Get Pixel Mat :: at

Question

I am trying to get a pixel from a Mat object. To check, I try to draw a diagonal line on a square and expect to get a perfect line, crossing the upper left and right vertices down.

for (int i =0; i<500; i++){ //I just hard-coded the width (or height) to make the problem more obvious (image2.at<int>(i, i)) = 0xffffff; //Draw a white dot at pixels that have equal x and y position. } 

The result, however, is not as expected. Here is the diagonal line drawn in the color picture. Here it is in the halftone picture. Does anyone see a problem?

Answer 1

 (image2.at<int>(i, i)) = 0xffffff; 

It looks like your color image is 24 bits, but your address pixels are in int terms that seem 32 bits.

Answer 2

The problem is that you are trying to access each pixel as an int (32-bit image per pixel), while your image is a three-channel unsigned char (24-bit image per pixel) or a single-channel unsigned char (8 bit per pixel image) for grayscale. You can try to access every pixel like this for a shade of gray.

 for (int i =0; i<image2.width; i++){ image2.at<unsigned char>(i, i) = 255; } 

or how is it for color

 for (int i =0; i<image2.width; i++){ image2.at<Vec3b>(i, i)[0] = 255; image2.at<Vec3b>(i, i)[1] = 255; image2.at<Vec3b>(i, i)[2] = 255; }