Determining whether a user specified point is inside a 2d polygon - c++

Question

I've done some research and there are many methods to doing this if the vertices and points are predetermined as seen here. However in my case, everything is specified by the user. The code I have put together (with the help of others), allows the user to create the polygon, and place points. I wrote functions to try and create vectors from the point to the vertices and then compute the angle. If it comes to 360 it should be inside and it should be colored green. Else it should be outside and red.
This is what I've been working on, but I cant seem to figure it out:
(Edit: Included my entire code)
GLint vert[100][2];
int width = 400, height = 600, n = 0, m = 0, type = GL_LINE_STRIP, v;
bool rubberbanding = false;
bool closed = false;
double testx, testy;
bool isitin;
double dotp(double x1, double y1, double x2, double y2) {
double a;
a = (x1 * x2) + (y1 * y2);
return a;
}
double mag(double x1, double y1, double x2, double y2) {
double a;
double x = (x2 - x1);
double y = (y2 - y1);
a = sqrt((x*x) + (y*y));
return a;
}
bool inpoly(int numv, GLint vx[][2], GLint vy[][2], double tx, double ty) {
double angle = 0.0;
int n = 0;
while (n != numv) {
//vector from point to vertex
double newv1x = vx[n][0] - tx;
double newv1y = vy[n][1] - ty;
double magv1 = mag(tx, ty, vx[n][0], vy[n][1]); //magnitude of vector
//vector from point to next vertex
double newv2x = vx[n + 1][0] - tx;
double newv2y = vy[n + 1][1] - ty;
double magv2 = mag(tx, ty, vx[n+1][0], vy[n+1][1]);//magnitude of vector
//dot product between the two vectors
double dp = dotp(newv1x, newv1y, newv2x, newv2y);
//angle between two vectors
double vang = acos(dp / (magv1*magv2));
angle += vang;
n++;
}
//vector from point to last vertex
double newv1x = vx[numv][0] - tx;
double newv1y = vy[numv][1] - ty;
double magv1 = mag(tx, ty, vx[numv][0], vy[numv][1]); //magnitude of vector
//vector from point to first vertex
double newv2x = vx[0][0] - tx;
double newv2y = vy[0][1] - ty;
double magv2 = mag(tx, ty, vx[0][0], vy[0][1]);//magnitude of vector
//dot product between the two vectors
double dp = dotp(newv1x, newv1y, newv2x, newv2y);
//angle between two vectors
double vang = acos(dp / (magv1*magv2));
angle += vang;
if (angle == 360.0) return true;
return false;
}
void display(){
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1, 1, 0);
glBegin(closed ? GL_LINE_LOOP : GL_LINE_STRIP);
for(int i = 0; i < m; i++){
glVertex2iv(vert[i]);
}
glEnd();
/*
glColor3f(0, 0, 1);
glBegin(GL_POINTS);
for (int i = m; i < n; i++) {
glVertex2iv(vert[i]);
}
*/
isitin = inpoly(m, vert, vert, testx, testy);
if (isitin == true) {
glColor3f(0, 1, 0);
glBegin(GL_POINTS);
for (int i = m; i < n; i++) {
glVertex2iv(vert[i]);
}
}
else {
glColor3f(1, 0, 0);
glBegin(GL_POINTS);
for (int i = m; i < n; i++) {
glVertex2iv(vert[i]);
}
}
glEnd();
glutSwapBuffers();
glutPostRedisplay();
}
void keyboard(unsigned char key, int x, int y){
switch(key){
case 'r': n = 0; m = 0; closed = false; break;
case 'c': closed = true; break;
}
glutPostRedisplay();
}
int findVertex(int x, int y){
int dx, dy;
for(int i = 0; i < n; i++){
dx = vert[i][0] - x;
dy = vert[i][1] - y;
if(dx*dx + dy*dy < 16) return i;
}
return - 1;
}
void mousemove(int x, int y)
{
testx = x;
testy = height - 1 - y;
}
void mouse(int button, int state, int x, int y){
switch(button){
case GLUT_LEFT_BUTTON:
if(state == GLUT_DOWN){
if (n < 100) {
v = n++;
vert[v][0] = x;
vert[v][1] = height - 1 - y;
// n++;
rubberbanding = true;
glutPostRedisplay();
if (!closed) m = n;
}
}
else{
rubberbanding = false;
}
break;
}
}
void motion(int x, int y){
if(rubberbanding){
vert[v][0] = x;
vert[v][1] = height - 1 - y;
glutPostRedisplay();
}
}
void main(int argc, char** argv){
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE);
glutInitWindowSize(width,height);
glutInitWindowPosition(50,100);
glutCreateWindow("Project 3");
glClearColor(0.0,0.0,0.0,0.0);
glColor3f( 1, 1, 0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, width, 0, height);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glutDisplayFunc(display);
glutKeyboardFunc(keyboard);
glutMouseFunc(mouse);
glutPassiveMotionFunc(mousemove);
glutMotionFunc(motion);
glutMainLoop();
}
When I run the program in Visual Studio, I can draw the polygon, and I can specify
points, but for some reason all the points appear red. If anyone has any ideas on how to fix this, it would be greatly appreciated.

Answer 1

Probably your error is that acos returns in radians, and you're testing if the sum equals 360 degrees.
Also, you shouldn't compare doubles that way, since that calculation is probably adding rounding error in each sum. See here for more information.