Count length of black pixels on image - opencv

Question

I am using OpenCV to manipulate some images.
Suppose that image.png is black/white image (only B or W for pixel colors).
For example, if we print the colors for 3rd line, it could be:
WWWWWBBBWWWWWWBBBBBBWWWWWBBWWWW
I'd like to save info on each sequence of black pixels, I mean, I'd like to be able to compute, for each row, the values:
number of black sequences in row i: (3 on example above)
x-coordinate of end pixels for each black sequence in row i: (6,8 and 15,20 and 26,27 on example above)
length of each black sequence on row i: (l1=3,l2=6,l3=2 on example above) (this is easy assuming item above is done)
I'm using some for loop and testing if color is black. When it is black, I save the x coordinate and start other loop inside to run from this coordinate to the end of line, testing if the color is white. When it finds white color, it stops and save the previous coordinate.
This works to compute only the length of first sequence of black pixels. I don't know how to go to next (I even don't know how many there are).
Here is the main part of code (with some trash code):
for(int y=0;y<img.rows;y++) //img.rows
{
for(int x=0;x<img.cols;x++)
{
Vec3b color = image.at<Vec3b>(Point(x,y));
printf("(%d,%d,%d)\n",color[1],color[2],color[3]);
if(color[0] == 0 && color[1] == 0 && color[2] == 0)
{
cor[0]='B';
ymax = y;
if (ymin == -1) { ymin = y; }
int xmin = x;
int diam_esq = img.cols/2-xmin;
double dist_esq = sqrt( (x-img.cols/2)*(x-img.cols/2) + (y-img.rows/2)*(y-img.rows/2) );
for(int z=x;z<img.cols;z++)
{
Vec3b colorz = image.at<Vec3b>(Point(z,y));
if(colorz[0] == 255 && colorz[1] == 255 && colorz[2] == 255)
{
int xmax = z-1;
int diam_dir = xmax-img.cols/2;
double dist_dir = sqrt( (z-1-img.cols/2)*(z-1-img.cols/2) + (y-img.rows/2)*(y-img.rows/2) );
int diam = xmax - xmin;
//printf("y=%*d, xmin=%*d, xmax=%*d, esq=%*d, dir=%*d, diam=%*d\n",5,y,5,xmin,5,xmax,5,diam_esq,5,diam_dir,5,diam);
printf("%*d%*d%*d%*d%*d%*d%*f%*f\n",5,y,5,xmin,5,xmax,5,diam_esq,5,diam_dir,5,diam,13,dist_esq,13,dist_dir);
break;
}
}
break;
}
}
//break; // only y=0
}

Answer 1

Here is some code that does what you want. It only prints out results. It doesn't save them anywhere but I assume you'll know how to do this.
To find black sequences in each row, there is no need to do nested for loops for each sequence, just keep track of whether or not you're inside a black sequence and if yes, of where it began. Also, I use Mat::ptr to allow for a more efficient traversal of your image, row by row.
for(int i = 0; i < image.rows; i++) {
Vec3b* ptr = image.ptr<Vec3b>(i);
bool blackSequence = false;
int blackSequenceStart = -1;
int numberOfBlackSequences = 0;
for(int j = 0; j < image.cols; j++) {
Vec3b color = ptr[j];
if(color[0] == 0 && !blackSequence) { // this is assuming that all pixels are either white or black
blackSequence = true;
blackSequenceStart = j;
numberOfBlackSequences++;
}
if(color[0] == 255 && blackSequence) {
blackSequence = false;
cout << "Row " << i << ": Sequence " << numberOfBlackSequences << " starts at " << blackSequenceStart
<< " and finishes at " << j - 1 << ". Total length: " << j - blackSequenceStart << endl;
}
if(j == image.cols - 1 && blackSequence) {
blackSequence = false;
cout << "Row " << i << ": Sequence " << numberOfBlackSequences << " starts at " << blackSequenceStart
<< " and finishes at " << j << ". Total length: " << j - blackSequenceStart + 1 << endl;
}
}
}