Roi extraction with cv::rect in opencv

Question

I am trying to extract the first quarter of a 100x100 pixel image. I was thinking of using the cv::rect function from a roi-definition (see code example below). Furthermore my original image is part of a vector-array (here in the code-example below, I only work with one image - later there will be several but this is probably irrelevant for the example here). What I am observing is a strange behaviour: The newly created image "Test" seems to be ok until about half of its rows. Then it shows completely random pixels (see example image). What is the cause ? Image-description: left: orig 100x100 image / right: created "Test" image with errors... ``` std::vector<cv::Mat> Image(1); // create vector array of one 100x100 pixel image Image[0] = cv::imread(argv[1]); // or in iOS: Image[0] = [in_image CVMat]; cv::Rect roi1 = cv::Rect(0, 0, Image[0].cols/2, Image[0].rows/2); cv::Mat Test = Image[0](roi1); imshow("final_result", Test); // the Test image has error rows !!!!!!! Why ???????? ``` Ano…

Accepted Answer

I finally found a workarround for this problem:

--> use the function "cvtColor"

```
UIImage *in_image = [UIImage imageNamed:@"image013.jpg"];    // 150 x 150 pixel image
cv::Mat cv_in_image = [in_image CVMat];

NSLog(@"cv_in_image cols = %i", cv_in_image.cols);
NSLog(@"cv_in_image rows = %i", cv_in_image.rows);

cv::Rect rect1;
rect1.x = 28;
rect1.y = 27;
rect1.width = 100;
rect1.height = 100;

NSLog(@"rect1 x = %i", rect1.x);
NSLog(@"rect1 y = %i", rect1.y);
NSLog(@"rect1 width = %i", rect1.width);
NSLog(@"rect1 height = %i", rect1.height);

cv::Mat Image1 = cv_in_image(rect1);

// This is the workarround !!!!!!!!!!!
/  -----------------------------------
// Use cvtColor-function twice in a row...
cv::cvtColor(Image1, Image1, cv::COLOR_RGB2BGR);  // after that function, the error is gone
cv::cvtColor(Image1, Image1, cv::COLOR_BGR2RGB);  // I use the function a second time to get back the original color-set...

NSLog(@"Image1 cols = %i", Image1.cols);
NSLog(@"Image1 rows = %i", Image1.rows);

self.imageView0.image = [[UIImage alloc] UIImageFromCVMat:(Image1)];
```

The ROI can finally be created as can be seen in the image documentation below : (...Really not sure why this suddenly works that way with the UIImage-View in iOS !! ????)

Roi extraction with cv::rect in opencv

Solution

Problem