Test harness for tweaking the dither algorithm

Makefile

@@ -52,7 +52,7 @@ OBJS := $(C_FILES:.c=.o) $(CPP_FILES:.cpp=.o)
 all: $(TARGET).hex
 
 $(TARGET).elf: $(OBJS) $(LDSCRIPT)
-	$(CC) $(LDFLAGS) -o $@ $(OBJS)
+	$(CC) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
 
 %.hex: %.elf
 	$(SIZE) $<

src/hcolor.h

@@ -92,15 +92,22 @@ struct HPixel {
 
     /// Temporal dithering algorithm. Returns a 24-bit RGB color.
     uint32_t dither() {
+
+        // Incorporate the residual from last frame
         int r16 = color.r + residual[0];
         int g16 = color.g + residual[1];
         int b16 = color.b + residual[2];
+
+        // Round to the nearest 8-bit value
         int r8 = std::min<int>(0xff, std::max<int>(0, (r16 + 0x80) >> 8));
         int g8 = std::min<int>(0xff, std::max<int>(0, (g16 + 0x80) >> 8));
         int b8 = std::min<int>(0xff, std::max<int>(0, (b16 + 0x80) >> 8));
-        residual[0] = r16 - (r8 | (r8 << 8));
-        residual[1] = g16 - (g8 | (g8 << 8));
-        residual[2] = b16 - (b8 | (b8 << 8));
+
+        // Compute the error, after expanding the 8-bit value back to 16-bit.
+        residual[0] = r16 - (r8 * 257);
+        residual[1] = g16 - (g8 * 257);
+        residual[2] = b16 - (b8 * 257);
+
         return (r8 << 16) | (g8 << 8) | b8;
     }
 };

src/main.cpp

@@ -6,9 +6,10 @@
  */
 
 #include <OctoWS2811.h>
+#include <math.h>
 #include "hcolor.h"
 
-static const int ledsPerStrip = 64;
+static const int ledsPerStrip = 16;
 static const int ledsTotal = ledsPerStrip * 8;
 
 DMAMEM int displayMemory[ledsPerStrip * 6];
@@ -19,15 +20,23 @@ HPixelBuffer<ledsTotal> pixbuf;
 void setup()
 {
     leds.begin();
+    Serial.begin(115200);
 
     for (unsigned i = 0; i < ledsTotal; ++i) {
-        pixbuf.pixels[i].color = HColor16(0x0080, 0x0080, 0x0080);
+        pixbuf.pixels[i].color = HColor16(0,0,0);
     }
 }
 
 void loop()
 {
-    pixbuf.pixels[0].color.b = millis() & 0xffff;
-
-    pixbuf.show(leds);
+	// XXX: Proof of concept
+	
+	for (int i = 0; i < 16; i++) {
+		unsigned c = pow(sin(i * 0.2 + millis() * 0.0005) * 0.5 + 0.5, 2.2) * 0x1000;
+	    pixbuf.pixels[i].color = HColor16(c>>2, c, c>>1);
+   	}
+	
+	for (int i = 0; i < 1000; i++) {
+		pixbuf.show(leds);
+	}
 }