Draw arrow on line

I have this code:

CGPoint arrowMiddle = CGPointMake((arrowOne.x + arrowTo.x)/2, (arrowOne.y + arrowTo.y)/2);

CGPoint arrowLeft = CGPointMake(arrowMiddle.x-40, arrowMiddle.y);
CGPoint arrowRight = CGPointMake(arrowMiddle.x, arrowMiddle.y + 40);

[arrowPath addLineToScreenPoint:ar开发者_高级运维rowLeft];
[arrowPath addLineToScreenPoint:arrowMiddle];
[arrowPath addLineToScreenPoint:arrowRight];
[[mapContents overlay] addSublayer:arrowPath];
[arrowPath release];

with this output:

http://img517.yfrog.com/img517/7690/schermafbeelding2010032.png

What have i to add to get the left and right the at same degree of the line + 30°.

If someone has the algorithm of drawing an arrow on a line, pleas give it. It doesn't matter what programming language it is...

Thanks

Here is what you do. First, take the vector of the line and normalize it by dividing it by its length — this will give you a vector of length 1 pointing in the direction of the line. Next, multiply it by the length you need it to be. Turn it by 120° and -120° to make the arrow. Finally, offset it by the coordinates where you want it to be. Here is how it would look like in code:

// calculate the position of the arrow
CGPoint arrowMiddle;
arrowMiddle.x = (arrowOne.x + arrowTo.x) / 2;
arrowMiddle.y = (arrowOne.y + arrowTo.y) / 2;

// create a line vector
CGPoint v;
v.x = arrowTo.x - arrowOne.x;
v.y = arrowTo.y - arrowOne.y;

// normalize it and multiply by needed length
CGFloat length = sqrt(v.x * v.x + v.y * v.y);
v.x = 40 * (v.x / length);
v.y = 40 * (v.y / length);

// turn it by 120° and offset to position
CGPoint arrowLeft = CGPointApplyAffineTransform(v, CGAffineTransformMakeRotation(3.14 * 2 / 3));
arrowLeft.x = arrowLeft.x + arrowMiddle.x;
arrowLeft.y = arrowLeft.y + arrowMiddle.y;

// turn it by -120° and offset to position
CGPoint arrowRight = CGPointApplyAffineTransform(v, CGAffineTransformMakeRotation(-3.14 * 2 / 3));
arrowRight.x = arrowRight.x + arrowMiddle.x;
arrowRight.y = arrowRight.y + arrowMiddle.y;

Thanks for respond! In the meanwhile I found also an solution.

It's Like this:

double slopy , cosy , siny;
                double Par = 10.0;  //length of Arrow (>)
                slopy = atan2( ( arrowOne.y - arrowTo.y ),
                              ( arrowOne.x - arrowTo.x ) );
                cosy = cos( slopy );
                siny = sin( slopy ); //need math.h for these functions


            CGPoint arrowMiddle = CGPointMake((arrowOne.x + arrowTo.x)/2, (arrowOne.y + arrowTo.y)/2);
            [arrowPath addLineToScreenPoint:arrowMiddle];

            CGPoint arrowLeft = CGPointMake( arrowMiddle.x + round( - Par * cosy - ( Par / 2.0 * siny ) ), arrowMiddle.y + round( - Par * siny + ( Par / 2.0 * cosy ) ) );
            [arrowPath addLineToScreenPoint:arrowLeft];

            CGPoint arrowRight = CGPointMake( arrowMiddle.x + round( - Par * cosy + ( Par / 2.0 * siny ) ),arrowMiddle.y - round( Par / 2.0 * cosy + Par * siny ) );
            [arrowPath addLineToScreenPoint:arrowRight];

            [arrowPath addLineToScreenPoint:arrowMiddle];
            [[mapContents overlay] addSublayer:arrowPath];
            [arrowPath release];

The only problem here is that i draw it like it's an RMPath(route-me framework) and that the arrow gets bigger/smaller when you zoom in/out.

But thanks for respond, I will look into it which code is the most perform.