Does dct2 function perform quantization and compression?

Hello group,

I am new a DSP novice and am trying to understand the DCT. I have a couple of questions. My understanding of the DCT is that there are three steps

1) DCT 2) Quantization 3) Compression (zig-zag style like JPEGs).

Does the dct2 function perform all of these functions automatically so or just the first step?

Some context of what I am trying to do:

Extract features from lip-images for use with visual speech recognition. In the literature, any lip-images are transformed using the DCT and usually the first 30 or 35 co-efficients taken as input vectors. I am trying to replicate some of their published experiments and am unclear as to what people mean when they talk about the "first" n co-efficients. Is it simply the first 30 [i,j] values after the DCT step or the first 30 co-efficients after the compression step? I recognise that my query in this regard might be off-topic, but any insights from DSP experts would be appreciated.

Best regards,

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