drawRadialGradient function

Image drawRadialGradient(Image destination, int x1, int y1, int x2, int y2, int cx, int cy, int c1, int c2, [ double scale = 1, ])

Fills a rectangular area delimited by x1, y1, x2 and y2 with a radial gradient blending from c1 to c2 centered at (cx, cy)

Implementation

Image drawRadialGradient(
    Image destination, int x1, int y1, int x2, int y2, int cx, int cy, int c1, int c2,
    [double scale = 1]) {
  x1 = x1.clamp(0, destination.width);
  x2 = x2.clamp(x1, destination.width);
  y1 = y1.clamp(0, destination.height);
  y2 = y2.clamp(y1, destination.height);

  int distanceSquared(int x1, int y1, int x2, int y2) =>
      (x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2);

  List<int> distances = [];

  if (cx > x1 && cx < x2 && cy > y1 && cy < y2) {
    distances.add(0);
  } else if (cx > x1 && cx < x2) {
    if (cy < y1) {
      distances.add(pow(y1 - cy, 2).floor());
    } else {
      distances.add(pow(cy - y2, 2).floor());
    }
  } else if (cy > y1 && cy < y2) {
    if (cx < x1) {
      distances.add(pow(x1 - cx, 2).floor());
    } else {
      distances.add(pow(cx - x2, 2).floor());
    }
  }

  distances.add(distanceSquared(x1, y1, cx, cy));
  distances.add(distanceSquared(x1, y2, cx, cy));
  distances.add(distanceSquared(x2, y1, cx, cy));
  distances.add(distanceSquared(x2, y2, cx, cy));

  int minDistance = distances.reduce(min);
  int maxDistance = distances.reduce(max);

  for (int x = x1; x < x2; x++) {
    for (int y = y1; y < y2; y++) {
      int distance = distanceSquared(x, y, cx, cy);

      num interpolation = pow((distance - minDistance) / (maxDistance - minDistance), scale);

      int fraction = (interpolation * 255).floor();

      destination.setPixelSafe(x, y, alphaBlendColors(c1, c2, fraction));
    }
  }

  return destination;
}