# glBegin

Definition: | include pGUI.e include opengl.e glBegin(integer mode) ... glEnd()
Description: | Deprecated, specifically not supported on WebGL. While still supported on desktop/Phix, these routines have been coloured illegal to indicate they cannot be handled by pwa/p2js. The glBegin() and glEnd() functions delimit the vertices that define a primitive or a group of like primitives. The glBegin() function accepts a single argument that specifies which of ten primitives the vertices compose. Taking n as an integer count starting at one, and N as the total number of vertices specified, the interpretations are as follows: mode: The primitive or primitives that will be created from vertices presented between glBegin() and the subsequent glEnd().
pwa/p2js: | Not supported.
Notes: | The following are accepted symbolic constants for mode and their meanings, as defined in opengl.e:
    global constant GL_POINTS         = 0,  -- Treats each vertex as a single point.
                                            -- Vertex n defines point n. N points are drawn.
                    GL_LINES          = 1,  -- 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_LOOP      = 2,  -- 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_LINE_STRIP     = 3,  -- 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_TRIANGLES      = 4,  -- 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 = 5,  -- 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   = 6,  -- Draws a connected group of triangles.
                                            -- One triangle is defined for each vertex presented after the
                                            -- first two vertices.
                                            -- Vertices 1, n + 1, n + 2 define triangle n.
                                            -- N - 2 triangles are drawn.
                    GL_QUADS          = 7,  -- 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     = 8,  -- 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/2 - 1 quadrilaterals are drawn.
                                            -- Note that the order in which vertices are used to construct a
                                            -- quadrilateral from strip data is different from independent data.
                    GL_POLYGON        = 9   -- Draws a single, convex polygon.
                                            -- Vertices 1 through N define this polygon.
If mode is not one of the above values, glGetError() will return GL_INVALID_ENUM. You can use only a subset of OpenGL functions between glBegin() and glEnd(). The functions you can use are: glColor, glEdgeFlag, glEvalCoord, glEvalPoint, glIndex, glMaterial, glNormal(), glTexCoord, glVertex You can also use glCallList or glCallLists to execute display lists that include only the preceding functions. If any other OpenGL function is called between glBegin() and glEnd(), the error flag GL_INVALID_OPERATION is set and the function is ignored. Regardless of the value chosen for mode in glBegin, there is no limit to the number of vertices you can define between glBegin() and glEnd(). 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:
Type of primitive | Minimum number of vertices
--- | ---
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).
See Also: | glGetError, glNormal, glVertex, int_to_bits, glCallList, glCallLists glColor, glEdgeFlag glEvalCoord glEvalPoint glIndex glMaterial glTexCoord