Editing JPEG File Interchange Format (section)

== File format structure ==
A JFIF file consists of a sequence of markers or marker segments (for details refer to [[JPEG#Syntax and structure|JPEG, Syntax and structure]]). The markers are defined in part 1 of the [[JPEG]] Standard.<ref name="itu_t81" /> Each marker consists of two bytes: an <code>FF</code> byte followed by a byte which is not equal to <code>00</code> or <code>FF</code> and specifies the type of the marker. Some markers stand alone, but most indicate the start of a marker segment that contains data bytes according to the following pattern:

<code>FF ''xx'' ''s1'' ''s2'' ''[data bytes]''</code>

The bytes ''s1'' and ''s2'' are taken together to represent a [[Endianness|big-endian]] 16-bit integer specifying the length of the following "data bytes" plus the 2 bytes used to represent the length. In other words, ''s1'' and ''s2'' specify the number of the following ''data bytes'' as <math>256 \cdot  s1 + s2 - 2</math>.

According to part 1 of the JPEG standard, applications can use APP marker segments and define an application specific meaning of the data. In the JFIF standard, the following APP marker segments are defined:
* JFIF APP0 marker segment (JFIF segment for short) (mandatory)
* JFIF extension APP0 marker segment (JFXX segment for short) (optional)
They are described below.

The JFIF standard requires that the JFIF APP0 marker segment immediately follows the SOI marker. If a JFIF extension APP0 marker segment is used, it must immediately follow the JFIF APP0 marker segment.<ref name="hamilton_1992" /> So a JFIF file will have the following structure:

{| class="wikitable"
|-
!colspan="3" style="text-align:left"|JFIF file structure
|-
!style="text-align:left"|Segment !! style="text-align:left"|Code !! style="text-align:left"|Description
|-
| SOI || <code>FF D8</code> || Start of Image
|-
| JFIF-APP0 || <code>FF E0 ''s1'' ''s2'' 4A 46 49 46 00 ...</code> || see below
|-
| JFXX-APP0 || <code>FF E0 ''s1'' ''s2'' 4A 46 58 58 00 ...</code> || optional, see below
|-
| colspan="3" | … additional marker segments<br /> (for example SOF, DHT, COM)
|-
| SOS ||  <code>FF DA</code>  ||  Start of Scan
|-
|          || compressed image data ||
|-
| EOI || <code>FF D9</code>  || End of Image
|}

=== JFIF APP0 marker segment ===

In the mandatory JFIF APP0 marker segment the parameters of the image are specified. Optionally an uncompressed thumbnail can be embedded.

{| class="wikitable"
|-
!colspan="3" style="text-align:left"|JFIF APP0 marker segment
|-
! style="text-align:left"|Field || style="text-align:left"|Size (bytes) || style="text-align:left"|Description
|-
| APP0 marker || 2 || <code>FF E0</code>
|-
| Length || 2 || Length of segment excluding APP0 marker
|-
| Identifier || 5 || <code>4A 46 49 46 00</code> = "JFIF" in [[ASCII]], terminated by a null byte
|-
| JFIF version || 2 || First byte for major version, second byte for minor version (<code>01 02</code> for 1.02)
|-
| Density units || 1 || Units for the following pixel density fields
* <code>00</code> : No units; width:height [[pixel aspect ratio]] = Ydensity:Xdensity
* <code>01</code> : Pixels per inch (2.54&nbsp;cm)
* <code>02</code> : Pixels per centimeter
|-
| Xdensity || 2 || Horizontal pixel density. Must not be zero
|-
| Ydensity || 2 || Vertical pixel density. Must not be zero
|-
| Xthumbnail || 1 || Horizontal pixel count of the following embedded RGB thumbnail. May be zero
|-
| Ythumbnail || 1 || Vertical pixel count of the following embedded RGB thumbnail. May be zero
|-
| Thumbnail data || 3 × ''n'' || Uncompressed 24 bit RGB (8 bits per color channel) raster thumbnail data in the order R0, G0, B0, ... Rn-1, Gn-1, Bn-1; with ''n'' = Xthumbnail × Ythumbnail
|}

=== JFIF extension APP0 marker segment ===
Immediately following the JFIF APP0 marker segment may be a JFIF extension APP0 marker segment. This segment may only be present for JFIF versions 1.02 and above. It allows to embed a thumbnail image in 3 different formats.

{| class="wikitable"
|-
!colspan="3" style="text-align:left"|JFIF extension APP0 marker segment
|-
! style="text-align:left"|Field || style="text-align:left"|Size (bytes) || style="text-align:left"|Description
|-
| APP0 marker || 2 || <code>FF E0</code>
|-
| Length || 2 || Length of segment excluding APP0 marker
|-
| Identifier || 5 || <code>4A 46 58 58 00</code> = "JFXX" in [[ASCII]], terminated by a null byte
|-
| Thumbnail format || 1 || Specifies what data format is used for the following embedded thumbnail:
* <code>10</code> : JPEG format
* <code>11</code> : 1 byte per pixel palettized format
* <code>13</code> : 3 byte per pixel RGB format
|-
| Thumbnail data || variable || Depends on the thumbnail format, see below
|}

The thumbnail data depends on the thumbnail format as follows:

{| class="wikitable"
|-
!colspan="3" style="text-align:left"| Thumbnail stored using JPEG encoding
|-
! style="text-align:left"|Field || style="text-align:left"|Size (bytes) || style="text-align:left"|Description
|-
| SOI || 2 || <code>FF D8</code>
|-
|          || variable || Must be JIF format using YCbCr or just Y, and must not contain JFIF or JFXX segments
|-
| EOI || 2 || <code>FF D9</code>
|}

{| class="wikitable"
|-
!colspan="3" style="text-align:left"| Thumbnail stored using one byte per pixel
|-
! style="text-align:left"|Field || style="text-align:left"|Size (bytes) || style="text-align:left"|Description
|-
| Xthumbnail || 1 || Horizontal pixel count of the following embedded thumbnail. Must not be zero
|-
| Ythumbnail || 1 || Vertical pixel count of the following embedded thumbnail. Must not be zero
|-
| Thumbnail palette || 768 || 256 palette entries, each containing a 24 bit RGB color value
|-
| Thumbnail data || ''n'' || One byte per pixel containing the index of the color within the palette,
with ''n'' = Xthumbnail × Ythumbnail
|}

{| class="wikitable"
|-
!colspan="3" style="text-align:left"| Thumbnail stored using three byte per pixel
|-
! style="text-align:left"|Field || style="text-align:left"|Size (bytes) || style="text-align:left"|Description
|-
| Xthumbnail || 1 || Horizontal pixel count of the following embedded thumbnail. Must not be zero
|-
| Ythumbnail || 1 || Vertical pixel count of the following embedded thumbnail. Must not be zero
|-
| Thumbnail data || 3 × ''n'' || Uncompressed 24 bit RGB (8 bits per color channel) raster thumbnail data in the order R0, G0, B0, ... Rn-1, Gn-1, Bn-1; with ''n'' = Xthumbnail × Ythumbnail
|}