Save OpenNI depth image as BMP

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Hi. I'm using OpenCV to capture RGB-D image with OpenNI. I can capture a depth image, however, I couldn't save it as bmp. I know it can be saved as 16-bit PNG. However, the computer I'm using is not so powerful one (embedded computer) for robot. I've tried to save depth as 16-bit PNG image, however, it is so slow. I also tuned the compression ration as lowest, it is still slow.

So I tested BMP for comparison, it was fast. I think I can save it as 16-bit BMP, however, when I save using imwrite, it just save 8-bit greyscale bmp. Does anyone know how to do that?