How to avoid restarting the polygon when redraw? - c++

Question

I'm writing the program that continuously draws a polygon until the user clicks right-click, but when I continue to draw something else on the screen the polygon disappears, how can I avoid this? This is my program:
float mouseX, mouseY;
vector<float> vecX(40);
vector<float> vecY(40);
int numPoints = 0;
int closed = 0;
void mouse(int button, int state, int x, int y)
{
mouseX = x;
mouseY = y;
if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
{
if (closed || numPoints > 40)
numPoints = 0;
closed = 0;
vecX[numPoints] = mouseX;
vecY[numPoints] = mouseY;
numPoints++;
}
if (button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN)
closed = 1;
}
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
if (numPoints)
{
glBegin(GL_LINE_STRIP);
for (int i = 0; i < numPoints; ++i)
glVertex2f(vecX[i], vecY[i]);
if (closed)
glVertex2f(vecX[0], vecY[0]);
else
glVertex2f(mouseX, mouseY);
glEnd();
}
glFlush();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(600, 400);
glutInitWindowPosition(0, 0);
glutCreateWindow("Testing");
gluOrtho2D(0, 600, 400, 0);
glutDisplayFunc(display);
glutMouseFunc(mouse);
glutMainLoop();
}
The vecX and vecY are used to store the coordinate of the mouse click.

Answer 1

Once a polyline is finished, you need to store it to a container. Use a std::vector of polylines. The type of a polyline is std::vector<float>. At the begin, just 1 empty polyline is in the container:
std::vector<std::vector<float>> polylines(1);
When you left-click, a new vertex coordinate is added to the last polyline in the container. When you right-click, a new polyline is added to the container:
void mouse(int button, int state, int x, int y)
{
mouseX = x;
mouseY = y;
if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
{
polylines.back().push_back(mouseX);
polylines.back().push_back(mouseY);
}
if (button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN)
{
polylines.push_back(std::vector<float>());
}
}
Draw the polylines in nested loops:
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
for (auto i=0; i < polylines.size(); ++ i)
{
bool is_last = i == polylines.size() - 1;
const auto& polyline = polylines[i];
glBegin(is_last ? GL_LINE_STRIP : GL_LINE_LOOP);
for (auto j = 0; j < polyline.size(); j += 2)
glVertex2f(polyline[j], polyline[j+1]);
if (is_last)
glVertex2f(mouseX, mouseY);
glEnd();
}
glutSwapBuffers();
}