Line Intersect Triangle

[kevin]

Line Intersect Triangle


  The demo shows a line and spinning triangle.  You can position the line head / tail by holding the left or right mouse buttons down

PlayBASIC Code: [Select]

        openscreen 1000,950,32,true
        // Mouse buttons to position the head / tail of the test line
        // Space key to end 

        Type tTriangle
                X1#,Y1#
                X2#,Y2#
                X3#,Y3#
        EndType

        dim poly as tTriangle pointer
        poly = new tTriangle
        
        Size# =500        
        Angle#=0
        poly.x1 = GetScreenWidth()/2
        poly.y1 = GetScreenHeight()/2
    
        x1# = 100
        y1# = 100
        
        x2# = 700
        y2# = 500
        
        Setfps 61.7
        
        do
                cls 

                angle#=wrapangle(angle#,0.2)
                    
                if LeftKey() then angle#=wrapangle(angle#-1)
                if RightKey() then angle#=wrapangle(angle#+1)

                poly.x2 = poly.x1 +cos(angle#-35)*Size#
                poly.y2 = poly.y1 +sin(angle#-35)*Size#
                poly.x3 = poly.x1 +cos(angle#+35)*Size#
                poly.y3 = poly.y1 +sin(angle#+35)*Size#

                if leftmousebutton()
                        x1#=mousex()
                        y1#=mousey()
                endif
                    
                if rightmousebutton()
                        x2#=mousex()
                        y2#=mousey()
                endif

                DrawPolygon(Poly,$334455)

                State=LineIntersectTriangle(x1#,y1#,x2#,y2#, poly)
    
                ThisRGB = -1
                if State=0 then ThisRGB = $ff0000 
                linec x1#,y1#,x2#,y2#,ThisRGB

                ink ThisRGB
                print "Line Intersect Triangle:"+str$(State)
                ink -1
                
            Sync
        loop spacekey()
    
    
        end
    


psub Cross(x1#,y1#,x2#,y2#,x3#,y3#)
        result=(((x2#-x1#)*(y3#-y1#))-((x3#-x1#)*(y2#-y1#)))
endpsub result




psub DrawPolygon(Poly as tTriangle pointer,ThisRGB)
        tric Poly.X1#,Poly.Y1,Poly.X2#,Poly.Y2#,Poly.X3#,Poly.Y3#, ThisRGB    
EndPsub



function LineIntersectTriangle(lx1#,ly1#,lx2#,ly2#, poly as tTriangle pointer)

        px1#=poly.x1#
        py1#=poly.y1#
        px2#=poly.x2#
        py2#=poly.y2#
        px3#=poly.x3#
        py3#=poly.y3#

        //  test #1 of vertex is clock wise (outside)
        P1 =  (Cross(lx1#,ly1#,px1#,py1#,px2#,py2#)>0) * 4
        P1+=  (Cross(lx1#,ly1#,px2#,py2#,px3#,py3#)>0) * 2    
        P1+=  (Cross(lx1#,ly1#,px3#,py3#,px1#,py1#)>0)
        if p1=%111 then ExitFunction true    

        P2 =  (Cross(lx2#,ly2#,px1#,py1#,px2#,py2#)>0) * 4
        P2+=  (Cross(lx2#,ly2#,px2#,py2#,px3#,py3#)>0) * 2    
        P2+=  (Cross(lx2#,ly2#,px3#,py3#,px1#,py1#)>0)
        if p2=%111 then ExitFunction true    

        if LinesIntersect(lx1#,ly1#,lx2#,ly2#,px1#,py1#,px2#,py2#) then ExitFunction true            
        if LinesIntersect(lx1#,ly1#,lx2#,ly2#,px2#,py2#,px3#,py3#) then ExitFunction true            
        State=LinesIntersect(lx1#,ly1#,lx2#,ly2#,px3#,py3#,px1#,py1#)
        
EndFunction State

  More reading:

    - Lines Intersect

    - 2D portal rendering