Using a PNG image as mask for GrabCut

Question

I have a png image with green and red lines and transparent background, which I need to use is as a mask for executing GrabCut. But I get unexpected results. Here's my code:

//find the mask
Mat mask;
mask.create( image.size(), CV_8UC1);
mask.setTo(Scalar::all(GC_BGD));
Mat maskImg = imread("messi5.png");

for(int i=0; i<maskImg.cols; i++)    
    for(int j=0; j<maskImg.rows; j++)    
    {               
        //if it's red, make it white
        if ((int)maskImg.at<cv::Vec3b>(j,i)[0]==0 && (int)maskImg.at<cv::Vec3b>(j,i)[1] == 0 && (int)maskImg.at<cv::Vec3b>(j,i)[2] == 255) {

            mask.at<cv::Vec3b>(j,i)[0]= GC_BGD;
            mask.at<cv::Vec3b>(j,i)[1] = GC_BGD;
            mask.at<cv::Vec3b>(j,i)[2] = GC_BGD;
        }           

        //if it's green, make it black
        if ((int)maskImg.at<cv::Vec3b>(j,i)[0]==0 && (int)maskImg.at<cv::Vec3b>(j,i)[1] == 255 && (int)maskImg.at<cv::Vec3b>(j,i)[2] == 0) {    
            mask.at<cv::Vec3b>(j,i)[0] = GC_FGD;
            mask.at<cv::Vec3b>(j,i)[1] = GC_FGD;
            mask.at<cv::Vec3b>(j,i)[2] = GC_FGD;
        }
    }

 ...

Here's the output: http://prntscr.com/40kt4e . I guess it's like there's no rectangle, it only sees the GC_FGD pixels, everything else is considered BG. And it looks somehow scaled, but I have no idea how to fix it.

Answer 1

i was trying to say in

GrabCut reading mask from PNG file in OpenCV (C++)

that your using the 3 channel accessor for a 1 channel image. this will mess things up a little, use the 1 channel version for the 1 channel mask:

    Mat image;
    image= cv::imread(file);

    //everything outside this box will be set to def. 
    //background GC_BGD, clearly from the image you can see that the players legs are outside the box, 
    //so this will cause problems. you need to either change the box, 
    //such that everything is outside the box is the background, or use your mask to scribble on the players legs in green.  
    cv::Rect rectangle(startX, startY, width, height);  
    cv::Mat bgModel,fgModel;

    //find the mask
    Mat mask;
    mask.create( image.size(), CV_8UC1);  //CV_8UC1 is single channel
    mask.setTo(Scalar::all(GC_BGD));  //you have set it to all def. background 

    Mat maskImg = imread("messi5.png");


    for(int i=0; i<maskImg.cols; i++)    
        for(int j=0; j<maskImg.rows; j++)    
        {               
            //if it's red, make it black
            if ((int)maskImg.at<cv::Vec3b>(j,i)[0]==0 && (int)maskImg.at<cv::Vec3b>(j,i)[1] == 0 && (int)maskImg.at<cv::Vec3b>(j,i)[2] == 255) {

                //the whole mask is black so this is redundant
                mask.at<uchar>(j,i)= GC_BGD;  //GC_BGD := 0 := black 

            }           

            //if it's green, make it white
            if ((int)maskImg.at<cv::Vec3b>(j,i)[0]==0 && (int)maskImg.at<cv::Vec3b>(j,i)[1] == 255 && (int)maskImg.at<cv::Vec3b>(j,i)[2] == 0) {    

                    mask.at<uchar>(j,i) = GC_FGD; //GC_FGD:= 1 := white 

            }
        }

For more efficient code for looping over images please see: http://docs.opencv.org/doc/tutorials/core/how_to_scan_images/how_to_scan_images.html

The LUT function my do the trick here: http://docs.opencv.org/doc/tutorials/core/how_to_scan_images/how_to_scan_images.html#the-core-function