I am writing a code for determining a JPEG image's color space. I have found two references that may help me implementing this. One is on the oracle.com, the other one is a C source code from the ijg.com which "is responsible for the reference implementation of the original JPEG standard".
However they do differ. E.g. in IJG when there is no Adobe marker and there are 4 channels it is assumed as CMYK, but in oracle it is YCCA. Also IJG's implementation doesn't look on subsampling, whereas for 4-channel subsampled it is YCCK in oracle specs, and so on.
Also there are many missings in ColorSpace class, when I implemented the oracle's logic I needed to specify 3 extra color spaces, like YCCK, YCCA, RGBA.
Another point is that I found information that JPEG does not support transparency in alpha channel here, why would oracle talk about YCCA and RGBA in the context of JPEG metadat specification?
In result when checking an image with IJG's logic it tells me it is CMYK (checked the image with ImageMagick on ubuntu and it also says it is CMYK), with oracle's logic it is YCCA. Who to believe? Why would oracle not rely on the original JPEG specification? Or there is something else I don't know?
After my comments on old JPEG standards, I finally found the answer.
On ISO/IEC 10918-6:2013 (E), section 6.1:
Images encoded with only one component are assumed to be grayscale data in which 0 is black and 255 is white.
Images encoded with three components are assumed to be RGB data encoded as YCbCr unless the image contains an APP14 marker segment as specified in 6.5.3, in which case the colour encoding is considered either RGB or YCbCr according to the application data of the APP14 marker segment. The relationship between RGB and YCbCr is defined as specified in Rec. ITU-T T.871 | ISO/IEC 10918-5.
Images encoded with four components are assumed to be CMYK, with (0,0,0,0) indicating white unless the image contains an APP14 marker segment as specified in 6.5.3, in which case the colour encoding is considered either CMYK or YCCK according to the application data of the APP14 marker segment. The relationship between CMYK and YCCK is defined as specified in clause 7.
and the APP14 flags is "Adobe\0", the AP12 has the transform flag:
Transform flag values of 0, 1 and 2 shall be supported and are interpreted as follows:
0 – CMYK for images that are encoded with four components in which all four CMYK values are complemented; RGB for images that are encoded with three components; i.e., the APP14 marker does not specify a transform applied to the image data.
1 – An image encoded with three components using YCbCr colour encoding.
2 – An image encoded with four components using YCCK colour encoding.
So, it depends: It should be CMYK, but it could be YCCK if APP14 and AP12 have the right values.