Porting bgn/end Commands

IRIS GL uses the begin/end paradigm but has a different function for each graphics primitive. For example, you probably use bgnpolygon and endpolygon to draw polygons, and bgnline and endline to draw lines. In OpenGL, you use the glBegin / glEnd structure for both. In OpenGL you draw most geometric objects by enclosing a series of functions that specify vertices, normals, textures, and colors between pairs of glBegin and glEnd calls. For example:

void glBegin( GLenum mode) ; 
   /* vertex list, colors, normals, textures, materials */ 
void glEnd( void );

The glBegin function takes a single parameter that specifies the drawing mode, and thus the primitive. Here's an OpenGL code sample that draws a polygon and then a line:

glBegin( GL_POLYGON ) ; 
   glVertex2f(20.0, 10.0); 
   glVertex2f(10.0, 30.0); 
   glVertex2f(20.0, 50.0); 
   glVertex2f(40.0, 50.0); 
   glVertex2f(50.0, 30.0); 
   glVertex2f(40.0, 10.0); 
glEnd(); 
glBegin( GL_LINES ) ; 
   glVertex2i(100,100); 
   glVertex2i(500,500); 
glEnd();

With OpenGL, you draw different geometric objects by specifying different parameters for glBegin. The following table lists the OpenGL glBegin parameters that correspond to equivalent IRIS GL functions.

IRIS GL function	Value of glBegin mode	Meaning
bgnpoint	GL_POINTS	Individual points.
bgnline	GL_LINE_STRIP	Series of connected line segments.
bgnclosedline	GL_LINE_LOOP	Series of connected line segments, with a segment added between first and last vertices.
	GL_LINES	Pairs of vertices interpreted as individual line segments.
bgnpolygon	GL_POLYGON	Boundary of a simple convex polygon.
	GL_TRIANGLES	Triples of vertices interpreted as triangles.
bgntmesh	GL_TRIANGLE_STRIP	Linked strips of triangles.
	GL_TRIANGLE_FAN	Linked fans of triangles.
	GL_QUADS	Quadruples of vertices interpreted as quadrilaterals.
bgnqstrip	GL_QUAD_STRIP	Linked strips of quadrilaterals.

 

For a detailed discussion of the differences between triangle meshes, strips, and fans, see Porting Triangles.

There is no limit to the number of vertices you can specify between a glBegin / glEnd pair.

In addition to specifying vertices inside a glBegin / glEnd pair, you can specify a current normal, current texture coordinates, and a current color. The following table lists the commands valid inside a glBegin / glEnd pair.

IRIS GL function	OpenGL function	Meaning
v2, v3, v4	glVertex	Sets vertex coordinates.
RGBcolor, cpack	glColor	Sets current color.
color	glIndex	Sets current color index.
n3f	glNormal	Sets normal vector coordinates.
	glEvalCoord	Evaluates enabled one- and two-dimensional maps.
callobj	glCallList, glCallLists	Executes display list(s).
t2	glTexCoord	Sets texture coordinates.
	glEdgeFlag	Controls drawing edges.
lmbind	glMaterial	Sets material properties.

 

Note

If you use any OpenGL function other than those listed in the preceding table inside a glBegin / glEnd pair, you'll get unpredictable results, or possibly an error.