How to Apply Mask to Image in OpenCV?

You don't apply a binary mask to an image. You (optionally) use a binary mask in a processing function call to tell the function which pixels of the image you want to process. If I'm completely misinterpreting your question, you should add more detail to clarify.

While @perrejba s answer is correct, it uses the legacy C-style functions. As the question is tagged C++, you may want to use a method instead:

inputMat.copyTo(outputMat, maskMat);

All objects are of type cv::Mat.

Please be aware that the masking is binary. Any non-zero value in the mask is interpreted as 'do copy'. Even if the mask is a greyscale image.

Also be aware that the .copyTo() function does not clear the output before copying.

If you want to permanently alter the original Image, you have to do an additional copy/clone/assignment. The copyTo() function is not defined for overlapping input/output images. So you can't use the same image as both input and output.

Well, this question appears on top of search results, so I believe we need code example here. Here's the Python code:

import cv2

def apply_mask(frame, mask):
    """Apply binary mask to frame, return in-place masked image."""
    return cv2.bitwise_and(frame, frame, mask=mask)

Mask and frame must be the same size, so pixels remain as-is where mask is 1 and are set to zero where mask pixel is 0.

And for C++ it's a little bit different:

cv::Mat inFrame; // Original (non-empty) image
cv::Mat mask; // Original (non-empty) mask

// ...

cv::Mat outFrame;  // Result output
inFrame.copyTo(outFrame, mask);