extract Grayscale from yuv in libjpeg

Question

On either TurboJpeg or the underlying libJpeg APIs - is there a simple function for extracting the Y component from the Ycbcr (after I've already read the image)?

I know I can loop over the data and extract the bytes. Isn't this already implemented?

Answer 1

When decompressing/decoding, use the TJPF_GRAY format, it discards the colour information and only the Y (luminescence) remains.

See the this part of the API for reference, as well as that part if you are willing to transform it , or this function if you are willing to decode it directly into a gray-scale image.