Get the whole pixel array inside the circle

You are looking for the following set of pixels:

where r is the radius of your circle and (m1, m2) is the center.

In order for these pixels to move over all positions and save those that match the criteria in the list:

List<int> indices = new List<int>();

for (int x = 0; x < width; x++)
{
    for (int y = 0; y < height; y++)
    {
        double dx = x - m1;
        double dy = y - m2;
        double distanceSquared = dx * dx + dy * dy;

        if (distanceSquared <= radiusSquared)
        {
            indices.Add(x + y * width);
        }
    }
}