Move pResidual calculation up one level

This amazingly helps quite a lot! The idle time between frames is down from 0.8ms to 0.66ms now.

firmware/fadecandy.cpp

@@ -54,7 +54,7 @@ ALWAYS_INLINE static inline uint32_t lutInterpolate(const uint16_t *lut, uint32_
     return (lut[index] * invAlpha + lut[index + 1] * alpha) >> 8;
 }
 
-static inline uint32_t updatePixel(uint32_t icPrev, uint32_t icNext, unsigned n)
+static inline uint32_t updatePixel(uint32_t icPrev, uint32_t icNext, unsigned n, int8_t *pResidual)
 {
     /*
      * Update pipeline for one pixel:
@@ -77,9 +77,6 @@ static inline uint32_t updatePixel(uint32_t icPrev, uint32_t icNext, unsigned n)
     iG = lutInterpolate(&buffers.lutCurrent[1 * 256], iG);
     iB = lutInterpolate(&buffers.lutCurrent[2 * 256], iB);
 
-    // Pointer to the residual buffer for this pixel
-    int8_t *pResidual = &residual[n * 3];
-
     // Incorporate the residual from last frame
     iR += pResidual[0];
     iG += pResidual[1];
@@ -133,7 +130,10 @@ static void updateDrawBuffer(unsigned interpCoefficient)
     uint32_t icPrev = 257 * (0x10000 - interpCoefficient);
     uint32_t icNext = 257 * interpCoefficient;
 
-    for (int i = 0; i < LEDS_PER_STRIP; ++i) {
+    // Pointer to the residual buffer for this pixel
+    int8_t *pResidual = residual;
+
+    for (int i = 0; i < LEDS_PER_STRIP; ++i, pResidual += 3) {
 
         // Six output words
         union {
@@ -151,7 +151,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
          * This generates fairly efficient code using the UBFX and BFI instructions.
          */
 
-        uint32_t p0 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p0 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0, pResidual + LEDS_PER_STRIP * 3 * 0);
 
         o5.p0d = p0;
         o5.p0c = p0 >> 1;
@@ -178,7 +178,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
         o0.p0b = p0 >> 22;
         o0.p0a = p0 >> 23;
 
-        uint32_t p1 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p1 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 1, pResidual + LEDS_PER_STRIP * 3 * 1);
 
         o5.p1d = p1;
         o5.p1c = p1 >> 1;
@@ -205,7 +205,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
         o0.p1b = p1 >> 22;
         o0.p1a = p1 >> 23;
 
-        uint32_t p2 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p2 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 2, pResidual + LEDS_PER_STRIP * 3 * 2);
 
         o5.p2d = p2;
         o5.p2c = p2 >> 1;
@@ -232,7 +232,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
         o0.p2b = p2 >> 22;
         o0.p2a = p2 >> 23;
 
-        uint32_t p3 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p3 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 3, pResidual + LEDS_PER_STRIP * 3 * 3);
 
         o5.p3d = p3;
         o5.p3c = p3 >> 1;
@@ -259,7 +259,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
         o0.p3b = p3 >> 22;
         o0.p3a = p3 >> 23;
 
-        uint32_t p4 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p4 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 4, pResidual + LEDS_PER_STRIP * 3 * 4);
 
         o5.p4d = p4;
         o5.p4c = p4 >> 1;
@@ -286,7 +286,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
         o0.p4b = p4 >> 22;
         o0.p4a = p4 >> 23;
 
-        uint32_t p5 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p5 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 5, pResidual + LEDS_PER_STRIP * 3 * 5);
 
         o5.p5d = p5;
         o5.p5c = p5 >> 1;
@@ -313,7 +313,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
         o0.p5b = p5 >> 22;
         o0.p5a = p5 >> 23;
 
-        uint32_t p6 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p6 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 6, pResidual + LEDS_PER_STRIP * 3 * 6);
 
         o5.p6d = p6;
         o5.p6c = p6 >> 1;
@@ -340,7 +340,7 @@ static void updateDrawBuffer(unsigned interpCoefficient)
         o0.p6b = p6 >> 22;
         o0.p6a = p6 >> 23;
 
-        uint32_t p7 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 0);
+        uint32_t p7 = updatePixel(icPrev, icNext, i + LEDS_PER_STRIP * 7, pResidual + LEDS_PER_STRIP * 3 * 7);
 
         o5.p7d = p7;
         o5.p7c = p7 >> 1;
@@ -383,7 +383,7 @@ extern "C" int main()
     while (1) {
         buffers.handleUSB();
 
-        updateDrawBuffer((millis() << 2) & 0xFFFF);
+        updateDrawBuffer((millis() << 4) & 0xFFFF);
         leds.show();
     }
 }