Thanks for the thorough explanation, I am now embedding the aircraft
supplied position parameters in the ImageDescription field. The next
flight has been postponed 24 hours and is now scheduled to take off at
0705 UTC Friday.
-scott
Andrey Filippov wrote:
Scott,
There is no easy way to write GPS data to Exif bypassing the program
in C that does it. The Elphel PHP extension includes parsing Exif data
into human readable form, but not the other way around.
Internally camera maintains condensed Exif - condensed is what
remains from the Exif header if you remove all the constant fields
(like camera model) and the Exif pointers and merge the remaining
fields together. These condensed Exif headers are copied from the
current data buffer for each frame that is stored in the output
video buffer so the Exif data can be re-assembled with the images when
applications read them from the circular video buffer.
When application (such as camogm, streamer or imgsrv) reads image
frame and requests Exif header that dynamic data (individual for each
acquired frame) is combined with the static template (generated at
boot time) and provided to an application as complete Exif header.
The php script that runs at startup:
http://elphel.cvs.sourceforge.net/viewvc/elphel/elphel353-8.0/apps/web/imgsrv/exif.php?view=markup
creates template from the XML file
http://elphel.cvs.sourceforge.net/viewvc/elphel/elphel353-8.0/apps/web/imgsrv/Exif_template.xml?view=markup
and builds the directory for the condensed Exif writable by the
applications. This directory does not include the data format of the
particular Exif fields - just the offset and length (in bytes) fo that
field, referenced by the modified Exif tag number (modified to include
both tag number and tag group in a single long number). It is up to
the particular application to format data according to Exif
conventions and matching the Exif_template.xml
Example of the PHP script
http://elphel.cvs.sourceforge.net/viewvc/elphel/elphel353-8.0/packages/web/353/camvc/camvc.php?view=markup
343 if ( $value!==null) elphel_set_exif_field(0x10e,
$value.chr(0));
writes string data from $value terminated by zero byte (chr(0) ) to
Exif field with the tag 0x10e (group 0) , the tag called
ImageDescription in Exif. The Exif_template.xml reserves 40 bytes
fro the image description, so if you need more you have to modify that
template file. It is also possible to add more fileds - you need to
define them in Exif_template.xml, then write with
elphel_set_exif_field (or directly using the device driver).
It should be possible to write GPS data fileds with PHP script, but
that would be more tricky than to wrikte to ImageDescription tag -
you'll need to write data in the Exif format, not just string data.
Making all such conversions in PHP may add considerable load if the
data is updated at high rate, you may need to write a C program,
modifying it from this one:
http://elphel.cvs.sourceforge.net/viewvc/elphel/elphel353-8.0/apps/garminusb2nmea-0.12a/nmea2exif.c?view=markup
So the quick fix that I would recommend is to format the required data
into a string of =40 bytes and use elphel_set_exif_field(0x10e,
$string_to_write.chr(0));
Andrey
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