Why can JPEG images contain several quantization tables?

Question

I checked and roughly understood the JPEG encoding algorithm from the wiki page about it.

1. Transform to YCbCr, downsample
2. Split into 8x8 blocks
3. Apply DCT on blocks
4. Divide resulting matrices by quantization table entries
5. Entropy encoding

I understand that the quantization table in a file depends on what created the image, e.g. a camera manufacturer likely has their own proprietary QT algorithms, photoshop etc have their own QTs, there are public ones, etc. Now, if one opens 'real' JPEG files they may contain several quantization tables. How can this be? I'd assume the decoding algorithm looks like this:

1. Decode entropy encoding, receive blocks
2. Multiply blocks by quantization table entries
3. revert other operations

What does the second/third/... QT do/when is it used? Is there an upper limit on the number of QTs in a JPEG file? When does it happen that a second QT is added to a JPEG file?

Answer 1

The quantization tables are used for different color components.

Like you already know, in a first step the image is transformed into YCbCr Color Space. In this color space you have three colors: Luminance (Y), Chrominance blue (Cb) and Chrominance red (Cr). As the human eye is less sensitive to colors but very sensitive for brightness, multiple quantization tables are used for the different components.

The Quantization Tables used for the Luminance consists of "lower" values, such that the dividing and rounding will not loose to much information on this component. Blue and red on the other hand have "higher" values as information is not needed that much.