Random access on 3-channel, 64-bit, integer image?

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Hello,

For a project I'm working on, we need to use a 3-channel, 64-bit, integer image. Since there is no 64-bit integer image type available to us in OpenCV, we build the image with type CV_64FC3 and then just fill it by accessing its locations by pointer. That is, the cv::Mat object thinks it is a double-precision type but we filled in integer data in a bitwise manner. This part seems to work.

What does not work is getting information out of this image in a random-access manner using OpenCV's built-in .at<...>(...) function. I'd like to be able to access a part of the image as a cv::Vec<...> type, without having to use reinterpret_cast< uint64_t>(...) constantly.

Here's the code demonstrating the point:

// Works.
cv::Vec3d vd = img.at< cv::Vec3d>( yy, xx );

// Works.
cv::Vec< double, 3> vd2 = img.at < cv::Vec< double, 3> >( yy, xx );

// Fails with OpenCV assertion failure.
cv::Vec< uint64_t, 3> vi = img.at< cv::Vec< uint64_t, 3> >( yy, xx );

The assertion failure I get with the last line is:

OpenCV Error: Assertion failed (((((sizeof(size_t)<<28)|0x8442211) >> ((DataType<_Tp>::depth) & ((1 << 3) - 1))*4) & 15) == elemSize1()) in cv::Mat::at, file c:\path\to\opencv_301\include\opencv2\core\mat.inl.hpp, line 930

If anyone has any input as to what I am doing wrong, I would love to hear it. I'd really like to do the random-access here using the .at<...>(...) function as illustrated in the failing line above.

Thank you for your time.