What are the different ways of constructing cv::Mat? - opencv

Question

OpenCV Version 3.2.0
I am reading Bradski and trying to make Different cv::Mat constructors - single channel.
Can someone please tell, why the constructors do not work?
float data1[6] = {1,2,3,4,5,6};
float data2[6] = {10,20,30,40,50,60};
float data3[6] = {100,200,300,400,500,600};
cv::Mat mat1(3,4,CV_32FC1); //OK
cv::Mat mat2(3,4,CV_32FC1,cv::Scalar(33.3)); //OK
cv::Mat mat3(3,4,CV_32FC1,data1,sizeof(float)); //OK
cv::Mat mat4(cv::Size(3,4),CV_32FC1); //OK
cv::Mat mat5(cv::Size(3,4),CV_32FC1,cv::Scalar(66.6)); //OK
cv::Mat mat6(cv::Size(3,4),CV_32FC1,data2,sizeof(float)); //OK
int sz[] = {8, 8, 8};
cv::Mat bigCube1(3, sz, CV_32FC1); // OK
cv::Mat bigCube2(3, sz, CV_32FC1, cv::Scalar::all(99)); // OK
cv::Mat bigCube3(3, sz, CV_32FC1, data3, 4); // Not OK, How to initialise a 3D from data?
std::cout << mat1 << std::endl << mat2 << std::endl << mat3 << std::endl << mat4 << std::endl << mat5 << std::endl << mat6 << std::endl; // OK
std::cout << bigCube1.at<float>(10,10,10) << std::endl << bigCube2.at<float>(10,10,10) << std::endl; // OK
cv::Mat img_rgb = cv::imread("lena.jpg", CV_LOAD_IMAGE_COLOR);
std::vector<cv::Range> ranges(3, cv::Range(2,3));
cv::Mat roiRange( img_rgb, cv::Range(100, 300), cv::Range(0, 512)); //OK
cv::Mat roiRect( img_rgb, cv::Rect(0,100,512,200)); // OK
cv::Mat roiRangeMultiple( bigCube1, ranges); // OK
cv::namedWindow("range", CV_WINDOW_AUTOSIZE);
imshow("range", roiRange); // OK
cv::namedWindow("rect", CV_WINDOW_AUTOSIZE);
imshow("rect", roiRect); // OK
std::cout << roiRangeMultiple.at<float>(0,1,1); // Not OK. Expecting a float value as answer
cv::waitKey(0);
The corresponding answers are:
[4.6634629e-10, 0, 0, 0;
0, 0, 0, 0;
127.62516, 2.8025969e-45, 0, 0]
[33.299999, 33.299999, 33.299999, 33.299999;
33.299999, 33.299999, 33.299999, 33.299999;
33.299999, 33.299999, 33.299999, 33.299999]
[1, 2, 3, 4;
2, 3, 4, 5;
3, 4, 5, 6]
[0, 0, 0;
0, 0, 0;
0, 0, 0;
0, 0, 0]
[66.599998, 66.599998, 66.599998;
66.599998, 66.599998, 66.599998;
66.599998, 66.599998, 66.599998;
66.599998, 66.599998, 66.599998]
[10, 20, 30;
20, 30, 40;
30, 40, 50;
40, 50, 60]
0 // bigCube1
99 // bigCube2
And then the corresponding answers for lena.jpg is the cropped version from Range and Rect. I dont know how to use the ranges though.

Answer 1

Multiple issues.
cv::Mat mat3(3,4,CV_32FC1,data1,sizeof(float));
This will crash in debug mode, failing an assertion. Even though this is not in the documentation, the step size must be at least the length of a row (i.e. no overlap is allowed).
The correct code for this scenario would be something like.
float data1[12] = { 1, 2, 3, 4, 2, 3, 4, 5, 3, 4, 5, 6 };
cv::Mat mat3(3, 4, CV_32FC1, data1, 4 * sizeof(float));
cv::Mat mat6(cv::Size(3,4),CV_32FC1,data2,sizeof(float));
Similar situation as in previous case. Also note that this produces a differently shaped array -- previous was 3 rows, 4 columns, this one has 4 rows and 3 columns (see docs of cv::Size).
float data2[12] = { 10, 20, 30, 40, 20, 30, 40, 50, 30, 40, 50, 60 };
cv::Mat mat6(cv::Size(3, 4), CV_32FC1, data2, 3 * sizeof(float));
cv::Mat bigCube1(3, sz, CV_8UC1);
std::cout << bigCube1 << std::endl;
Formatting of arrays with more than 2 dimensions is not supported.
You can test that the array was correctly created by manually printing all the values:
for (auto const& v : cv::Mat1b(bigCube2)) {
std::cout << uint(v) << " ";
}
std::cout << "\n";
cv::Mat bigCube3(3, sz, CV_32FC1, data3, 4);
The problem here is the last parameter. From the docs
cv::Mat::Mat(int ndims,
const int * sizes,
int type,
void * data,
const size_t * steps = 0
)
steps - Array of ndims-1 steps in case of a multi-dimensional array (the last step is always set to the element size). If not specified, the matrix is assumed to be continuous.
You're not passing an array of steps as the last parameter (only a single integer)
you don't pass enough data
and the rows would again overlap.
One way to do this would be something like
float data3[8 * 8 * 8];
// Populate the data with sequence 0..511
std::iota(std::begin(data3), std::end(data3), 0.0f);
int sz[] = { 8, 8, 8 };
size_t steps[] = { 8 * 8 * sizeof(float), 8 * sizeof(float) };
cv::Mat bigCube3(3, sz, CV_32FC1, data3, steps);
cv::Mat bigCube1(3, sz, CV_8UC1);
// ...
std::cout << roiRangeMultiple.at<float>(0,1,1); // Not OK. Expecting a float value as answer
The data type is CV_8UC1, so each element is an unsigned char. That means you shouldn't be extracting float values from it. Your expectation is incorrect. (Now I see you changed the code in your question).
Also, note that with cv::Range "start is an inclusive left boundary of the range and end is an exclusive right boundary of the range". Since you extract cv::Range(2,3) in each axis, the resulting Mat is 1 x 1 x 1. Hence, you're accessing elements out of range (again, this would trigger a debug mode assertion).
std::cout << roiRangeMultiple.at<unsigned char>(0,0,0);
After the change you have the correct type. However notice that you never initialize bigCube1. You most likely get a 0.0f as a result, which will print as 0. You can try this yourself, just execute std::cout << 0.0f; and see.