Editing JPEG File Interchange Format (section)

=== 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"
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!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"
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!colspan="3" style="text-align:left"| Thumbnail stored using one byte per pixel
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! 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
|}