Begin( Mode)
The Begin and End functions delimit the vertices of a primitive or a group of like primitives. They appear within the glDraw protocol in Mathscript scripts:
glDraw {
begin( mode)
...
end()
}
And they may also appear (without the draw) in Actor definition Scripts.
Mode
The primitive or primitives that will be created from vertices presented between Begin and the subsequent End. The following are accepted symbolic constants and their meanings:
GL_POINTS
Treats each vertex as a single point. Vertex n defines point n. N points are drawn.
GL_LINES
Treats each pair of vertices as an independent line segment. Vertices 2n - 1 and 2n define line n. N/2 lines are drawn.
GL_LINE_STRIP
Draws a connected group of line segments from the first vertex to the last. Vertices n and n+1 define line n. N - 1 lines are drawn.
GL_LINE_LOOP
Draws a connected group of line segments from the first vertex to the last, then back to the first. Vertices n and n+1 define line n. The last line, however, is defined by vertices N and 1. N lines are drawn.
GL_TRIANGLES
Treats each triplet of vertices as an independent triangle. Vertices 3n - 2, 3n -1, and 3n define triangle n. N/3 triangles are drawn.
GL_TRIANGLE_STRIP
Draws a connected group of triangles. One triangle is defined for each vertex presented after the first two vertices. For odd n, vertices n, n + 1, and n + 2 define triangle n. For even n, vertices n + 1, n, and n + 2 define triangle n. N - 2 triangles are drawn.
GL_TRIANGLE_FAN
Draws a connected group of triangles. One triangle is defined for each vertex presented after the first two vertices. Vertices 1, n + 1, and n + 2 define triangle n. N - 2 triangles are drawn.
GL_QUADS
Treats each group of four vertices as an independent quadrilateral. Vertices 4n - 3, 4n - 2, 4n - 1, and 4n define quadrilateral n. N/4 quadrilaterals are drawn.
GL_QUAD_STRIP
Draws a connected group of quadrilaterals. One quadrilateral is defined for each pair of vertices presented after the first pair. Vertices 2n - 1, 2n, 2n + 2, and 2n + 1 define quadrilateral n. N quadrilaterals are drawn. Note that the order in which vertices are used to construct a quadrilateral from strip data is different from that used with independent data.
GL_POLYGON
Draws a single, convex polygon. Vertices 1 through N define this polygon.
Remarks
The Begin and End functions delimit the vertices that define a primitive or a group of like primitives. The Begin function accepts a single argument that specifies which of ten ways the vertices are interpreted. Taking n as an integer count starting at one, and N as the total number of vertices specified, the interpretations are as follows:
· You may use only a subset of Mathscript interactive OpenGL functions between Begin and End. The OpenGL functions you may use are:
If any other interactive OpenGL function is called between Begin and End, the error flag is set and the function is ignored. On the other hand, any other (non-OpenGL) Mathscript Statements are allowed between the Begin and End. These are ignored by OpenGL.
· Regardless of the value chosen for mode in Begin, there is, in Mathscript, the limit of 65535 to the number of vertices you can define between Begin and End. Lines, triangles, quadrilaterals, and polygons that are incompletely specified are not drawn. Incomplete specification results when either too few vertices are provided to specify even a single primitive or when an incorrect multiple of vertices is specified. The incomplete primitive is ignored; the complete primitives are drawn.
· The minimum specification of vertices for each primitive is:
Minimum Number of Vertices
Type of Primitive: point 1
line 2
triangle 3
quadrilateral 4
polygon 3
Modes that require a certain multiple of vertices are
GL_LINES (2),
GL_TRIANGLES (3),
GL_QUADS (4), and
GL_QUAD_STRIP (2).