Added limit velocity and drag to GPU particles

dist/preview release/what's new.md

@@ -34,6 +34,7 @@
   - Added support for angular speed gradients. [Doc](https://doc.babylonjs.com/babylon101/particles#rotation)
   - Added support for velocty gradients. [Doc](https://doc.babylonjs.com/babylon101/particles#velocity-over-time)
   - Added support for limit velocty gradients. [Doc](https://doc.babylonjs.com/babylon101/particles#limit-velocity-over-time)
+  - Added support for drag gradients. [Doc](https://doc.babylonjs.com/babylon101/particles#drag)
   - Added support for noise textures. [Doc](http://doc.babylonjs.com/babylon101/particles#noise-texture)
   - Added support for emit rate gradients. [Doc](http://doc.babylonjs.com/babylon101/particles#emit-rate-over-time)
   - Start size gradient support for particles. [Doc](http://doc.babylonjs.com/babylon101/particles#start-size-over-time) ([TrevorDev](https://github.com/TrevorDev))

src/Particles/babylon.gpuParticleSystem.ts

@@ -519,9 +519,9 @@
                 attributes: ["position", "age", "life", "seed", "size", "color", "direction", "initialDirection", "angle", "cellIndex"],
                 uniformsNames: ["currentCount", "timeDelta", "emitterWM", "lifeTime", "color1", "color2", "sizeRange", "scaleRange","gravity", "emitPower",
                                 "direction1", "direction2", "minEmitBox", "maxEmitBox", "radius", "directionRandomizer", "height", "coneAngle", "stopFactor", 
-                                "angleRange", "radiusRange", "cellInfos", "noiseStrength"],
+                                "angleRange", "radiusRange", "cellInfos", "noiseStrength", "limitVelocityDamping"],
                 uniformBuffersNames: [],
-                samplers:["randomSampler", "randomSampler2", "sizeGradientSampler", "angularSpeedGradientSampler", "velocityGradientSampler", "noiseSampler"],
+                samplers:["randomSampler", "randomSampler2", "sizeGradientSampler", "angularSpeedGradientSampler", "velocityGradientSampler", "limitVelocityGradientSampler", "noiseSampler", "dragGradientSampler"],
                 defines: "",
                 fallbacks: null,  
                 onCompiled: null,
@@ -773,7 +773,15 @@
             
             if (this._velocityGradientsTexture) {
                 defines += "\n#define VELOCITYGRADIENTS";
-            }                    
+            }        
+
+            if (this._limitVelocityGradientsTexture) {
+                defines += "\n#define LIMITVELOCITYGRADIENTS";
+            }                
+            
+            if (this._dragGradientsTexture) {
+                defines += "\n#define DRAGGRADIENTS";
+            }              
             
             if (this.isAnimationSheetEnabled) {
                 defines += "\n#define ANIMATESHEET";
@@ -919,6 +927,14 @@
         private _createVelocityGradientTexture() {
             this._createFactorGradientTexture(this._velocityGradients, "_velocityGradientsTexture");
         }          
+
+        private _createLimitVelocityGradientTexture() {
+            this._createFactorGradientTexture(this._limitVelocityGradients, "_limitVelocityGradientsTexture");
+        }          
+
+        private _createDragGradientTexture() {
+            this._createFactorGradientTexture(this._dragGradients, "_dragGradientsTexture");
+        }            
             
         private _createColorGradientTexture() {
             if (!this._colorGradients || !this._colorGradients.length || this._colorGradientsTexture) {
@@ -959,6 +975,8 @@
             this._createSizeGradientTexture();
             this._createAngularSpeedGradientTexture();
             this._createVelocityGradientTexture();
+            this._createLimitVelocityGradientTexture();
+            this._createDragGradientTexture();
 
             this._recreateUpdateEffect();
             this._recreateRenderEffect();
@@ -1030,6 +1048,15 @@
                 this._updateEffect.setTexture("velocityGradientSampler", this._velocityGradientsTexture);      
             }
 
+            if (this._limitVelocityGradientsTexture) {      
+                this._updateEffect.setTexture("limitVelocityGradientSampler", this._limitVelocityGradientsTexture);  
+                this._updateEffect.setFloat("limitVelocityDamping", this.limitVelocityDamping);    
+            }            
+
+            if (this._dragGradientsTexture) {      
+                this._updateEffect.setTexture("dragGradientSampler", this._dragGradientsTexture);      
+            }
+
             if (this.particleEmitterType) {
                 this.particleEmitterType.applyToShader(this._updateEffect);
             }

src/Shaders/gpuUpdateParticles.vertex.fx

@@ -118,6 +118,15 @@ uniform sampler2D angularSpeedGradientSampler;
 uniform sampler2D velocityGradientSampler;
 #endif
 
+#ifdef LIMITVELOCITYGRADIENTS
+uniform sampler2D limitVelocityGradientSampler;
+uniform float limitVelocityDamping;
+#endif
+
+#ifdef DRAGGRADIENTS
+uniform sampler2D dragGradientSampler;
+#endif
+
 #ifdef NOISE
 uniform vec3 noiseStrength;
 uniform sampler2D noiseSampler;
@@ -300,6 +309,11 @@ void main() {
 #ifdef VELOCITYGRADIENTS
     directionScale *= texture(velocityGradientSampler, vec2(ageGradient, 0)).r;
 #endif
+
+#ifdef DRAGGRADIENTS
+    directionScale *= texture(dragGradientSampler, vec2(ageGradient, 0)).r;
+#endif
+
     outPosition = position + direction * directionScale;
     
     outLife = life;
@@ -318,7 +332,20 @@ void main() {
 #ifndef BILLBOARD    
     outInitialDirection = initialDirection;
 #endif
-    outDirection = direction + gravity * timeDelta;
+
+    vec3 updatedDirection = direction + gravity * timeDelta;
+
+#ifdef LIMITVELOCITYGRADIENTS
+    float limitVelocity = texture(limitVelocityGradientSampler, vec2(ageGradient, 0)).r;
+
+    float currentVelocity = length(updatedDirection);
+
+    if (currentVelocity > limitVelocity) {
+        updatedDirection = updatedDirection * limitVelocityDamping;
+    }
+#endif
+
+    outDirection = updatedDirection;
 
 #ifdef NOISE
     vec3 localPosition = outPosition - emitterWM[3].xyz;