1. The Canvas Class Lecture 8 Fri, Sep 12, 2003

2. 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.

3. 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.

4. Some Supporting Classes Some classes supporting the Canvas class are Point2 – 2-dimensional points. Rect – Rectangles with vertex coordinates of type T (e.g., int or float). Some future supporting classes Point3, Point4, Vector.

5. Classes point2.h rect.h canvas.h canvas.cpp

6. The moveTo() Function void Canvas::moveTo(float x, float y) { currPt.set(x, y); } void Canvas::moveTo(Point2 p) { currPt = p; }

7. 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); }

8. The Canvas Class DrawHouse.cpp

9. 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();

10. 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; }

11. 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.

12. 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(); }

13. 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.

14. Example: Drawing Arcs SmileyFace.cpp PieChart.cpp