OpenGL - How can I move my printed lines? - c++

Question

I have set up a mouse function which is called in my main() like so:
struct point
{
int x;
int y;
};
std::vector <point> points;
point OnePoint;
void processMouse(int button, int state, int x, int y)
{
if ((button == GLUT_LEFT_BUTTON) && (state == GLUT_DOWN))
{
int yy;
yy = glutGet(GLUT_WINDOW_HEIGHT);
y = yy - y; /* In Glut, Y coordinate increases from top to bottom */
OnePoint.x = x;
OnePoint.y = y;
points.push_back(OnePoint);
}
glutPostRedisplay();
}
and to print a line through the vertices I have written some code in my display function that allows me to do just that:
glBegin(GL_LINES);
glColor3f(1.0, 0.0, 0.0);
for (int i = 0; i < points.size();i++)
{
glVertex2i(points[i].x, points[i].y);
}
glEnd();
However now what I want to do is move all the lines either left or right when I click the correct arrow keys to do so, but I can't figure out how.
I'm aware that it could possibly be something like:
glVertex2i(points[i].x + 10, points[i].y); // moving points 10 along the x axis
However since i is out of the for loop, I get error messages

Answer 1

You should introduce a new variable:
std::vector <point> points;
point offset; // <- how much to offset points
Make sure to set it to zero during initialization.
Then in the drawing code you add that offset to each point:
glBegin(GL_LINES);
glColor3f(1.0, 0.0, 0.0);
for (int i = 0; i < points.size();i++)
glVertex2i(points[i].x + offset.x, points[i].y + offset.y);
glEnd();
Or use a translation matrix to do that automagically:
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glTranslatef(offset.x, offset.y, 0);
glBegin(GL_LINES);
glColor3f(1.0, 0.0, 0.0);
for (int i = 0; i < points.size();i++)
glVertex2i(points[i].x, points[i].y);
glEnd();
glPopMatrix();
On the key-press handler you just change the offset:
// left-arrow:
offset.x -= 1;
// right-arrow:
offset.x += 1;
...