Finding closest non-black pixel in an image fast

Question

I have a 2D image randomly and sparsely scattered with pixels.
given a point on the image, I need to find the distance to the closest pixel that is not in the background

Answer 1

Ok, this sounds interesting. I made a c++ version of a soulution, I don't know if this helps you. I think it works fast enough as it's almost instant on a 800*600 matrix. If you have any questions just ask.

Sorry for any mistakes I've made, it's a 10min code... This is a iterative version (I was planing on making a recursive one too, but I've changed my mind). The algorithm could be improved by not adding any point to the points array that is to a larger distance from the starting point then the min_dist, but this involves calculating for each pixel (despite it's color) the distance from the starting point.

Hope that helps

//(c++ version)
#include
#include
#include
using namespace std;
//ITERATIVE VERSION

//picture witdh&height
#define width 800
#define height 600
//indexex
int i,j;

//initial point coordinates
int x,y;
//variables to work with the array
int p,u;
//minimum dist
double min_dist=2000000000;
//array for memorising the points added
struct point{
  int x;
  int y;
} points[width*height];
double dist;
bool viz[width][height];
// direction vectors, used for adding adjacent points in the "points" array.
int dx[8]={1,1,0,-1,-1,-1,0,1};
int dy[8]={0,1,1,1,0,-1,-1,-1};
int k,nX,nY;
//we will generate an image with white&black pixels (0&1)
bool image[width-1][height-1];
int main(){
    srand(time(0));
    //generate the random pic
    for(i=1;i<=width-1;i++)
        for(j=1;j<=height-1;j++)
            if(rand()%10001<=9999) //9999/10000 chances of generating a black pixel
            image[i][j]=0;
            else image[i][j]=1;
    //random coordinates for starting x&y
    x=rand()%width;
    y=rand()%height;
    p=1;u=1;
    points[1].x=x;
    points[1].y=y;
    while(p<=u){
        for(k=0;k<=7;k++){
          nX=points[p].x+dx[k];
          nY=points[p].y+dy[k];
          //nX&nY are the coordinates for the next point
          //if we haven't added the point yet
          //also check if the point is valid
          if(nX>0&&nY>0&&nX