Grayscale JPEG to YCbCr JPEG

Question

How can I convert a grayscale JPEG to a YCbCr JPEG?

I am looking for a possibility without converting the JPEG file to RGB and then back to JPEG.

Answer 1

Y is the gray channel. All you would need to do is change the number of components in the SOF marker from 1 to three and add dummy Cb and Cr channels that produce all zeros.