Drawing Basic Graphics Primitives

1. Drawing Basic Graphics Primitives Lecture 4 Wed, Sep 3, 2003

2. OpenGL Functions • OpenGL function names always begin with gl, glu, or glut, depending on the library. • Names of functions that allow a variable number of arguments are followed by a digit indicating the number. • Names of functions that allow various argument types are followed by a letter indicating the type.

3. State Variables • OpenGL maintains a large number of state variables. • These state variables are global – they are in effect and accessible within all functions. • Some states are • Colors used in various situations. • Shading effects. • Lighting effects.

4. Setting the Color • One of the states maintained is the current color used for drawing. • The color is a combination of red, green, and blue. • Use glColor*() to set the current color. • glColor3f(1.0, 0.0, 0.0) – bright red. • glColor3f(1.0, 1.0, 0.0) – bright yellow. • glColor3f(0.5, 0.5, 0.5) – medium gray.

5. Drawing Dots • To color a single pixel, use the glVertex*() function. • glVertex2i(int, int). • glVertex3f(float, float, float), etc. • The pixel is colored with the current color, as set by glColor(). • The coordinates are in “world” coordinates, not screen coordinates.

6. Primitive Objects • The pixel-drawing function is called glVertex() because the pixel is typically a vertex in a polygon. • The programmer must indicate to the GPU whether the point is an isolated point or part of a line or polygon. • To do this, use glBegin() and glEnd() to enclose the vertices of the primitive objects.

7. Primitive Objects • glBegin(type) defines the type of object to be drawn. • Some values of typeare • GL_POINTS • GL_LINES • GL_TRIANGLES • GL_POLYGON • glEnd() marks the end of the object.

8. Creating Points • The following program segment will draw three pixels. glBegin(GL_POINTS); glVertex2i(10, 20); glVertex2i(50, 20); glVertex2i(50, 50); glEnd():

9. Creating Triangles • The following program segment will draw and fill a triangle. glBegin(GL_TRIANGLES); glVertex2i(10, 20); glVertex2i(50, 20); glVertex2i(50, 50); glEnd():

10. Creating Triangles • The following program segment will draw, but not fill, a triangle. glBegin(GL_LINE_LOOP); glVertex2i(10, 20); glVertex2i(50, 20); glVertex2i(50, 50); glEnd():

11. Drawing Primitives • For points, the size of the drawing pen is set by the function glPointSize(size). • For lines, the pen width is defined by the function glLineWidth(width). • The size and width are measured in pixels.

12. Example: DrawDots • DrawDots.cpp • Why does each dot disappear when the next dot is drawn? • How could we modify the program so that all the dots remained? • Do not clear the buffer, but keep adding to it? • Store all the points in a list and draw each one every time? • What happens when the window is resized?

13. Making Line Drawings • Drawing lines is similar to drawing points and triangles. • Use glBegin(GL_LINES); • Every pair of points drawn between glBegin() and glEnd() is rendered as a line. • We may list many pairs of points.

14. Example: Draw an X • The following program segment will draw two line segments that form an X. glBegin(GL_LINES); glVertex2i(50, 50); glVertex2i(100, 100); glVertex2i(50, 100); glVertex2i(100, 50); glEnd();

15. Example: Draw an X • DrawX.cpp

16. Drawing Polygons • If we use GL_POLYGON in the glBegin() function, then the entire list of points is used to draw a single polygon. • What would the last example look like? • Why not use GL_POLYGON to draw several polygons in one group, just as with GL_LINES?

17. Example: Draw an Octagon • The following program segment will draw a regular octagon. int cx = 100, cy = 100; glBegin(GL_POLYGON); for (int i = 0; i < 8; i++) { float dx = cos(i*PI/4); float dy = sin(i*PI/4); glVertex2i(x + 100*dx, y + 100*dy); } glEnd();

18. Example: Draw an Octagon • DrawOctagon.cpp