This document discusses OpenGL primitives for drawing geometric shapes. It explains that OpenGL uses vertices and primitive types to describe objects. Point, line, triangle, quad and polygon primitives are covered. Code examples are provided to demonstrate each primitive type, including how to specify vertices, colors, line widths and drawing modes. The document concludes by inviting the reader to design their own 2D scene on graph paper and then implement it using OpenGL primitives.

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PRACTICING OPENGL-
PRIMITIVES

2. OPENGL OUTPUT PRIMITIVES
Each geometric object is described by a set of vertices and the type of primitive to be
drawn. Whether and how the vertices are connected is determined by the primitive
type.

3. OPENGL PRIMITIVES- GL_POINTS
Lab Activity (1): Add the following code to display() method.
glPointSize(4.0); /* specify point to be 4 pixels thick */
glBegin(GL_POINTS);
glVertex2f(0.0f, 2.0f); //note 2D form
glVertex2f(1.0f, 2.0f);
glVertex2f(0.0f, -2.0f);
glVertex2f(-1.0f, 0.0f);
glEnd();

4. OPENGL PRIMITIVES- LINES
 Three different line primitives can be created:
GL_LINES: glColor3f(0.0,0.0,0.0);
glLineWidth(5.0);
glBegin(GL_LINES);
glVertex2f (-20.0, -20.0);
glVertex2f (20.0, 20.0);
glColor3f(1.0,1.0,0.0);
glVertex2f (10.0, -20.0);
glVertex2f (10.0, 20.0);
glEnd();

5. OPENGL PRIMITIVES- LINES
GL_LINE_STRIP: Draws a connected set of line segments from the first vertex to the
last.
glColor3f(0.5,0.6,0.0);
glLineWidth(4);
glBegin(GL_LINE_STRIP);
glVertex2f (5.0, 10.0);
glVertex2f (10.0, 10.0);
glVertex2f (10.0, 0.0);
glVertex2f (5.0, 0.0);
glEnd();

6. OPENGL PRIMITIVES- LINES
GL_LINE_LOOP:
glColor3f(0.9,0.8,0.0);
glLineWidth(4);
glBegin(GL_LINE_LOOP);
glVertex2f (-10.0, 9.0);
glVertex2f (5.0, 9.0);
glVertex2f (5.0, -1.0);
glVertex2f (-10.0, -1.0);
glEnd();

7. OPENGL PRIMITIVES –GL_TRIANGES
glColor3f(1.0,0.0,1.0);
glLineWidth(4); -The default is GL_FILL for both front-and back-facing
polygons.
glBegin(GL_TRIANGLES);
glVertex2f (-10.0, -10.0);
glVertex2f (-5.0, -10.0);
glVertex2f (-5.0, -5.0);
glEnd();
 Insert this command before glLineWidth, and note the effect.
glPolygonMode(GL_FRONT, GL_LINE);
GL_BACK GL_POINT
or or
GL_FRONT_AND_BACK GL_FILL

8. OPENGL PRIMITIVES –
GL_TRIANGLE_STRIP
glColor3f(0.0,1.0,0.0);
glLineWidth(4);
glPolygonMode(GL_FRONT, GL_LINE);
glBegin(GL_TRIANGLE_STRIP);
glVertex2f (-10.0, -10.0);
glVertex2f (-19.0, -10.0);
glVertex2f (-10.0, -20.0);
glVertex2f (-18.0,-19.0); //create 2ndtriangle
glVertex2f (-5.0, -25.0); //create 3rd triangle
glVertex2f (-20.0, -29.0); //create 4th triangle
glEnd();
Note:
• Order of points does matter!
• If the vertices are defined clockwise, the front of the polygon will
be shown. Otherwise, the back of the polygon will be shown.

9. OPENGL PRIMITIVES –
GL_TRIANGLE_FAN
Draws a connected set of triangles. One triangle is defined for each vertex
presented after the first two vertices.
Note: Order of points does matter!
glColor3f(0.0,0.0,1.0);
glLineWidth(4);
/* draw only the outline of polygon */
glPolygonMode(GL_FRONT, GL_LINE);
glBegin( GL_TRIANGLE_FAN);
glVertex2f (12.0, -30.0);
glVertex2f(30.0, -30.0);
glVertex2f (30.0, -20.0);
glVertex2f (22.0, -15.0);
glVertex2f (12.0, -12.0);
glEnd();

10. OPENGL PRIMITIVES –GL_QUADS
Note: Order of points does matter!
/* creating 2 quadrilaterals */
glColor3f(0.0,0.0,0.0);
glPolygonMode(GL_FRONT, GL_LINE);
glLineWidth(4);
glBegin( GL_QUADS );
glVertex2f (-28.0, 25.0);
glVertex2f (-28.0, 10.0);
glVertex2f (-20.0, 10.0);
glVertex2f (-20.0, 20.0);
glVertex2f (-15.0, 20.0);
glVertex2f (-2.0, 20.0);
glVertex2f (-2.0, 28.0);
glVertex2f (-15.0, 28.0);
glEnd();

11. OPENGL PRIMITIVES –
GL_QUAD_STRIP
/* creating 2 quadrilaterals using GL_QUAD_STRIP */
glColor3f(0.0,1.0,1.0);
glPolygonMode(GL_FRONT, GL_LINE);
glLineWidth(4);
glBegin( GL_QUAD_STRIP);
glVertex2f (20.0, 10.0);
glVertex2f (29.0, 13.0);
glVertex2f (18.0, 20.0);
glVertex2f (26.0, 20.0);
glVertex2f (22.0, 25.0);
glVertex2f (30.0, 25.0);
glVertex2f (15.0, 30.0);
glVertex2f (35.0, 30.0);
glEnd();

12. OPENGL PRIMITIVES –
GL_POLYGON
glBegin(GL_POLYGON);
glVertex2f(2.0,1.0);
glVertex2f(12.0,1.0);
glVertex2f(14.0,3.0);
glVertex2f(1.0,3.0);
glEnd();

13. CREATIVE DRAWING USING OPENGL
PRIMITIVES

14. LAB EXERCISE: TIME TO SHOW YOUR
CREATIVITY
“Sample 2D Scenary are given Lab folder”
1. Show your best to Design your virtual World (2D Scene) onto graph
paper.
 With Well defined coordinates Points of each object in Scene.
 Also Define Color of each Object Of Your Scene
1. Use Open GL built in primitives to Implement your
Designed virtual world (2D Scene).
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