Losslessly remove chrominance channels iff JPEG is grayscale

[cow10]

Is there a way of losslessly reducing a JPEG image's bit-depth to 8 if it's already in grayscale?

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magick convert image image

losslessly chooses the appropriate bit-depth for a grayscale PNG but results in recompression for JPEG. I'm currently using

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jpegtran -grayscale -outfile tmpImage

and checking to see if

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magick compare -metric rmse image tmpImage NUL

is "0 (0)" before replacing the image, though I'd prefer an IM-only solution if possible.

[snibgo]

IM is a general-purpose raster image processor. So it reads a file and decompresses it, processes the image, then compresses and writes a file. If the output format is JPEG, this will almost certainly be lossy.

Some operations on JPEG can be lossless, but only by doing those operations with intimate knowledge of how JPEG works. IM doesn't do that. Specialist programs such as jpegtran do.