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Before the image data is ever loaded when a JPEG image is selected for viewing the markers must be read. In a JPEG image, the very first marker is the SOI, or Start Of Image, marker. This is the first "hey, I'm a JPEG" declaration by the file. The JPEG standard, as written by the Joint Picture Expert's Group, specified the JPEG interchange format. This format had several shortcomings for which the JFIF (JPEG File Interchange Format) was an attempted remedy. The JFIF is the format used by almost all JPEG file readers/writers. It tells the image readers, "Hey, I'm a JPEG that almost anyone can understand."
Most markers will have additional information following them. When this is the case, the marker and its associated information is referred to as a "header." In a header the marker is immediately followed by two bytes that indicate the length of the information, in bytes, that the header contains. The two bytes that indicate the length are always included in that count.
A marker is prefixed by FF (hexadecimal). The marker/header information that follows does not specify all known markers, just the essential ones for baseline JPEG.
A component is a specific color channel in an image. For instance, an RGB image contains three components; Red, Green, and Blue.
Start of Image (SOI) marker -- two bytes (FFD8)
JFIF marker (FFE0)
Define Quantization table marker (FFDB)
Define Huffman table marker (FFC4)
Start of frame marker (FFC0)
The H and V sampling factors dictate the final size of the component they are associated with. For instance, the color space defaults to YCbCr and the H and V sampling factors for each component, Y, Cb, and Cr, default to 2, 1, and 1, respectively (2 for both H and V of the Y component, etc.) in the Jpeg-6a library by the Independent Jpeg Group. While this does mean that the Y component will be twice the size of the other two components--giving it a higher resolution, the lower resolution components are quartered in size during compression in order to achieve this difference. Thus, the Cb and Cr components must be quadrupled in size during decompression.
Start of Scan marker (FFDA)
Comment marker (FFFE)
End of Image (EOI) marker (FFD9)
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