How to get intersections between a polygon and a straight line?

beginners
#1

I want to get intersections between a blob’s outline and a line. I find that I can draw out the intersection of the blob’s outline and a very thin rectangle by doing this:

int num = contours.nBlobs;//get blob's contour points number

if(num>0){
    npts=contours.blobs[0].nPts;
    
    ofSetHexColor(0xb5de10);
    ofPushMatrix();
    ofSetPolyMode(OF_POLY_WINDING_ABS_GEQ_TWO);

    //ofSetPolyMode(OF_POLY_WINDING_NONZERO);
    ofBeginShape();
   for(int i=0; i<npts; i++){     //draw the blobs
        int x = contours.blobs[0].pts[i].x;
        int y = contours.blobs[0].pts[i].y+500;
        ofVertex(x,y);

    }
    
    ofNextContour(true);     
   ofVertex(w/2,h+500);   // a very thin rectangle
    ofVertex(w/2+2,h+500);
    ofVertex(w/2+2,h/2);
    ofVertex(w/2,h/2);
    
    ofEndShape(true);
    ofPopMatrix();
    
    
}

But still cannot get the upper intersection point and lower intersection point’s coordinates. Any one have an idea how to do that??
Thank so much.

#2

There are a bunch of ways to do this. A few references:

Check out the clipLineToPath method. It takes a path and chops a line up based on intersections.
https://github.com/bakercp/ofxFont/blob/master/src/ofFrameSetter.cpp#L204

You might also check https://github.com/alexiswolfish/ofxSweepLine