What is .jpg file?
Joint Photographic Experts Group JFIF is a raster image.
Images are compressed.
File extension is .jpg.
Software handling .jpg files
Rest7 Free Online Converter
Structure of .jpg images
JPEG images don't have a header. They have segments.
Each segment has a different meaning and each starts with 0xFF. After this byte
there is another byte which defined segment meaning.
First segment SOI (FF D8)
This is the first segment of JPEG image. It's just 2 bytes long.
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xD8
|
If a file has these 2 bytes at the beginning then it's most likely a JPEG image.
(Should be second) segment APP0 (FF E0)
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xE0
|
| Length
| UInt16
| Length of this segment (excluding first 2 bytes- the Marker)
|
| Identifier
| Char8[5]
| = 'JFIF\0'
|
| MajorVersion
| UInt8
| Eg. 01
|
| MinorVersion
| UInt8
| Eg. 02
|
| DensityUnits
| UInt8
| 0 = no units, 1 = pixels per inch (2.54 cm), 2 = pixels per cm
|
| XDensity
| UInt16
| Horizontal pixel density. Must not be 0
|
| YDensity
| UInt16
| Vertical pixel density. Must not be 0
|
| XThumbnail
| UInt8
| Horizontal pixel density of a thumbnail. Can be 0
|
| YThumbnail
| UInt8
| Vertical pixel density a thumbnail. Can be 0
|
| Thumbnail
| ...
| Uncompressed 24 bit RGB. Number of pixels = XThumbnail * Ythumbnail
So a 200x100 thumbnail needs 60000 bytes.
|
Additional APP0 segment (FF E0)
This segment is optional. If it is present it should be after the first APP0 segment.
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xE0
|
| Length
| UInt16
| Length of this segment (excluding first 2 bytes- the Marker)
|
| Identifier
| Char8[5]
| = 'JFXX\0'
|
| ThumbnailFormat
| UInt8
| 10 = JPEG, 11 = 8 bpp palettized, 13 = RGB 24 bpp
|
| Thumbnail
| ...
| Depends on the thumbnail format
|
If thumbnail format = 10 then a new segment follows:
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xD8
|
| Thumbnail
| ...
| JIF format image using YCbCr or just Y, and must not contain JFIF or JFXX segments
|
| EndOfSegment
| UInt16
| = 0xFF 0xD9
|
Or if thumbnail format = 11 then a new segment follows:
| Name
| Type
| Description
|
| Xthumbnail
| UInt8
| Width of thumbnail
|
| Ythumbnail
| UInt8
| Height of thumbnail
|
| ThumbnailPalette
| UInt24[256]
| 256 entries of the palette. Each entry is 24 bit RGB
|
| Thumbnail
| ...
| Size in bytes = Xthumbnail × Ythumbnail
|
OR if thumbnail format = 13 then a new segment follows:
| Name
| Type
| Description
|
| Xthumbnail
| UInt8
| Width of thumbnail
|
| Ythumbnail
| UInt8
| Height of thumbnail
|
| Thumbnail
| ...
| Size in bytes = 3 * Xthumbnail × Ythumbnail, (RGB 24 bit)
|
(Often second or third) segment APP1 (FF E1)
This is used to store EXIF data. If you don't need EXIF data you can skip parsking this segment.
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xE1
|
| Length
| UInt16
| Length of this segment (excluding first 2 bytes- the Marker)
|
| ExifData
| ...
| Size in bytes = "Length"
|
Segment COM (FF FE)
This is an optional comment. If you don't need the comment can skip parsking this segment.
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xFE
|
| Length
| UInt16
| Length of this segment (excluding first 2 bytes- the Marker)
|
| Comment
| ...
| Size in bytes = "Length"
|
Segment SOF0 (FF C0)
Start Of Frame. If this segment is present it means the image is baseline DCT-based
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xC0
|
| Length
| UInt16
| Length of this segment (excluding first 2 bytes- the Marker)
|
| BitsPerSample
| UInt8
| Usually 8, 12 and 16 are rarely supported
|
| Width
| UInt16
|
|
| Height
| UInt16
|
|
| ComponentsCount
| UInt8
| 1 = grayscale, 3 = color YcbCr or YIQ, 4 = CMYK
|
| Compontents
| UInt24[ComponentsCount]
|
|
Segment SOF2 (FF C2)
Start Of Frame. If this segment is present it means the image is progressive DCT-based
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xC2
|
| Length
| UInt16
| Length of this segment (excluding first 2 bytes- the Marker)
|
| Data
| ...
|
|
Segment DHT (FF C4)
Define Huffman Table
Segment DQT (FF DB)
Define Quantization Table
Segment DAC (FF ??)
Define Arithmetic Table
Most programs don't support this arithmetic-coded images.
Segment SOS (FF DA)
Start Of Scan
Last segment EOI (FF D9)
This is the last segment of JPEG image. It's just 2 bytes long.
| Name
| Type
| Description
|
| Marker
| UInt16
| = 0xFF 0xD9
|
Data types:
UInt8, UInt16, UInt32, UInt64 - Unsigned Integer 8 bit, 16 bit, 32 bit, 64 bit
Int8, Int16, Int32, Int64 - Signed Integer 8 bit, 16 bit, 32 bit, 64 bit
UMint16, UMint32 - Unsigned Motorola Integer 16 bit, 32 bit (Big Endian)
Char8, Char16 - Char 8 bit, 16 bit
Real world .jpg samples
Here are some real world example pictures.
