Normals to a drawn line at runtime (incorrect math)?

Question

Hello.I have an equation which seemingly gives back Normals (line segments) to a line drawn with ActionScript and the mouse.If I go from lesser mouseX position to greater mouseX position it seems to work fine.line1Arr.rotation=45*Math.atan2(mouseYArr-mouseYArr[i-1], mouseXArr-mouseXArr[i-1]);line1Arr is the instance of the line segment Normal that is being attached to the line and mouseXArr and mouseXArr[i-1] is the most recent point and the one before it.The problem is that after having done a web search I think my math is not correct to calculate the normal.  I have found a thread somewhere which mentions to use derivative of x and derivative of y to calculate the normals.  Another member in the thread posted that one can also use sine and cosine to calculate this.This is the problematic equation that I tried when going mouseX in the opposite directionline1Arr.rotation=(-45)*Math.atan2(mouseXArr[i-1]-mouseXArr[i-0], mouseYArr[i-1]-mouseYArr[i-0]);Any help with this math problem is greatly appreciated.

kglad · Accepted Answer

because you're using a series of lines, there's no need to use trig: basic cartesian geometry is sufficient.

for example, if you have an array of [x,y] points used in your drawing, calling normalsF will add perpendiculars to the midpoint of those segments. (for an example, http://www.kglad.com/Files/forums/normals.fla)

function normalsF():void{

for(var i=0;i<curveA.length-1;i++){

normalF(curveA,curveA[i+1]);

}

function normalF(p1:Array,p2:Array):void{

m = -(p1[0]-p2[0])/(p1[1]-p2[1]);

b = (p1[1]+p2[1])/2 - m*(p1[0]+p2[0])/2;

drawnormalF(m,b,[(p1[0]+p2[0])/2, (p1[1]+p2[1])/2]);

}

function drawnormalF(m:Number,b:Number,pt:Array):void{

len = Math.sqrt(400/(4+4*m*m));

with(curveParent.graphics){

moveTo(pt[0]-len,m*(pt[0]-len)+b);

lineTo(pt[0]+len,m*(pt[0]+len)+b);

}

kglad · Answer

are you trying to find a normal to a line or a curve?do you have the equation for the line or curve? if yes, is that a standardized (eg, y=..) or parametric or something else equation?

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