Merge pull request #6720 from ctrlaltdavid/20760

Fix particle radius and rgba start, middle, finish interpolation

examples/example/entities/particlesTest.js

@@ -71,7 +71,7 @@
                 Entities.editEntity(particles, {
                     radiusSpread: 0.0,
                     radiusStart: 0.0,
-                    particleRadius: 2 * PARTICLE_RADIUS,  // Bezier interpolation used means that middle value isn't intersected
+                    particleRadius: PARTICLE_RADIUS,
                     radiusFinish: 0.0
                 });
                 break;

libraries/entities-renderer/src/textured_particle.slv

@@ -56,13 +56,50 @@ float bezierInterpolate(float y1, float y2, float y3, float u) {
     return (1.0 - u) * (1.0 - u) * y1 + 2.0 * (1.0 - u) * u * y2 + u * u * y3;
 }
 
-vec4 interpolate3Vec4(vec4 y1, vec4 y2, vec4 y3, float u) {
-    return vec4(bezierInterpolate(y1.x, y2.x, y3.x, u),
-                bezierInterpolate(y1.y, y2.y, y3.y, u),
-                bezierInterpolate(y1.z, y2.z, y3.z, u),
-                bezierInterpolate(y1.w, y2.w, y3.w, u));
+float interpolate3Points(float y1, float y2, float y3, float u) {
+    // Makes the interpolated values intersect the middle value.
+
+    if ((u <= 0.5f && y1 == y2) || (u >= 0.5f && y2 == y3)) {
+        // Flat line.
+        return y2;
+    }
+
+    float halfSlope;
+    if ((y2 >= y1 && y2 >= y3) || (y2 <= y1 && y2 <= y3)) {
+        // U or inverted-U shape.
+        // Make the slope at y2 = 0, which means that the control points half way between the value points have the value y2.
+        halfSlope = 0.0f;
+
+    } else {
+        // L or inverted and/or mirrored L shape.
+        // Make the slope at y2 be the slope between y1 and y3, up to a maximum of double the minimum of the slopes between y1
+        // and y2, and y2 and y3. Use this slope to calculate the control points half way between the value points.
+        // Note: The maximum ensures that the control points and therefore the interpolated values stay between y1 and y3.
+        halfSlope = (y3 - y1) / 2.0f;
+        float slope12 = y2 - y1;
+        float slope23 = y3 - y2;
+        if (abs(halfSlope) > abs(slope12)) {
+            halfSlope = slope12;
+        } else if (abs(halfSlope) > abs(slope23)) {
+            halfSlope = slope23;
+        }
+    }
+
+    float stepU = step(0.5f, u);  // 0.0 if u < 0.5, 1.0 otherwise.
+    float slopeSign = 2.0f * stepU - 1.0f; // -1.0 if u < 0.5, 1.0 otherwise
+    float start = (1.0f - stepU) * y1 + stepU * y2;  // y1 if u < 0.5, y2 otherwise
+    float middle = y2 + slopeSign * halfSlope;
+    float finish = (1.0f - stepU) * y2 + stepU * y3; // y2 if u < 0.5, y3 otherwise
+    float v = 2.0f * u - step(0.5f, u);  // 0.0-0.5 -> 0.0-1.0 and 0.5-1.0 -> 0.0-1.0
+    return bezierInterpolate(start, middle, finish, v);
 }
 
+vec4 interpolate3Vec4(vec4 y1, vec4 y2, vec4 y3, float u) {
+    return vec4(interpolate3Points(y1.x, y2.x, y3.x, u),
+                interpolate3Points(y1.y, y2.y, y3.y, u),
+                interpolate3Points(y1.z, y2.z, y3.z, u),
+                interpolate3Points(y1.w, y2.w, y3.w, u));
+}
 
 void main(void) {
     TransformCamera cam = getTransformCamera();
@@ -73,21 +110,21 @@ void main(void) {
     // Which quad vertex pos?
     int twoTriID = gl_VertexID - particleID * NUM_VERTICES_PER_PARTICLE;
 
-	// Particle properties
+    // Particle properties
     float age = inColor.x / particle.lifespan;
     float seed = inColor.y;
 
     // Pass the texcoord and the z texcoord is representing the texture icon
     varTexcoord = vec2((UNIT_QUAD[twoTriID].xy + 1.0) * 0.5);
     varColor = interpolate3Vec4(particle.color.start, particle.color.middle, particle.color.finish, age);
-    
+
     // anchor point in eye space
-    float radius = bezierInterpolate(particle.radius.start, particle.radius.middle, particle.radius.finish, age);
+    float radius = interpolate3Points(particle.radius.start, particle.radius.middle, particle.radius.finish, age);
     vec4 quadPos = radius * UNIT_QUAD[twoTriID];
 
     vec4 anchorPoint;
     <$transformModelToEyePos(cam, obj, inPosition, anchorPoint)$>
-    
+
     vec4 eyePos = anchorPoint + quadPos;
     <$transformEyeToClipPos(cam, eyePos, gl_Position)$>
 }