Drawing Basic Graphics Primitives

Lecture 4Wed, Sep 3, 2003

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.

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.

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.

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.

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.

Primitive Objects

glBegin(type) defines the type of object to be drawn.Some values of type are GL_POINTS GL_LINES GL_TRIANGLES GL_POLYGON

glEnd() marks the end of the object.

Creating Points

The following program segment will draw three pixels.

glBegin(GL_POINTS); glVertex2i(10, 20); glVertex2i(50, 20); glVertex2i(50, 50);glEnd():

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():

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():

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.

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?

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.

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();

Example: Draw an X

DrawX.cpp

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?

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();

Example: Draw an Octagon

DrawOctagon.cpp