problematic scaling

gistfile1.cpp

#include<FastLED.h>

#define LED_PIN     13
#define BRIGHTNESS  100
#define LED_TYPE    WS2812B
#define COLOR_ORDER GRB

const uint8_t kMatrixWidth  = 16;
const uint8_t kMatrixHeight = 16;
const bool    kMatrixSerpentineLayout = true;

// some counters for moving the edge
byte c1 = 0;
byte c2 = 64;
byte c3;

#define NUM_LEDS (kMatrixWidth * kMatrixHeight)
#define MAX_DIMENSION ((kMatrixWidth>kMatrixHeight) ? kMatrixWidth : kMatrixHeight)

CRGB leds[kMatrixWidth * kMatrixHeight];

uint8_t noise[MAX_DIMENSION][MAX_DIMENSION];

CRGBPalette16 currentPalette( RainbowStripeColors_p );

uint8_t       colorLoop = 0;

void setup() {
  delay(500);
  LEDS.addLeds<LED_TYPE,LED_PIN,COLOR_ORDER>(leds+5,NUM_LEDS-5);
  LEDS.setBrightness(BRIGHTNESS);
}

// fills the array with 0-255
void fillnoise8() {
  for(int i = 0; i < MAX_DIMENSION; i++) {
    for(int j = 0; j < MAX_DIMENSION; j++) {
      byte data = i * kMatrixWidth + j;
      noise[i][j] = data;
    }
  }
}

void MapNoiseToRectangle(uint8_t x1, uint8_t y1, uint8_t x2, uint8_t y2)
{
  static uint8_t ihue=0;
  for(int i = x1; i <= x2; i++) {
    for(int j = y1; j <= y2; j++) {

      uint8_t index = noise[i * (kMatrixWidth-1) / (x2-x1)][j * (kMatrixHeight-1) / (y2-y1)];
      uint8_t bri =   255;

      if( colorLoop) {
        index += ihue;
      }

      CRGB color = ColorFromPalette( currentPalette, index, bri);
      leds[XY(i,j)] = color;
    }
  }
  ihue+=1;
}

void CLS() {
  for(int i = 0; i <NUM_LEDS; i++) {
    leds[i]=0x000000;
  }
}

void loop() {
  CLS();
  fillnoise8();
  
  c3++;
  if (c3%4==0) {
    c1++;
    c2+=2;
  }

  MapNoiseToRectangle(sin8(c1)/20, sin8(c2)/20, 15, 15);
  
  LEDS.show();
}


uint16_t XY( uint8_t x, uint8_t y)
{
  uint16_t i;
  if( kMatrixSerpentineLayout == false) {
    i = (y * kMatrixWidth) + x;
  }
  if( kMatrixSerpentineLayout == true) {
    if( y & 0x01) {
      uint8_t reverseX = (kMatrixWidth - 1) - x;
      i = (y * kMatrixWidth) + reverseX;
    } 
    else {
      i = (y * kMatrixWidth) + x;
    }
  }
  return i;
}