Compressed Image File Formats: JPEG, PNG, GIF, XBM, BMP

The encoding process depends upon whether you have a sequential scan or a progressive scan. The details of the encoding process are too complicated to fit within an answer here.

I highly recommend this book:

https://www.amazon.com/Compressed-Image-File-Formats-JPEG/dp/0201604434/ref=sr_1_2?ie=UTF8&qid=1531091178&sr=8-2&keywords=JPEG&dpID=5168QFRTslL&preST=_SX258_BO1,204,203,200_QL70_&dpSrc=srch

It is the only source I know of that explains JPEG end-to-end in plain English.

JPEG is extremely complex. The only source I know of that explains this for programmers is:

https://www.amazon.com/Compressed-Image-File-Formats-JPEG/dp/0201604434

This is the only book that actually explains the process for programmers without academic mumbo-jumbo

https://www.amazon.com/Compressed-Image-File-Formats-JPEG/dp/0201604434/ref=sr_1_2?ie=UTF

https://www.amazon.com/Compressed-Image-File-Formats-JPEG/dp/0201604434/ref=pd_bxgy_14_img_2?_encoding=UTF8&psc=1&refRID=VPB1FTFQG5PJH1AFZ51M

https://www.amazon.com/dp/0201604434/