|Description of Exif file format||2005-05-18|
Most of the digital camera's use Exif file format to store images. This specification was made by
JEIDA. Some information can be found on the internet. I could
not find any upto date document. So I started collecting and published my findings on the web.
Here is a tiny description about Exif.
Sometimes DCF is mentioned (Design rule for Camera File system). DCF defines whole file-system of digital camera: directory structure, file naming method, character set and file format etc. The file format of DCF is based on Exif2.1 specification and can be found on EXIF.org. This document is more or less based on Exif2.1/DCF specification
This is a FREE / OPEN document, you may use this document for any purpose (commercial / non-commercial) of 'all' / 'part of' this document.
If you have information about
- 'unknown' items
- fautly data
- missing items
Please e-mail me, Ger Vermeulen g e v e 0 0 1@ c a s e m a . n l
rev. 1.1 2005-06-03
|1 What is Exif file format?|
|The Exif file format is based on the JPEG file format. Exif inserts some of image / digital camera information and thumbnail image to JPEG in conformity to JPEG specification. Therefore you can view Exif formatted image files by JPEG compliant Internet browser / Picture viewer / Photo retouch software etc. as normal JPEG image files.|
|2. JPEG format and Marker|
Every JPEG file starts from binary value '0xFFD8', ends with binary value '0xFFD9'. There are
several binary 0xFFXX data in JPEG data, they are known as 'Marker', and it
starts a new part of JPEG information. 0xFFD8 means SOI (Start of image), 0xFFD9 means EOI
(End of image). These two special
Markers have no data following, the other markers have data with it.
Basic format of Marker is below.
The 'Data' contains Data Size descriptor and Information, Size (2 Bytes) has 'Motorola' byte align. The data size is the size of the total Data: Size + Info ( value = 2 + n).
FF xx - 00 0C - 01 02 03 04 05 06 07 08 09 0A
In JPEG format, some of Markers describe data, then SOS (Start Of Stream) marker placed. After the SOS marker, JPEG image stream starts and terminated by EOI (End Of Image) marker.
|3. EXIF marker|
The marker 0xFFE0~0xFFEF is named 'Application Marker', not necessary for decoding JPEG image. They are
used by user application. For example, older olympus / canon / casio /
agfa digital camera use JFIF (JPEG File Interchange
Format) for storing images. JFIF uses APP0 (0xFFE0) marker for inserting digital camera configuration data and
For inserting data Exif uses the Application Marker APP1 (0xFFE1) marker to avoid a conflict with the JFIF format. Every Exif file format starts from this marker.
It starts with SOI (0xFFD8) marker, so it's a JPEG file. Then an APP1 marker follows immediately. All the data of Exif is stored in the APP1 data area. The APP1 Data part starts with size 'ssss' (Exif data area). The size 'ssss' includes the size field also.
After the 'SSSS', APP1 data starts. This part is explained in the next section.
After the APP1 marker area other JPEG markers follow.
|4. Exif data structure|
The structure of Exif data (APP1) is shown below. This is a case of 'Intel' byte aligns and it contains
JPEG format thumbnail. The Exif data starts from ASCII character 'Exif' and 2bytes of 0x00,
and then Exif data follows. Exif uses TIFF format to store data. For more details of TIFF format, please refer
to 'TIFF6.0 specification'.
|4.1 TIFF header structure|
First 8 bytes of TIFF format is the TIFF header.
First 2 bytes defines byte align of TIFF data. 0x4949='I I' is 'Intel' type byte align, 0x4d4d='MM' is 'Motorola' type byte align. For example, decimal value '305.419.896' is noted as hexadecimal 0x12345678. If Motorola aligned, it is noted as 0x12345678, if it's Intel aligned it is noted as 0x78563412. A lot of digital camera uses Intel align, but not all. Therefore when we need the value of Exif data, we MUST check byte align field every time. Though JPEG data uses Motorola align only, Exif allows both alignments. I can't understand why Exif didn't fix a byte align to Motorola.
Next 2 bytes always have the value of 0x002A.
The last 4 bytes of TIFF header is the offset to the first IFD (Image File Directory, described in next chapter). Includes this offset, all the offset value used in TIFF format counts offset bytes from the first byte of the TIFF header ('I I' or 'MM'). Usually the first IFD starts immediately next to TIFF header, so this offset has often the value '0x00000008'.
|4.2 IFD : Image File Directory|
Next to TIFF header, there is the first IFD : Image File Directory - IFD-0. It contains image information data.
The first 2 bytes ('eeee') contains the number of directory entries in this IFD.
Then directory entries (12 bytes per entry) follows.
After last directory entry, there is a 4bytes of data ('oooooooo'), it has the offset to next IFD. If its value is '0x00000000', it means this is the last IFD and there is no linked IFD.
Data format ('ffff') is defined as see table.
'rational' means a fractional value, it contains 2-signed/unsigned long integer values, and the first represents the numerator, and the second, the denominator.You can get the total data byte length by multiplies a 'bytes/components' value by number of components stored 'nnnnnnnnn' area. If total data length is less or equal than 4bytes, 'dddddddd' contains the value of that Tag. If its size is over 4bytes, 'dddddddd' contains the offset to data stored address.
|4.3 IFD data structure example|
At Exif format, the first IFD is IFD0 (IFD of main image), then it links to
IFD1 (IFD of thumbnail image) and
IFD link is terminated. But IFD0/IFD1 doesn't contain any digital
camera's information such as shutter speed, focal
length etc. IFD0 always contains special Tag Exif Offset (0x8769), it shows an offset to Exif SubIFD.
Exif SubIFD is IFD formatted data also, it contains digital camera's information.
In case of Extended Exif format (Exif2.1/DCF), Exif SubIFD contains special Tag Exif Interoperability Offset (0xa005). It also points to the Interoperability IFD. By the DCF specification, this tag is mandatory and both SubIFD (main image IFD) and IFD1 (thumbnail image IFD) may have Interoperability IFD. In usual, only main image have this tag.
And some of digital camera uses IFD data format for Makernote; Maker-specific magic data area. It's very hard to decide whether makernote is IFD format or not, be careful to coding program. See Appendix for information of Makernote.
0000: 49 49 2A 00 08 00 00 00-02 00 1A 01 05 00 01 00 0010: 00 00 26 00 00 00 69 87-04 00 01 00 00 00 11 02 0020: 00 00 40 00 00 00 48 00-00 00 01 00 00 00If the first part of TIFF data is above, it can read as:
|4.4 Thumbnail image|
|Exif format contains a thumbnail of the image (except Ricoh RDC-300Z). Usually it is located next to the IFD1. There are 3 formats for thumbnails; JPEG format (JPEG uses YCbCr), RGB TIFF format, YCbCr TIFF format. It seems that JPEG format and 160x120 pixels of size are recommended thumbnail format for Exif2.1 or later. By the DCF specification, thumbnail image MUST use JPEG format and image size is fixed to 160x120 pixels.|
|4.4.1 JPEG format thumbnail|
|If the value of Compression (0x0103) Tag in IFD1 is '6', thumbnail image format is JPEG. Most of Exif image uses JPEG format for thumbnail. In that case, you can get offset of thumbnail from JpegIFOffset (0x0201) Tag in IFD1, size of thumbnail from JpegIFByteCount (0x0202) Tag. Data format is ordinary JPEG format, starts from 0xFFD8 and ends by 0xFFD9.|
|4.4.2 TIFF format thumbnail|
If the value of Compression (0x0103) Tag in IFD1 is '1', thumbnail image format is no
TIFF image). Start point of thumbnail data is StripOffset (0x0111) Tag, size of thumbnail is the sum of
StripByteCounts (0x0117) Tag.
If thumbnail uses no compression and PhotometricInterpretation
(0x0106) Tag in IFD1 has a value '2',
thumbnail uses RGB format. In that case, you can see thumbnail image by simply copy data to computer's RGB
format (such as BMP format, or copy to VRAM directory). Kodak DC-210/220/260 uses this format. Be mention that
at TIFF stores pixel data as 'RGB' order, but BMP stores 'BGR' order.
At DX-5/7/9, YCbCrSubsampling(0x0212) has values of '2,1', PlanarConfiguration(0x011c) has a value '1'. So the data align of this image is below.
Y(0,0),Y(1,0),Cb(0,0),Cr(0,0), Y(2,0),Y(3,0),Cb(2,0),Cr(3.0), Y(4,0),Y(5,0),Cb(4,0),Cr(4,0). . . .
The numeric in parenthesis is pixel coordinates. DX series' YCbCrCoefficients(0x0211) has values '0.299/0.587/0.114', ReferenceBlackWhite(0x0214) has values '0,255,128,255,128,255'. Therefore to convert from Y/Cb/Cr to RGB is;
Horizontal subsampling is a value '2', so you can calculate B(1,0)/R(1,0)/G(1,0) by using the Y(1,0) and Cr(0,0)/Cb(0,0). Repeat this conversion by value of ImageWidth(0x0100) and ImageLength(0x0101).