Convert 12-bit Bayer image to 8-bit RGB using OpenCV

I was able to convert my data to 8-bit RGB using the following code:

// Copy the data into an OpenCV Mat structure
cv::Mat bayer16BitMat(height, width, CV_16UC1, inputBuffer);

// Convert the Bayer data from 16-bit to to 8-bit
cv::Mat bayer8BitMat = bayer16BitMat.clone();
// The 3rd parameter here scales the data by 1/16 so that it fits in 8 bits.
// Without it, convertTo() just seems to chop off the high order bits.
bayer8BitMat.convertTo(bayer8BitMat, CV_8UC1, 0.0625);

// Convert the Bayer data to 8-bit RGB
cv::Mat rgb8BitMat(height, width, CV_8UC3);
cv::cvtColor(bayer8Bit, rgb8BitMat, CV_BayerGR2RGB);

I had mistakenly assumed that the 12-bit data I was getting from the camera was tightly packed, so that two 12-bit values were contained in 3 bytes. It turns out that each value was contained in 2 bytes, so I didn't have to do any unpacking to get my data into a 16-bit array that is supported by OpenCV.

Edit: See @petr's improved answer that converts to RGB before converting to 8-bits to avoid losing any color information during the conversion.

For anyone struggling with this, the above solution only works if your image actually comes in 16bit otherwise, as already suggested by the comments you should chop-off the 4 least significant bits. I achieved that with this. It's not very clean but it works.

unsigned short * image_12bit = (unsigned short*)data;
char out[rows * cols];

for(int i = 0; i < rows * cols; i++) {
  out[i] = (char)((double)(255 * image_12bit[i]) / (double)(1 << 12));
}

cv::Mat bayer_image(rows, cols, CV_8UC1, (void*)out);
cv::cvtColor(bayer_image, *res, cv::COLOR_BayerGR2BGR);

The Gillfish's answer technically works but during the conversion it uses smaller data structure (CV_8UC1) than the input (which is CV_16UC1) and loses some color information.

I would suggest first to decode the Bayer encoding but stay in 16-bits per channel (from CV_16UC1 to CV_16UC3) and later convert to CV_8UC3.

The modified Gillfish's code (assuming the camera gives image in 16bit Bayer encoding):

// Copy the data into an OpenCV Mat structure
cv::Mat mat16uc1_bayer(height, width, CV_16UC1, inputBuffer);

// Decode the Bayer data to RGB but keep using 16 bits per channel
cv::Mat mat16uc3_rgb(width, height, CV_16UC3);
cv::cvtColor(mat16uc1_bayer, mat16uc3_rgb, cv::COLOR_BayerGR2RGB);

// Convert the 16-bit per channel RGB image to 8-bit per channel
cv::Mat mat8uc3_rgb(width, height, CV_8UC3);
mat16uc3_rgb.convertTo(mat8uc3_rgb, CV_8UC3, 1.0/256); //this could be perhaps done more effectively by cropping bits