Why several FFDA markers in jpeg file

Question

In my jpeg file there are few FFDA markers. From which marker my data starts, and how do I know from which marker I decode the file?

Answer 1

Your JPEG is probably progressive which means you have to decode the data after at least the first FFDA marker, which will bring you an intermediate result.

If this is your first attempt at making a JPEG decoder you should choose another image and try to implement a baseline decoder instead. Progressive images adds a lot of complexity to the problem.

Answer 2

The JPEG standard has many options that are not used very often. A typical color image will have 3 color components (Y, Cr, Cb) interleaved in a single scan (one FFDA marker). They can also be stored in any combination in separate scans. A progressive JPEG image encodes multiple scans with more and more detail (AC coefficients) in each successive scan, but the standard allows any combination of color components and coefficients to be mixed in different scans. I have only seen one case of a non-progressive JPEG with separate scans for each color component; it came from an IP camera.