Can't save image in JPG with white background OpenCV - c++

Question

I write a simple app in OpenCV that delete black background of an image and save it with white background in JPG. However, it's always saved with black background.
This is my code:
Mat Imgsrc = imread("../temp/temp1.jpg",1) ;
mat dest;
Mat temp, thr;
cvtColor(Imgsrc, temp, COLOR_BGR2GRAY);
threshold(temp,thr, 0, 255, THRESH_BINARY);
Mat rgb[3];
split(Imgsrc,rgb);
Mat rgba[4] = { rgb[0],rgb[1],rgb[2],thr };
merge(rgba,4,dest);
imwrite("../temp/r5.jpg", dest);

Answer 1

You can simply use setTo with a mask to set some pixels to a specific value according to a mask:
Mat src = imread("../temp/temp1.jpg",1) ;
Mat dst;
Mat gray, thr;
cvtColor(src, gray, COLOR_BGR2GRAY);
// Are you sure to use 0 as threshold value?
threshold(gray, thr, 0, 255, THRESH_BINARY);
// Clone src into dst
dst = src.clone();
// Set to white all pixels that are not zero in the mask
dst.setTo(Scalar(255,255,255) /*white*/, thr);
imwrite("../temp/r5.jpg", dst);
Also a few notes:
You can directly load an image as grayscale using: imread(..., IMREAD_GRAYSCALE);
You can avoid to use all those temporary Mats.
Are you sure you want to use 0 as threshold value? Because in this case you can avoid entirely to apply theshold, and set to white all pixels that are 0 in the grayscale image: dst.setTo(Scalar(255,255,255), gray == 0);
This is how I'd do:
// Load the image
Mat src = imread("path/to/img", IMREAD_COLOR);
// Convert to grayscale
Mat gray;
cvtColor(src, gray, COLOR_BGR2GRAY);
// Set to white all pixels that are 0 in the grayscale image
src.setTo(Scalar(255,255,255), gray == 0)
// Save
imwrite("path/to/other/img", src);