JFIF (JPEG File Interchange Format)

Definition: The JPEG File Exchange Format is a minimal file format that allows the exchange of JPEG bitstreams among different applications and platforms.

It uses a number of markers to break a JPEG stream down into its component structures. Markers are two bytes in length and the first byte always contains the value 0×FF; the second byte contains a code that specifies the marker type. Table 1 shows some of the most important JFIF markers, whose explanation follows.

1. SOI: This marker is found at the start of a JFIF file.
2. APP0: Contains JFIF information, including image thumbnail.
3. APPn: Contains application specific information.
4. DQT: Contains the quantization table for quantization and dequantization operations which are performed after FDCT and before IDCT.
5. SOF0: Contains image bits per pixel, height, and width among other information.
6. DHT: For Huffman encoding after quantization and Huffman decoding before dequantization.
7. SOS: Contains compressed, encoded image data.
8. Page 372  EOI: Signifies end of image.
9. COM: Contains user defined plaintext comments, often containing image metadata.

The standard also defines the RGB to YCbCr conversion and color subsampling steps.