NAME

imexif.c - EXIF support for Imager

SYNOPSIS

  if (im_decode_exif(im, app1data, app1datasize)) {
    // exif block seen
  }

DESCRIPTION

This code provides a basic EXIF data decoder. It is intended to be called from the JPEG reader code when an APP1 data block is found, and will set tags in the supplied image.

PUBLIC FUNCTIONS

These functions are available to other parts of Imager. They aren't intended to be called from outside of Imager.

im_decode_exif

im_decode_exif(im, data_base, data_size);

The data from data_base for data_size bytes will be scanned for EXIF data.

Any data found will be used to set tags in the supplied image.

The intent is that invalid EXIF data will simply fail to set tags, and write to the log. In no case should this code exit when supplied invalid data.

Returns true if an EXIF header was seen.

INTERNAL FUNCTIONS

EXIF Processing

save_ifd0_tags

save_ifd0_tags(im, tiff, &exif_ifd_offset, &gps_ifd_offset)

Scans the currently loaded IFD for tags expected in IFD0 and sets them in the image.

Sets *exif_ifd_offset to the offset of the EXIF IFD if found.

save_exif_ifd_tags

save_exif_ifd_tags(im, tiff)

Scans the currently loaded IFD for the tags expected in the EXIF IFD and sets them as tags in the image.

process_maker_note

This is a stub for processing the maker note tag.

Maker notes aren't covered by EXIF itself and in general aren't documented by the manufacturers.

High level TIFF functions

To avoid relying upon tifflib when we're not processing an image we have some simple in-memory TIFF file management.

tiff_init

imtiff tiff; if (tiff_init(tiff, data_base, data_size)) { // success }

Initialize the tiff data structure.

Scans for the byte order and version markers, and stores the offset to the first IFD (IFD0 in EXIF) in first_ifd_offset.

tiff_final

tiff_final(&tiff)

Clean up the tiff structure initialized by tiff_init()

tiff_load_ifd

if (tiff_load_ifd(tiff, offset)) { // process the ifd }

Loads the IFD from the given offset into the tiff objects ifd.

This can fail if the IFD extends beyond end of file, or if any data offsets combined with their sizes, extends beyond end of file.

Returns true on success.

tiff_clear_ifd

tiff_clear_ifd(tiff)

Releases any memory associated with the stored IFD and resets the IFD pointers.

This is called by tiff_load_ifd() and tiff_final().

tiff_get_tag_double

  double value;
  if (tiff_get_tag(tiff, index, &value)) {
    // process value
  }

Attempts to retrieve a double value from the given index in the current IFD.

The value must have a count of 1.

tiff_get_tag_double

  double value;
  if (tiff_get_tag(tiff, index, &value)) {
    // process value
  }

Attempts to retrieve a double value from the given index in the current IFD.

The value must have a count of 1.

tiff_get_tag_int_array

  int value;
  if (tiff_get_tag_int_array(tiff, index, &value, array_index)) {
    // process value
  }

Attempts to retrieve an integer value from the given index in the current IFD.

tiff_get_tag_int

  int value;
  if (tiff_get_tag_int(tiff, index, &value)) {
    // process value
  }

Attempts to retrieve an integer value from the given index in the current IFD.

The value must have a count of 1.

Table-based tag setters

This set of functions checks for matches between the current IFD and tags supplied in an array, when there's a match it sets the appropriate tag in the image.

copy_int_tags

Scans the IFD for integer tags and sets them in the image,

copy_rat_tags

Scans the IFD for rational tags and sets them in the image.

copy_string_tags

Scans the IFD for string tags and sets them in the image.

copy_num_array_tags

Scans the IFD for arrays of numbers and sets them in the image.

copy_name_tags

This function maps integer values to descriptions for those values.

In general we handle the integer value through copy_int_tags() and then the same tage with a "_name" suffix here.

Low level data access functions

These functions use the byte order in the tiff object to extract various types of data from the tiff data.

These functions will abort if called with an out of range offset.

The intent is that any offset checks should have been done by the caller.

tiff_get16

Retrieve a 16 bit unsigned integer from offset.

tiff_get32

Retrieve a 32-bit unsigned integer from offset.

tiff_get_bytes

Retrieve a byte string from offset.

This isn't used much, you can usually deal with the data in-situ. This is intended for use when you need to modify the data in some way.

tiff_get16s

Retrieve a 16-bit signed integer from offset.

tiff_get32s

Retrieve a 32-bit signed integer from offset.

tiff_get_rat

Retrieve an unsigned rational from offset.

tiff_get_rats

Retrieve an signed rational from offset.

SEE ALSO

Imager, jpeg.c

http://www.exif.org/

AUTHOR

Tony Cook <tonyc@cpan.org>

REVISION

$Revision$