Tweak blazing trail decay

- Rewrite decay code to fade brightness smoothly with an x^4
  relationship
- Fade hue linearly from orange to red rather than for a subset of
  time from red to orange

Signed-off-by: Bart Nagel <bart@tremby.net>

src/kaleidoscope/plugin/LED-Stalker.cpp

@@ -105,27 +105,28 @@ cRGB Haunt::compute(uint8_t *step) {
 // BlazingTrail
 BlazingTrail::BlazingTrail(void) {
 }
+uint8_t hue_start = 50.0 / 360 * 0xff;
+uint8_t hue_end = 0;
 
 cRGB BlazingTrail::compute(uint8_t *step) {
   cRGB color;
 
-  if (*step >= 0xff - 30) {
-    color = hsvToRgb(0xff - *step, 255, 255);
-  } else {
-    color = hsvToRgb(30, 255, 255);
+  // Get a float from 0 to 1 for our position in the animation
+  float pos = (0xff - *step) / (float) 0xff;
 
-    color.r = min(*step * color.r / 255, 255);
-    color.g = min(*step * color.g / 255, 255);
-  }
+  // Fade hue linearly from orange to red
+  uint8_t hue = hue_start + pos * (hue_end - hue_start);
 
-  if (*step >= 0xff - 30)
-    *step -= 1;
-  else if (*step >= 0x40)
-    *step -= 16;
-  else if (*step >= 32)
-    *step -= 32;
-  else
+  // Fade value from full following a -x^4+1 curve
+  uint8_t val = 0xff * (1 - pos * pos * pos * pos);
+
+  color = hsvToRgb(hue, 0xff, val);
+
+  if (*step < 4) {
     *step = 0;
+  } else {
+    *step -= 4;
+  }
 
   return color;
 }