grass fire algorithm taking way too long, how to optimize? - c++

Question

So I am working with openCV and trying to write a bunch of algorithms "from scratch" so to speak so that I can really understand what the library is doing. I wrote a modified grass fire algorithm to segment BLOBs from an image that I have already digitized. However, the algorithm takes over 2 minutes to run on my very capable laptop (16 gigs ram, quad core i7, etc...). What am I doing here that is making it so complex? Alternately, is there a better algorithm for extracting BLOBs from a digitized image?
THANKS!
Here is the algorithm
std::vector<boundingBox> grassFire(cv::Mat digitalImage){
std::vector<boundingBox> blobList;
int minY, minX, maxY, maxX, area, yRadius, xRadius, xCenter, yCenter;
for(int curRow = 0; curRow<digitalImage.rows; curRow++){
for(int curCol = 0; curCol<digitalImage.cols; curCol++){
//if there is something at that spot in the image
if((int)digitalImage.at<unsigned char>(curRow, curCol)){
minY = curRow;
maxY = curRow;
minX = curCol;
maxX = curCol;
area = 0;
yRadius = 0;
xRadius = 0;
for(int fireRow=curRow; fireRow<digitalImage.rows; fireRow++){
//is in keeps track of the row and started keeps track of the col
//is in will break if no pixel in the row is part of the blob
//started will break the inner loop if a nonpixel is reached AFTER a pixel is reached
bool isIn = false;
bool started = false;
for(int fireCol = curCol; fireCol<digitalImage.cols; fireCol++){
//make sure that the pixel is still in
if((int)digitalImage.at<unsigned char>(fireRow, fireCol)){
//signal that an in pixel has been found
started = true;
//signal that the row is still in
isIn = true;
//add to the area
area++;
//reset the extrema variables
if(fireCol > maxX){maxX = fireCol;}
if(fireCol < minX){minX = fireCol;}
if(fireRow > maxY){maxY = fireRow;}
//no need to check min y since it is set already by the loop trigger
//set the checked pixel values to 0 to avoid double counting
digitalImage.at<unsigned char>(fireRow, fireCol) = 0;
}
//break if the next pixel is not in and youve already seen an in pixel
//do nothing otherwise
else{if(started){break;}}
//if the entire blob has been detected
if(!isIn){break;}
}
}
}else{}//just continue the loop if the current pixel is not in
//calculate all blob specific values for the blob at hand
xRadius =(int)((double)(maxX - minX)/2.);
yRadius =(int)((double)(maxY - minY)/2.);
xCenter = maxX - xRadius;
yCenter = maxY - yRadius;
//add the blob to the vector in the appropriate position (largest area first)
int pos = 0;
for(auto elem : blobList){
if(elem.getArea() > area){
pos++;
}
else{break;}
}
blobList.insert(blobList.begin() + pos, boundingBox(area, xRadius, yRadius, xCenter, yCenter));
}
}
return blobList;
}

Answer 1

You say `just continue the loop if the current pixel is not in but you don't continue the loop there, and fall thru to the code that adds another element to blobList (which code will access past the end of the lit of no element satisfies the condition in that for loop).
And using this
for(const auto &elem : blobList)
would avoid making copies of all those boundingBoxes.