manipulating lines in a GL_LINE_STRIP

Question

I am making a simple 2d drawing program in which i have an array of vertices(say a1,a2,a3,.....) which is to be drawn using GL_LINE_STRIP .Problem is that , say there are 3 consecutive vertices : p1, p2, p3, now i want that if (p2==some specfic vertex value) then the GL_LINE_STRIP should end at p1 and a new GL_LINE_STRIP start at p3, ignoring p2. I want to make a break at p2 in the strip. How do i do that?? PS: I tried making (line width = 0 at p2 in the loop of line strip but found out that since opengl is a state machine, we cant change width inside glBegin and GLEnd. any other alternative guys???

HERE IS SOME FAILED SAMPLE CODE:

GLfloat thickness;
glBegin(GL_LINE_STRIP);
for(int i = 0; i < points.size(); i++)//points has all vertices stored...points is an array where each element is a structure containing x and y values
{
    if(points[i].x==randomPoint.x || points[i-1].x==randomPoint.x){//checking if it is equal to p2(i am only checking x coordinate,which is sufficient for me to know that this point is to be excluded)
        thickness = 0;

    }else {
        thickness = 4;

    }

    glLineWidth(thickness);
    glVertex2i(points[i].x, points[i].y);
}

SporreKing SporreKing · Accepted Answer · 2016-03-08T15:19:22

If I've got the question right you want to stop the linestrip at a vertex if it is a specific value, and start a new one on the next vertex. (Correct me if I've misunderstood)

Call glEnd() after the last vertex you want to connect and start a new glBegin() when you want to start a new strip.

As for the width part you would have to set the width before the call to glBegin(). A linestrip (afaik) can not have it's width changed in a draw state as you say. You would have to end the draw call, change the width, go 1 vertex back and start a new draw call.

manipulating lines in a GL_LINE_STRIP

1 Answers