
The smallest and largest supported width of aliased lines.
The smallest and largest supported width of antialiased lines.