Is the output of the libjpeg always RGB (or luminance for monochrome images)?

Question

I use he libjpeg to manipulate jpeg images.
My question is simple :
Is the output of the libjpeg always RGB (or luminance for monochrome images) ?
I'm not an expert with the color spaces... Thanks !

Answer 1

Strictly speaking, JPEG is always YCbCr (YUV), so that's the output. You may apply other colorspaces, but typically only RGB or (less commonly) CMYK are supported, and the underlying data in the file will remain YCbCr. Grayscale simply leaves the second two channels out and only encodes Y.