Error rotated error indexing

C++
1

Say I have a grayscale image 2 pixels width 3 pixels height. And I want to rotate it on some angle. For example, I will rotate it on 90 degrees. I will use the matrices for the forward and backward transformations like this:

col1 col2
0 -1
1 0
col1 col2
0 1
-1 0

First I calculate the rotated bounding rect, by transforming the points of the input rect and calculating boundingRect on them and for the current example it simply becomes like this:



cv::Rect inputRect(0, 0, 2, 3);
// calculations
cv::Rect rotatedRect(-3, 0, 3, 2);

Next I want to use a function from opencv: cv::remap to calculate the values of all the pixels in an image. But to do so, first I need to calculate the mapping matrices. I do it simply like this:



cv::Size rotated(3, 2);
cv::Matx<float, 2, 2> backward {0, 1, -1, 0};
auto mapX = cv::Mat_<float>::zeroes(rotated);
auto mapY = cv::Mat_<float>::zeroes(rotated);

for (int row = 0; row < rotated.height; row++)
    for (int col = 0; col < rotated.width; col++)
    {
        cv::Vec<float, 2> input(rotatedRect.x + col, rotatedRect.y + row)
        cv::Vec<float, 2> output = backward * input;
        mapX(row, col) = output[0];
        mapY(row, col) = output[1];
    }

The problem is that this code generates the following x and y map matrices:

col1 col2 col3
0 0 0
1 1 1
col 1 col 2 col3
3 2 1
3 2 1

which lead to an image with the black line of pixels on the left side because y coordinates of 3 are out of the input matrix range.

I absolutely understand that I met the problem with the difference of matrix size/pixel index, but I can’t understand how to fix it…

2

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