Maybe I'm not looking hard enough, but everything seems to want me to use an array. Thus, how do I get the channel value for a particular pixel for foo if foo is something like Mat foo = imread("bar.png")
?
OpenCV get pixel channel value from Mat image
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There are 3 best solutions below
3

The below code works for me, for both accessing and changing a pixel value.
For accessing pixel's channel value :
for (int i = 0; i < image.cols; i++) {
for (int j = 0; j < image.rows; j++) {
Vec3b intensity = image.at<Vec3b>(j, i);
for(int k = 0; k < image.channels(); k++) {
uchar col = intensity.val[k];
}
}
}
For changing a pixel value of a channel :
uchar pixValue;
for (int i = 0; i < image.cols; i++) {
for (int j = 0; j < image.rows; j++) {
Vec3b &intensity = image.at<Vec3b>(j, i);
for(int k = 0; k < image.channels(); k++) {
// calculate pixValue
intensity.val[k] = pixValue;
}
}
}
`
Source : Accessing pixel value
0

The pixels array is stored in the "data" attribute of cv::Mat. Let's suppose that we have a Mat matrix where each pixel has 3 bytes (CV_8UC3).
For this example, let's draw a RED pixel at position 100x50.
Mat foo;
int x=100, y=50;
Solution 1:
Create a macro function that obtains the pixel from the array.
#define PIXEL(frame, W, x, y) (frame+(y)*3*(W)+(x)*3)
//...
unsigned char * p = PIXEL(foo.data, foo.rols, x, y);
p[0] = 0; // B
p[1] = 0; // G
p[2] = 255; // R
Solution 2:
Get's the pixel using the method ptr.
unsigned char * p = foo.ptr(y, x); // Y first, X after
p[0] = 0; // B
p[1] = 0; // G
p[2] = 255; // R
Assuming the type is CV_8UC3 you would do this:
Here is the documentation for Vec3b. Also, don't forget OpenCV stores things internally as BGR not RGB.
EDIT :
For performance reasons, you may want to use direct access to the data buffer in order to process the pixel values:
Here is how you might go about this:
Or alternatively: