Added a design proposal from metadata parse and mux.

Original commit message from CVS:
Added a design proposal from metadata parse and mux.

1 files changed, 88 insertions, 0 deletions

diff --git a/ext/metadata/README b/ext/metadata/README
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+### design proposal for metadata mux and parser ###
+
+## use cases ##
+
+1- Create metadata
+
+[ App send tag events to the pipeline ]
+  ||               /\
+  ||               ||
+  ||          jpegenc send tag messages like (width, height and compression info)
+  ||               ||
+  \/               ||
++---------+    +---------+    +-------------+    +----------+
+| v4l2src | -> | jpegenc | -> | metadatamux | -> | filesink |
++---------+    +---------+    +-------------+    +----------+
+
+* elements like videoscale should also send (width and height tags)
+* should metadatamux get info from caps (width and height) and use if not receive a event with such tags?
+
+
+2- View metadata
+
++---------+    +---------------+    +---------+    +-------------+
+| filesrc | -> | metadataparse | -> | jpegdec | -> | xvimagesink |
++---------+    +---------------+    +---------+    +-------------+
+
+or
+
+                    +--> whole chunk send as event to next element
+                    |
++---------+    +---------------+    +----------+
+| filesrc | -> | metadataparse | -> | fakesink |
++---------+    +---------------+    +----------+
+                    ||
+                    \/
+               1- individual tags send as messages (what about not mapped tags??)
+               2- whole chunk send as message (probably the application will ignore this)
+
+* in any case metadataparse strips out metadata chunks (different from current implementation)
+
+3- Modify (add, change, delete tags)
+
+
+                    +--> whole chunk send as event to next element -- ... --+
+                    |                                                       |
+                    |                                                       V
++---------+    +---------------+                                     +-------------+    +----------+
+| filesrc | -> | metadataparse | --------------- ... --------------> | metadatamux | -> | filesink |
++---------+    +---------------+                                     +-------------+    +----------+
+  /\                ||
+  ||                \/
+  ||           1- individual tags send as messages (what about not mapped tags??) ==============\\
+  ||           2- whole chunk send as message (probably the application will ignore this)       ||
+  ||                                                                                            ||
+[ App send tag events to the pipeline ] <=======================================================//
+
+* metadataparse strips out metadata chunks (different from current implementation)
+* application receives individual tag messages (what about not mapped ones?)
+  * the application only send events back to the pipeline for the tags the application wants to modify or keep.
+    The tags not sent will not be included in metadata chunks
+* the metadatamux modify the whole chunk event received before to just keep the individual tags sent by application
+* the metadatamux could have a property to say if it should remove or keep tags not sent by application. 
+
+Obs: By looking at the proposed design (1- view and 2- modify) seems that the metadataparse and metadatamux should not know about mapped tags.
+     Only the application map the tags to/from metadata specific format. This is because, if you notice, only tags sent by application will be keep
+     on new file.
+
+4- Convert (for example from jpeg to png)
+
+                      +--> whole chunk send as event to next element -- ... --+
+                      |                                                       |
+                      |                                                       V
++---------+    +---------------+                                        +-------------+    +----------+
+| filesrc | -> | metadataparse | -> | jpegdec | -> ... -> | pngenc | -> | metadatamux | -> | filesink |
++---------+    +---------------+                                        +-------------+    +----------+
+  /\                ||
+  ||                \/
+  ||           1- individual tags send as messages (what about not mapped tags??) ==============\\
+  ||           2- whole chunk send as message (probably the application will ignore this)       ||
+  ||                                                                                            ||
+[ App send tag events to the pipeline ] <=======================================================//
+
+* in this case the application set the metadatamux property to always keep the tags by default (application don't not need to send tag events)
+* metadatamux needs additional smartness to check what tags still does make sense (some from original jpeg may not make sense anymore)
+* if the image is resized, the videoscale should send messages to the application with new width (in the same way pngenc should also) and the
+  application could send width and height tag events to the pipeline
+* should metadatamux get info from caps (width and height) and use if not receive a event with such tags?
+