I never really gave it much thought but my recent research into the images supported by Xerox WorkCentres caused me to ponder the issue of DCTDecode. DCTDecode is used to encode an image using the JPEG compression, and is also a fully functional JPEG image, however, in PDF, one should consider the object as a TIFF bitmap, encoded using JPEG compression. I recently noticed the relevance of the notation in the Xerox 7655 specification:
- TIFF (6.0 Old JPEG and New JPEG)
It helps understand why JFIF is not a required format, as the PDF format itself contains the information that would typically be encoded in the APP14 tag.
A minor mystery laid to rest. Also note that the YCbCr tag is obviously inserted by some older software to help with managing the JPEG encoding, as JPEG itself failed to provide for ways to indicate the colorspace used. While PDF resolved this by encoding this in the PDF data (/ColorSpace tag), JFIF was proposed to extend the original JPEG standard.
Now it makes all much better sense as the relevance of DCTDecode objects, which are JPEG encoded data streams used to encode a TIFF image.
The YCbCr comment tag is likely added to inform the PDF generator as to the nature of the DCTDecode object. It all starts to make sense
This also makes sense, realizing that Xerox uses the same DCTDecode object to encode LinearGray images which is a color space not supported by JPEG.
Wherever the data takes us…