Chapter 4. Drawing Lines
In cairo, drawing often starts with the construction of a path consisting of a sequence of points. For example, the function draw_triangle for drawing a path connecting three points is given as follows:
fun
draw_triangle{l:agz}
(
cr: !cairo_ref l
, x0: double, y0: double
, x1: double, y1: double
, x2: double, y2: double
) : void = () where {
val () = cairo_move_to (cr, x0, y0)
val () = cairo_line_to (cr, x1, y1)
val () = cairo_line_to (cr, x2, y2)
val () = cairo_close_path (cr)
} (* end of [draw_triangle] *)
The functions involved in the body of draw_triangle are assigned the following types in ATS:
fun cairo_move_to {l:agz} (cr: !cairo_ref l, x: double, y: double): void
fun cairo_line_to {l:agz} (cr: !cairo_ref l, x: double, y: double): void
fun cairo_close_path {l:agz} (cr: !cairo_ref l): void
When called, cairo_move_to starts a new (sub)path whose initial point is (x, y) and cairo_line_to connects the current point on the current path to (x, y) and then set (x, y) to be the current point. The function cairo_close_path simply adds a segment connecting the current point to the initial point of the current (sub)path.
There is also a function cairo_rel_line_to of the following type:
This function is similar to cairo_line_to except for (x, y) being relative to the current point on the current (sub)path.
Once a path is constructed, cairo_stroke can be called to draw line segments along the path:
There are a few line attributes that can be set in cairo. For instance, the styles of line cap and line join as well as the width of line can be set by the following functions:fun cairo_set_line_cap {l:agz} (cr: !cairo_ref l, line_cap: cairo_line_cap_t): void
fun cairo_set_line_join {l:agz} (cr: !cairo_ref l, line_join: cairo_line_join_t): void
fun cairo_set_line_width {l:agz} (cr: !cairo_ref l, width: double): void
The following styles of line cap are supported:
and the following lines, from left to right, are drawn according to these styles, respectively:
The following styles of line join are supported:
and the following triangles, from left to right, are drawn according to these styles, respectively:
There is also a function cairo_set_dash for setting up line dash pattern.