JPEG Header Format

Strictly speaking, JPEG files do not have formal headers, but fg_jpeghead() and fgi_jpeghead() return relevant information from the file's start of frame segment. We call it a header for consistency with other image file formats.

offset

 size

 description
0

 2

 JPEG SOI marker (FFD8 hex)
2

 2

 image width in pixels
4

 2

 image height in pixels
6

 1

 number of components (1 = grayscale, 3 = RGB)
7

 1

 horizontal/vertical sampling factors for component 1
8

 1

 sampling factors for component 2 (if RGB)
9

 1

 sampling factors for component 3 (if RGB)

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copyright 2001 Ted Gruber Software, Inc.