How can I change the 8bit rgb images into 12bit rgb image

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Thanks for the reply. We have tried above-mentioned formula on 8-bit image but we are getting a black image as output. but we use OpenCV method ( convertTo) by setting alpha as 255 then getting readable image

don't forget, that you need a larger type to hold 12 bits, such as CV_16U, so:

img8.convertTo( img16, CV_16U, 4095.0/255 );

(just multiplying by 255 will make it 16bit range)