The Canvas Class

Lecture 8Fri, Sep 12, 2003

The Canvas Class

We will create a simple Canvas class that will encapsulate the commands that create and manage the drawing environment.The Canvas class user interface will make no references to OpenGL.

Disclaimer

This class is for demonstration purposes only.It would take more effort than it is worth to develop this class properly.It ought to include an interactive (mouse, keyboard) interface.You are welcome to develop it on your own.

Some Supporting Classes

Some classes supporting the Canvas class are Point2 – 2-dimensional points. Rect<T> – Rectangles with vertex

coordinates of type T (e.g., int or float).

Some future supporting classes Point3, Point4, Vector.

Classes

point2.hrect.hcanvas.hcanvas.cpp

The moveTo() Function

void Canvas::moveTo(float x, float y){ currPt.set(x, y);}

void Canvas::moveTo(Point2 p){ currPt = p;}

The lineTo() Function

void Canvas::lineTo(float x, float y){ glBegin(GL_LINES); glVertex2f(currPt.x, currPt.y); glVertex2f(x, y); glEnd(); currPt.set(x, y);}

The Canvas Class

DrawHouse.cpp

Drawing Circles

We have seen that to draw a circle, we should draw a polygon with many sides.float dx, dy, angle = 0.0;

glBegin(GL_POLYGON); for (int i = 0; i < 40; i++) { dx = cos(angle); dy = sin(angle); glVertex2f(cen.x + rad*dx, cen.y + rad*dy); angle += dAngle; }glEnd();

Drawing Circles

The following program segment uses the Canvas class to draw a circle.canvas.moveTo(cen.x + rad, cen.y);float angle = 0.0;float dAngle = PI/20.0;for (int i = 0; i < 40; i++){ float dx = cos(angle); float dy = sin(angle); canvas.lineTo(cen.x + rad*dx, cen.y + rad*dy); angle += dAngle;}

Drawing Circles

The previous example is inefficient.Why? It draws every point twice.

It would be better to include a drawCircle() member function in the Canvas class that uses GL_POLYGON for efficiency.

The drawCircle() Function

void Canvas::drawCircle(Point2 cen, float r, int n){ float angle = 0.0; float dAngle = 2*PI/n; glBegin(GL_POLYGON); for (int i = 0; i < n; i++) { float dx = cos(i*angle); float dy = sin(i*angle); glVertex2f(cen.x + rad*dx, cen.y + r*dy); angle += dAngle; } glEnd();}

Drawing Arcs

An arc is a portion of a circle.It can be described by a center a radius a start angle an end angle

Use the same algorithm as drawCircle(), but limit the range of the angle.

Example: Drawing Arcs

SmileyFace.cppPieChart.cpp