Graphics.Rendering.OpenGL.GL.LineSegments

This module corresponds to section 3.4 (Line Segments) of the OpenGL 2.1 specs.

Line Rasterization

lineWidth :: StateVar GLfloat

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 smoothLineWidthRange and smoothLineWidthGranularity, respectively. For aliased lines, query the supported range with aliasedLineWidthRange.

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 maximum. Query aliasedLineWidthRange to determine the maximum width.

An InvalidValue is generated if lineWidth is set to a value less than or equal to zero.

An InvalidOperation is generated if lineWidth is set during renderPrimitive.

Line Stipple

lineStipple :: StateVar (Maybe (GLint, GLushort))

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) is False, 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.

The initial value of lineStipple is Nothing, i.e. line stippling is disabled.

An InvalidOperation is generated if lineStipple is set during renderPrimitive.

Line Antialiasing

lineSmooth :: StateVar Capability

Controls whether line antialiasing is enabled. The initial state is Disabled.

Implementation-Dependent Limits

aliasedLineWidthRange :: GettableStateVar (GLfloat, GLfloat)

The smallest and largest supported width of aliased lines.

smoothLineWidthRange :: GettableStateVar (GLfloat, GLfloat)

The smallest and largest supported width of antialiased lines.

smoothLineWidthGranularity :: GettableStateVar GLfloat

The antialiased line width granularity, i.e. the size difference between supported widths.