From 7b4dedf0ce66c405d6a4c4006862350053c71127 Mon Sep 17 00:00:00 2001
From: mattkae <mattkae@protonmail.com>
Date: Sat, 29 Jan 2022 10:44:32 -0500
Subject: Doing undamped simulation correctly

---
 2d/softbody/softbody_1/undamped.cpp | 37 +++++++++++++++++++++----------------
 1 file changed, 21 insertions(+), 16 deletions(-)

(limited to '2d/softbody/softbody_1/undamped.cpp')

diff --git a/2d/softbody/softbody_1/undamped.cpp b/2d/softbody/softbody_1/undamped.cpp
index b041a6f..68e0636 100644
--- a/2d/softbody/softbody_1/undamped.cpp
+++ b/2d/softbody/softbody_1/undamped.cpp
@@ -36,13 +36,11 @@ namespace Undamped {
 
         // -- Spring initial variables
         float32 k = 4; // Spring Constant, in N / m
-        float32 initialDisplacement = 3.f;
-        float32 initialVelocity = 0.f;
 
         // -- Spring runtime variables
-        float32 angularVelocity = 0.f;
-        float32 displacement = 0.f;
-        float32 timeElapsed = 0.f;
+		float32 force = 0.f;
+        float32 velocity = 0.f;
+        float32 position = 0.f;
 
         void load(Renderer2d* renderer, SpringWeight* inWieight, float32 length, float32 inInitialDisplacement, float32 inK, float32 loopRadius);
         void update(float32 dtSeconds);
@@ -106,6 +104,7 @@ namespace Undamped {
     void SpringWeight::load(Renderer2d* renderer, float32 inMass, Vector4 startColor, Vector4 endColor) {
         mass = inMass;
         radius = mass * 16.f;
+		if (radius > 42) radius = 42.f;
         const int32 numSegments = 96;
         const float32 radiansPerSegment = (2.f * PI) / static_cast<float>(numSegments);
         const int32 numVertices = numSegments * 3;
@@ -149,14 +148,12 @@ namespace Undamped {
         shape.unload();
     }
 
-    void Spring::load(Renderer2d* renderer, SpringWeight* inWeight, float32 length, float32 inInitialDisplacement, float32 inK, float32 loopRadius) {
+    void Spring::load(Renderer2d* renderer, SpringWeight* inWeight, float32 length, float32 initialDisplacement, float32 inK, float32 loopRadius) {
         weight = inWeight;
-        initialDisplacement = inInitialDisplacement;
-		displacement = 0;
+	    position = initialDisplacement;
         k = inK;
-
-        angularVelocity = sqrtf(k / weight->mass);
-        timeElapsed = 0.f;
+		force = -k * initialDisplacement;
+        velocity = sqrtf(k / weight->mass);
         
         const int32 verticesPerSegment = 6;
         numSegments = 256;
@@ -170,12 +167,19 @@ namespace Undamped {
         const float32 offset = 0.25f;
 
         int32 vidx = 0;
+		float32 dx = position;
         for (int pidx = 0; pidx < numSegments; pidx++) {
             float32 y1 = lengthIncrement * pidx;
             float32 x1 = loopWidth * sinf(frequency * y1 + offset);
 
             float32 y2 = y1 + lengthIncrement;
             float32 x2 = loopWidth * sinf(frequency * y2 + offset);
+
+			float32 y1Offset = dx * (1.f - pidx / static_cast<float32>(numSegments));
+            float32 y2Offset = dx * (1.f - (pidx + 1) / static_cast<float32>(numSegments));
+
+			y1 += y1Offset;
+			y2 += y2Offset;
         
             vertices[vidx++].position = Vector2(x1, y1);
             vertices[vidx++].position = Vector2(x1, y2);
@@ -188,15 +192,16 @@ namespace Undamped {
         shape.load(vertices, numVertices, renderer, GL_DYNAMIC_DRAW);
         shape.model = Mat4x4().translateByVec2(Vector2(400, 300));
 
-        weight->shape.model = shape.model.translateByVec2(Vector2(0, -weight->radius));
+        weight->shape.model = shape.model.translateByVec2(Vector2(0, -weight->radius + dx));
     }
 
     void Spring::update(float32 dtSeconds) {
-        timeElapsed += dtSeconds;
-        float32 lastDisplacement = displacement;
-        displacement = initialDisplacement * cosf(angularVelocity * timeElapsed - initialVelocity);
-        float32 dx = displacement - lastDisplacement;
+		float32 lastPosition = position;
+		force = -k * position;
+	    velocity = velocity + (force * dtSeconds) / weight->mass;
+		position = position + velocity * dtSeconds;
 
+		float32 dx = position - lastPosition;
         int32 vidx = 0;
         for (int pidx = 0; pidx < numSegments; pidx++) {
             float32 y1Offset = dx * (1.f - pidx / static_cast<float32>(numSegments));
-- 
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