From 78090c2f6232444ba9d2727af6617a52bcfaf349 Mon Sep 17 00:00:00 2001
From: Chris Greene <archgoon@gmail.com>
Date: Sun, 21 Apr 2013 11:10:10 -0700
Subject: [PATCH] Bounds checking modified to remove spinning

By having the particles stop at the first point where they
cross the boundary line, we prevent the artificial torque
exerted on the semi-rigid body.
---
 images/error_type1.png | Bin 0 -> 5630 bytes
 js/verlet-js/verlet.js |  51 ++++++++++++++++++++++++++++++++++-------
 2 files changed, 43 insertions(+), 8 deletions(-)
 create mode 100644 images/error_type1.png

diff --git a/images/error_type1.png b/images/error_type1.png
new file mode 100644
index 0000000000000000000000000000000000000000..bdcde3b374485ef04bb12ed7873c8c887f215550
GIT binary patch
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diff --git a/js/verlet-js/verlet.js b/js/verlet-js/verlet.js
index 44a8710..8cc817d 100644
--- a/js/verlet-js/verlet.js
+++ b/js/verlet-js/verlet.js
@@ -44,6 +44,34 @@ Particle.prototype.draw = function(ctx) {
 	ctx.fill();
 }
 
+/**
+*
+* Computes time of intersection of a particle with a wall
+*
+* @param {Vec2} line    wall's root position
+* @param {Vec2} p       particle's position
+* @param {Vec2} dir     walls's direction
+* @param {Vec2} v       particle's velocity
+*/
+var intersectionTime = function(wall, p, dir, v) {
+    if (dir.x != 0) {
+        var denominator = v.y - dir.y * v.x / dir.x ;
+        if (denominator == 0) return undefined; // Movement is parallel to wall
+        var numerator = wall.y + dir.y * (p.x - wall.x) / dir.x - p.y;
+        return numerator / denominator;
+    } else { 
+        if (v.x == 0) return undefined; // parallel again
+        var denominator = v.x;
+        var numerator = wall.x - p.x;
+        return numerator / denominator;
+    }
+}
+
+var intersectionPoint = function(wall, p, dir, v) {
+    var t = intersectionTime(wall, p, dir, v);
+    return new Vec2(p.x + v.x * t, p.y + v.y * t);
+}
+
 var VerletJS = function(width, height, canvas) {
 	this.width = width;
 	this.height = height;
@@ -54,16 +82,24 @@ var VerletJS = function(width, height, canvas) {
 	this.draggedEntity = null;
 	this.selectionRadius = 20;
 	this.highlightColor = "#4f545c";
-	
+	var v = new Vec2();
 	this.bounds = function (particle) {
-		if (particle.pos.y > this.height-1)
-			particle.pos.y = this.height-1;
+        v.mutableSet(particle.pos);
+        v.mutableSub(particle.lastPos);
+		if (particle.pos.y > this.height-1) {
+            particle.pos.mutableSet(
+                intersectionPoint(new Vec2(0,this.height-1), particle.lastPos, new Vec2(1,0), v));
+        }
 		
-		if (particle.pos.x < 0)
-			particle.pos.x = 0;
+		if (particle.pos.x < 0) {
+            particle.pos.mutableSet(
+                intersectionPoint(new Vec2(0,0), particle.pos, new Vec2(0,1), v));
+        }
 
-		if (particle.pos.x > this.width-1)
-			particle.pos.x = this.width-1;
+		if (particle.pos.x > this.width-1) {
+			particle.pos.mutableSet(
+                intersectionPoint(new Vec2(this.width-1,0), particle.pos, new Vec2(0,1), v));
+        }
 	}
 	
 	var _this = this;
@@ -227,4 +263,3 @@ VerletJS.prototype.nearestEntity = function() {
 	
 	return entity;
 }
-