opencv matrix multi channel access and fill

Question

I have a matrix with 3 channels, and I have a problem to access and change the value of the elements. I have this code:

Mat m(4,4, CV_8UC3);   
Vec3b a;
a[0] = 255;
a[1] = 0;
a[2] = 0;
m.at<Vec3b>(0,0) = a;

when I print the matrix m, I have this :

[255, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0;
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0;
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0;
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

which is good, but the image that I get when I store the matrix with imwrite is this one:

and I don't understand why it's blue and why I have a square 2x2 blue, I should have had only the first pixel red no?

Have I done a mistake somewhere?

Answer 1

As mentioned earlier, pixels next to the high-intensity pixel take non-zero values after JPG-compression because JPEG is a lossy algorithm. In fact, JPEG-compression discards information precisely in image regions with strong gradient.

As for your second question: your pixels are blue because of an Opencv quirk: color channels are not stored in R, G, B order, but in B, G, R order:

For color images, the channel ordering is normally Blue, Green, Red

Source: http://docs.opencv.org/modules/core/doc/drawing_functions.html?highlight=green

(this is one of those things that make life miserable for the Opencv programmer)

Answer 2

I don't understand why it's blue ... not red?

In OpenCV, the image format is set by the flag when you create the image e.g. CV_8UC3 means 8bit pixels, unsigned, 3 color channels. In a color image the pixel order is BGR, data is stored in row order. Similarly, BGRA for CV_8UC4. So the value you set is blue instead of red, that's why you got blue.

I should have had only the first pixel ...?

The reason is that, for compressed image types e.g. JPG, they can not guarantee to keep the content precisely. Try to imwrite() to a un-compressed e.g. BMP image (typically). You will get the image like: