[Libexif-devel] TIFF in Exif

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I just joined the list, and found the following in the archives.

>> Is it possible to use libexif to access the exif information embedded in tiff 
>> files?  I`ve figured out that the offset to the exif info is stored in tiff 
>> tag 0x8769, but I`m still figuring out how to get libexif to read that 
>> information, so any hints would be greatly appreciated.
>> 
>> Thanks,
>> 
>> Paul Nolan, CEO Idruna Software Inc.
>> http://www.idruna.com

It *is* possible, and actually fairly easy.  I'd love to see "libexif"
support it, and I can offer a few words which may help. See below.

<ASIDE>
 It's important to state up front that I'm not an "expert"
in image file formats; nonetheless, ever since I bought my Nikon
990 about a year and a half ago, I've been dragging down every
(open-source) software distribution I could find that claims to
deal with EXIF, and poring over the TIFF, JPEG and EXIF specs trying
to understand the incestuous relation between them.  I do have
software (in addition to "libexif") that allows me to manipulate
Exif/Jpeg images (preserving the Exif information) but none of the
distributions so far seem to recognize that the EXIF spec also
covers the uncompressed TIFF format files produced by some cameras
at the highest quality setting...probably because few of us are
eager to chew up nearly 10MB/pix on our flash cards (or whatever).
The point of this lengthy aside is that it would be *very* nice if
"libexif" included support for reading (and *writing*) the Exif
information in TIFF files, and *then* distributions like PHP,
ImageMagick, etc.  used this common interface.  What follows is my
current understanding of how to access Exif information in TIFF
files.  I've used this to locally modify the "epinfo" program
included in the "photopc" distribution (Crosser/Bowen) to access
Exif information in TIFF files from my camera.  A knowledgeable
"libexif" developer should be able to munge "libexif" in a very
short time.
</ASIDE>

Here's the deal.  The existing routines in "libexif" should read
TIFF files jes' fine...you just have to start sooner.  It may be
necessary to recognize additional TIFF tags (I'm not up to speed
on "libexif" yet).

I'm not entirely sure how to explain this, so please bear with me.

When reading an EXIF/Jpeg file, you read a JPEG SOI (2 bytes),
followed by an APP1 header (10 bytes).  The APP1 begins with a TIFF
header, followed by two IFDS.  The first IFD contains an entry which
points to an EXIF IFD.

If the file is TIFF format, the first 12 bytes aren't there.  The
file starts with the TIFF header, including the offset to the first
IFD (which usually follows immediately, but might not).  Reading
the IFDs is exactly the same.  The TIFF IFD0 will contain some TIFF
markers (e.g. ImageWidth, ImageLength, Compression, etc.) which
won't be found in the JPEG IFD0, but that's not a problem if the
reader is prepared to recognize all valid TIFF markers.  At the
same time, The EXIF IFD  in the TIFF file will not include markers
for information already included in IFD0 (e.g PixelXdimension won't
be given because the information has already been included as
ImageWidth).  This won't be a problem for the library, but will
need to be handled at the application level for programs (like
ImageMagick) that want to do image format conversions.

That's it.  Just recognize that the file starts with the TIFF header
and proceed normally.  There are additional TIFF tags which should
be recognized as described in the EXIF spec (2.6.4 "TIFF Rev 6.0
Attribute Information", page 21).  The actual format is described
in the spec starting on page 11, section 2.5.2 "Basic Structure of
Uncompressed RGB data".  The following page includes a nice figure
showing the structure of an entire file.

For completeness, there is one other item which should be dealt
with.  One tag included in TIFF files is the 'Compression' tag.
The TIFF6 spec permits 3 values for this ('1' for uncompressed, 2
for "Modifed Huffman RLE compression", and 32773 for "Packbits RLE
compression").  The JPEG spec include a value ('6') for "JPEG
compressed thumbnail" (no tag for main image), and the TIFF_EP spec
includes a value ('7') for JPEG (baseline) compressed primary image
(that's a TIFF file with the image data in JPEG compressed format).
There's some confusion in my mind about what "libexif" intends to
support, and how it should interface with "libtiff" and "libjepg",
so if I'm boring you with details, sorry (I think what I'd really
like to see is a "libjpextif" that handles every TIFF, JPEG and
EXIF tag ever invented).

At the risk of boring you further, and requiring that this message
be divided into several volumes, I'm going to include some output
from a small program I've been using to try to understand the
structure of jpeg/tiff/exif files of various sorts.  If you are
able to interpret them, they may make the difference between reading
Exif from a jpeg format file and from a tiff format file clearer.

First, here's the start of an EXIF/JPEG file:
(The lines are longer than 80 characters, so beware of line wrap.)

@0000000=00000000:  JPEG SOI 0xffd8 
@0000002=0x000002:  <JPEG_APP1> length 14405, "Exif"
@0000012=0x00000c:  TIFF(II) 0x4949 0x42 (ifd offset = 0000008=0x0008)
@0000020=0x000014:  <IFD0 > 11 entries (at offset 0000022=0x000016)
@0000022=0x000016:      <0x010e=  270> [ImageDescription         ] 0x2=ASCII      11    @158
@0000034=0x000022:      <0x010f=  271> [Make                     ] 0x2=ASCII       6    @190
@0000046=0x00002e:      <0x0110=  272> [Model                    ] 0x2=ASCII       5    @214
@0000058=0x00003a:      <0x0112=  274> [Orientation              ] 0x3=SHORT       1    =1
@0000070=0x000046:      <0x011a=  282> [XResolution              ] 0x5=RATIONAL    1    @228
@0000082=0x000052:      <0x011b=  283> [YResolution              ] 0x5=RATIONAL    1    @236
@0000094=0x00005e:      <0x0128=  296> [ResolutionUnit           ] 0x3=SHORT       1    =2
@0000106=0x00006a:      <0x0131=  305> [Software                 ] 0x2=ASCII       9    @244
@0000118=0x000076:      <0x0132=  306> [DateTime                 ] 0x2=ASCII      20    @276
@0000130=0x000082:      <0x0213=  531> [YcbCrPositioning         ] 0x3=SHORT       1    =2
@0000142=0x00008e:      <0x8769=34665> [Exif IFD Pointer         ] 0x4=LONG        1    =284
@0000158=0x00009e:    ============= VALUES, IFD0 ============
@0000158=0x00009e:  ImageDescription:             
@0000190=0x0000be:  Make:   NIKON
@0000214=0x0000d6:  Model:   E990
@0000228=0x0000e4:  XResolution:  300/1
@0000236=0x0000ec:  YResolution:  300/1
@0000244=0x0000f4:  Software:   E990v1.0
@0000276=0x000114:  DateTime:   2002:06:20 12:13:26
    @00296=0x0128:  <EXIF IFD > 24 entries (at offset 284)
    @00298=0x012a:      <0x829a=33434> [ExposureTime             ] 0x5=RATIONAL    1    @590
    @00310=0x0136:      <0x829d=33437> [FNumber                  ] 0x5=RATIONAL    1    @598
        .
        .
        .
        etc.

Now a similar portion of an EXIF/TIFF file

@0000000=00000000:  TIFF(II) 0x4949 0x42 (ifd offset = 0000008=0x0008)
@0000008=0x000008:  <IFD0 > 19 entries (at offset 0000010=0x00000a)
@0000010=0x00000a:      <0x0100=  256> [ImageWidth               ] 0x4=LONG        1    =2048
@0000022=0x000016:      <0x0101=  257> [ImageLength              ] 0x4=LONG        1    =1536
@0000034=0x000022:      <0x0102=  258> [BitsPerSample            ] 0x3=SHORT       3    @242
@0000046=0x00002e:      <0x0103=  259> [Compression              ] 0x3=SHORT       1    =1
@0000058=0x00003a:      <0x0106=  262> [PhotometricInterpretation] 0x3=SHORT       1    =2
@0000070=0x000046:      <0x010e=  270> [ImageDescription         ] 0x2=ASCII      11    @248
@0000082=0x000052:      <0x010f=  271> [Make                     ] 0x2=ASCII       6    @280
@0000094=0x00005e:      <0x0110=  272> [Model                    ] 0x2=ASCII       5    @304
@0000106=0x00006a:      <0x0111=  273> [StripOffsets             ] 0x4=LONG      192    @1322
@0000118=0x000076:      <0x0112=  274> [Orientation              ] 0x3=SHORT       1    =1
@0000130=0x000082:      <0x0115=  277> [SamplesPerPixel          ] 0x3=SHORT       1    =3
@0000142=0x00008e:      <0x0116=  278> [RowsPerStrip             ] 0x4=LONG        1    =8
@0000154=0x00009a:      <0x0117=  279> [StripByteCounts          ] 0x4=LONG      192    @2090
@0000166=0x0000a6:      <0x011a=  282> [XResolution              ] 0x5=RATIONAL    1    @318
@0000178=0x0000b2:      <0x011b=  283> [YResolution              ] 0x5=RATIONAL    1    @326
@0000190=0x0000be:      <0x0128=  296> [ResolutionUnit           ] 0x3=SHORT       1    =2
@0000202=0x0000ca:      <0x0131=  305> [Software                 ] 0x2=ASCII       9    @334
@0000214=0x0000d6:      <0x0132=  306> [DateTime                 ] 0x2=ASCII      20    @366
@0000226=0x0000e2:      <0x8769=34665> [Exif IFD Pointer         ] 0x4=LONG        1    =386
@0000242=0x0000f2:    ============= VALUES, IFD0 ============
@0000242=0x0000f2:  BitsPerSample:  8,8,8
@0000248=0x0000f8:  ImageDescription:             
@0000280=0x000118:  Make:   NIKON
@0000304=0x000130:  Model:   E990
@0001322=0x00052a:  StripOffsets:  72009,121161,170313,... -> 2090
@0002090=0x00082a:  StripByteCounts:  49152,49152,49152,... -> 2858
@0000318=0x00013e:  XResolution:  300/1
@0000326=0x000146:  YResolution:  300/1
@0000334=0x00014e:  Software:   E990v1.0
@0000366=0x00016e:  DateTime:   2001:03:30 17:16:29
    @00386=0x0182:  <EXIF IFD > 19 entries (at offset 386)
    @00388=0x0184:      <0x829a=33434> [ExposureTime             ] 0x5=RATIONAL    1    @620
    @00400=0x0190:      <0x829d=33437> [FNumber                  ] 0x5=RATIONAL    1    @628
        .
        .
        .
        etc.

Hint: the first column is offset from beginning of file in decimal=hex.

-----------
dhh @ androcles.com