OpenCV: how to set alpha transparency of a pixel

Question

I have an image I am trying to segment by colouring each pixel either red green or blue. I have calculated a confidence score for each pixel and want to adjust the alpha tra

Answer 1

There are many ways to accomplish this. One possible way is to access and modify each individual pixel. Assuming image is a four-channel, 8-bit cv::Mat:

auto& pixel = image.at<cv::Vec4b>(i,j);
pixel[3] = confidence;

where i and j are the indices of the pixel in the image.

There are other methods that are probably more elegant, but they will depend on your current code.

UPDATE: The behavior you describe is to be expected. Apparently cv::imshow() does not support transparency. This explains why your displayed image is all blue.

As for the saved image, it is important to remember that the image is of type CV_8UC4. That means that each channel element is stored as a uchar. Assigning a value of 0.5 will truncate to zero, hence the fully transparent saved image.

If your confidence is a float value in the range [0,1], scale it by 255 to put it in the range supported by 8-bit images. Thus,

v[3] = 0.5;

becomes

v[3] = 0.5 * 255;

Answer 2

I've also had the same problem but when was drawing transparent shapes and solved it by blending images. I've fount this in OpenCV's documentation

if you want to paint semi-transparent shapes, you can paint them in a separate buffer and then blend it with the main image.

For this purpose I suggest you to take a look at this documentation article. Hope this will help.