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# The Canvas Class

The Canvas Class. Lecture 8 Fri, 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.

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## The Canvas Class

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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<T> – 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

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