Approximate image from byte array

Question

I wrote an application where some dots are floating around and if i assign a dot a Point, it will move on this position. Now i want to load an image, convert it to a monochrome image (Only pure black or white pixels - no shades of gray) and make each dot floating to a position where its representing a black pixel. I've already done loading and converting a image that way and extraceted the pixels as a 1 dimensional byte[]. I've managed to iterate though this array with the following code:

int stride = width * 4;
for (int y = 0; y < height; y++)
    for (int x = 0; x < width; x++)
    {
        int index = y * stride + 4 * x;
        // array[index] <- Red
        // array[index + 1] <- Green
        // array[index + 2] <- Blue
        // array[index + 3] <- Alpha
    }

The byte array holds every pixel with 4 bytes (RGBA). So the array length is ImageHeight*ImageWidth*4 bytes. Either a pixel is Black (0, 0, 0, 255) or White (255, 255, 255, 255).

My problem now is that i'm not able to correctly approximate the black areas of the image with just n dots. In most cases I will have much less floating dots than there are black pixels in the array. So what i need is a method that gives me a Point[] that contains n Points that will represent only the black areas of the image as good as possible. Can someone help me out?

Answer 1

Loop though the array and find the points that their red, green and blue are 0 to get the Black dots:

List<Point> blackPoints = new List<Points>()
for(int i=0; i<array.Length; i+=4)
    if(array[i] == 0 && array[i+1] == 0 && array[i+2] ==0) //alpha is not important
    {     
         int tmp = i / 4;
         blackPoints.Add(new Point(tmp%width, tmp/width));
    }

Create Methods, to get the weight of a pixel based on its own and neighbors colors, and also a Method to find the weight of a block:

public static class Exts
{
    public static int Weight(this points ps, int x, int y, int width, int height)
    {
         int weight = 0;
        for(int i=Math.Max(x - 1, 0); i<Math.Min(width, x+1))
            for(int j= Math.Max(y-1, 0), j<Math.Min(height, y+1))
                 if(ps.Any(a => a.X == i && a.Y == j)) weight++;
        return weight;
    }

    public static int BlockWeight(this Point[] ps, int x, int y)
    {
        return ps.Count(a => a.X <= x+2 && a.Y<= y+2);
    }   
}

Now loop through the bitmap, with blocks of nine pixels (3x3) and if a blocks wieght is more than half (in this case more than or equal to 5), select a the point in this block that has heighest weight, to represent the black point:

List<Point> result = new List<Point>();

for(int i=0; i<width; i+=3)
    for(int j=0; j< height; j+=3)
        if(points.BlockWeight(i,j) >= 5)
            result.Add(ps.Where(a => a.X <= x+2 && a.Y<= y+2).OrderByDescending(a => a.Weight(i, j, width, height)).First())