Assertion error when accessing pointer to pixels openCV - c++

Question

so I am struggling to understand why I am getting this assertion error from opencv when accessing a pointer in the next col/row of an image. Let me tell you what is happening and provide some code.
I am taking a ROI from the image, which is a cv::Mat or a header to a section of a bigger cv::Mat.
I constructed some pointers to access the value of my ROI. So lets say my ROI is filled with values of pixels and is a 3x3 Mat
with the following Dimensions (index starting at 0,0)
---------
| 1 | 2 | 3 |
| 4 | 5 | 6 |
| 7 | 8 | 9 |
first of all I need to initialize my pointers to point to their positions respectively. I took the ptr function of the cv::Mat and their location in the grid via cv::Point.
Problem faced:
When I try to access the pixel of the next neighbor, I get an assertion error.
Diagnostics by me:
I thought it might be the range, but I made sure that wouldn't be the case by defining the for loop conditions according to my dimensions.
The item I am trying to access doesn't exist, but how I understand it when I go through the ROI, i already have the values in a new Matrix and I should be able to access all values around my desired pixels.
PART OF THE CODE:
cv::Mat ROI =disTrafo(cv::Rect(cv::Point(x,y),cv::Size(3,3)));
cv::minMaxLoc(ROI,&minVal,&maxVal,&minCoord,&maxCoord);
auto* maxPtr_x = &maxCoord.x;
auto* maxPtr_y = &maxCoord.y;
auto* maxPtr_value = &maxVal;
uchar diff1 = 0;
uchar diff2= 0;
uchar diff3 = 0;
uchar diff4 = 0;
uchar max_diff = 0;
for(int j = 1; j < ROI.rows ; j++){
auto current = ROI.ptr<uchar>(maxCoord.y);
auto neighbor_down = ROI.ptr<uchar>(maxCoord.y+1); //THE PROB IS HERE according to debugging
auto neighbor_up = ROI.ptr<uchar>(maxCoord.y-1);
cv::Point poi ; //point of interest
for(int i= 0; i < ROI.cols; i++){
switch(maxCoord.x){ //PROOF FOR LOGIC
case 0:
if(maxCoord.y == 0){ //another switch statement maybe ??
diff1 = std::abs(current[maxCoord.x+1] - current[maxCoord.x]);
diff2 = std::abs(neighbor_down[maxCoord.x] - current[maxCoord.x]);
if(diff2 > diff1){
cv::Point(maxCoord.x,maxCoord.y+1) = poi;
} else {
cv::Point(maxCoord.x+1,maxCoord.y) = poi;
}
};
ASSERTION FAILED when running it: OpenCV Error: Assertion failed(y == 0|| < data && dims >= 1 && (unsigned)y < (unsigned)size.p[0])) in cv::Mat::ptr, file //... indicates path to header file mat.hpp// line 428
I can't put my finger on the problem, could you please be of assistance. And please give me some knowledge when working with pointers and pixels in case I misunderstood something.
Thank you

Answer 1

try this
for(int j = 1; j < ROI.rows ; j++){
auto current = ROI.ptr<uchar>(maxCoord.y);
auto neighbor_down = ROI.ptr<uchar>(maxCoord.y+1); //THE PROB IS HERE according to debugging
auto neighbor_up = ROI.ptr<uchar>(maxCoord.y-1);
cv::Point poi ; //point of interest
cv::Point bordess(Point(0,0));
for(int i= 0; i < ROI.cols; i++){
switch(maxCoord.x){ //PROOF FOR LOGIC
case 0:
if(maxCoord.y == 0){ //another switch statement maybe ??
diff1 = std::abs(current[maxCoord.x+1] - current[maxCoord.x]);
diff2 = std::abs(neighbor_down[maxCoord.x] - current[maxCoord.x]);
if(diff2 > diff1){
cv::Point(maxCoord.x,maxCoord.y+1) = poi & bordess;
} else {
cv::Point(maxCoord.x+1,maxCoord.y) = poi & bordess;
}
};

Answer 2

Ok so basically I figured out that my pointer definition was wrong due to the nature of the input image. I have done some preprocessing with the image and the ranges of the values inside changed from ucharto some other value. When I changed the auto neighbor_down = ROI.ptr<uchar>(maxCoord.y+1);for example to auto neighbor_down = ROI.ptr<float>(maxCoord.y+1); everything ran normal.