[gstreamer-bugs] [Bug 486659] metadata parser for image files

[GStreamer (bugzilla.gnome.org)]

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  GStreamer | gst-plugins-bad | Ver: HEAD CVS




------- Comment #7 from Edgard Lima  2007-10-16 11:19 UTC -------


1. Use helper libraries to create tag lists from metadachunk

* lets think just about the design, no matter if it is ext or gst

GstTagList *gst_tag_list_from_exifimage_chunk(const GstBuffer * buffer);
GstTagList *gst_tag_list_from_iptc_chunk(const GstBuffer * buffer);
GstTagList *gst_tag_list_from_xmp_chunk(const GstBuffer * buffer);

GstBuffer *gst_exifimage_chunk_from_tag_list(GstTagList *tag_list);
GstBuffer *gst_iptc_chunk_from_tag_list(GstTagList *tag_list);
GstBuffer *gst_xmp_chunk_from_tag_list(GstTagList *tag_list);

1.1 those helper libraries would be called by decoders
(tag_list_from_metadata_chunk) and sent as a message. On the other side,
Encoders recive tag messsages and write it as chunks inside the file been
converted to (metadata_chunk_from_tag_list)

1.1.1- Advantages
i - autoplugable
ii- there is only one central code (library) for all file formats. The only
thing the file format has to do is to find the metadata chunk inside it.

1.1.2- Disadvanges
i - The encoder doesn't have any idea about what kind of netadata it is so it
will have to write duplicate information to all the metadata chunks exif, iptc,
xmp and future ones.

1.1.3- Open issues
i- Applications would need some extra Gst-Interface to encoders if it want to
decide to write exif and/or iptc and/or xmp

examples:

-----------    -----------    ------------
| v4l2src | -> | jpegenc | -> | filesink |
-----------    -----------    ------------

The application could send metadata (as any other element in pipepilene). The
end file will be a file with 3 metadata chunks (exif, iptc, xmp) with
duplicated info
>From the application point of view, there is no problem because the metadata is
merged. Unless the application has 4 sidebars, one for general tags, one for
iptc tags and one for xmp tags. In this case the application can't identify
which metadata is from exif, iptc, and so on.

-----------    -----------    ----------    ------------
| filesrc | -> | jpegdec | -> | pngenc | -> | filesink |
-----------    -----------    ----------    ------------

The same problem, the application doesn't have control of what metadata will be
created

-----------    -----------    ---------------
| filesrc | -> | jpegdec | -> | xvimagesink |
-----------    -----------    ---------------

The same problem, if the application wants to show tags separately it can't
(lets just think of an app like Eye of Gnome for example) !!!

So, to solve these limitations
i- app can't know what kind of metadata it is
ii - encoders don't know what kind of metadata to write

I propose the following changes to GstTagList

1- A tag list has one id, name and description, i.e. GST_TAG_CATEGORY_EXIF,
"Exif" and "Exif metadata for images"
1.1 - Ids starting from some value, for example 4000 are reserved for
application specific
1.2 - there is a GST_TAG_CATEGORY_GENERAL

2- A tag values can contains not only unique values, like INT, STRING and so,
but can also be of type GROUP
2.1-  GROUP tags has a name, and description i.e. "Rights" "Information
regarding the legal restrictions" and also a list of other tags

With such a thing, the Application could show metadata all together or make it
friendly like this:

Exif
  Camera
    Make
    Model
    XResolution
    YResolution
  Image Data
    Orientation
    DateTime
    Compression
  MakerNote
    Object Distance
    Time Zone
XMP
  Basic
    Advisory
    BaseURL
    CreateDate
    CreatorTool
  Rights
    Certificate
    Marked
    Owner
    UsageTerms


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