doc/filter subplugin: brief guide on how to write subplugin

To clarify which mechanism to use for new tensor-filter subplugin,
added a short guide on how to write tensor-filter subplugin.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>

diff --git a/ext/nnstreamer/tensor_filter/README.md b/ext/nnstreamer/tensor_filter/README.md
index 2c50fde..d397841 100644 (file)
--- a/ext/nnstreamer/tensor_filter/README.md
+++ b/ext/nnstreamer/tensor_filter/README.md
@@ -46,3 +46,44 @@ By default, this subplugin loads ```./libtensorflow2-lite-custom.so```, which is
 ## Tensorrt
 ## TRIx-engine
 ## TVM
+
+
+# How to write a new subplugin
+
+If you want to support a new deep neural network framework or a runtime/backend library of a new hardware accelerator, you need a new tensor-filter subplugin.
+
+## Write a subplugin in C
+
+Reference (example): [/ext/nnstreamer/tensor_filter/tensor_filter_nnfw.c]
+Interface: [/gst/nnstreamer/include/nnstreamer_plugin_api_filter.h]
+
+
+If the framework or backend/runtime library has C APIs and you want to write the subplugin in C, use ```#include <nnstreamer_plugin_api_filter.h>```.
+Your C subplugin is supposed to fill in ```GstTensorFilterFramework``` struct and register the struct with ```nnstreamer_filter_probe (GstTensorFilterFrameworkEventData *)``` function, which is supposed to be called with ```((constructor))``` initializer (```init_filter_nnfw (void)``` function in the reference).
+If your subplugin has custom properties to be supplied by users, describe their usages with ```nnstreamer_filter_set_custom_property_desc ()``` function.
+Then, call ```nnstreamer_filter_exit ()``` function with ```((desctructor))``` terminator (```fini_filter_nnfw (void)``` function in the reference).
+
+
+In ```GstTensorFilterFramework```, there are two different ways, ```v0 (version == GST_TENSOR_FILTER_FRAMEWORK_V0)``` and ```v1 (version == GST_TENSOR_FILTER_FRAMEWORK_V1)```. In the struct, there is a ```union``` of ```v0``` and ```v1```, and it is recommended to use ```v1``` and ```set version = GST_TENSOR_FILTER_FRAMEWORK_V1``` (v1). ```v0``` is supposed to be used by old subplugins for backward compatibilty and any new subplugins should use ```v1```, which is simpler and richers in features.
+
+
+However, note that if you are going to use framework/library with C++ APIs, please do not use ```nnstreamer_plugin_api_filter.h```, but use the base tensor-filter-subplugin C++ class as in the next section.
+
+## Write a subplugin in C++
+
+
+Reference (example): [/ext/nnstreamer/tensor_filter/tensor_filter_snap.cc]
+Interface: [/gst/nnstreamer/include/nnstreamer_cppplugin_api_filter.hh]
+
+
+If the framework or backend/runtime library has C++ APIs or you want to write the subplugin in C++, use ```#include <nnstreamer_cppplugin_api_filter.hh>``` and inherit the base class, ```nnstreamer::tensor_filter_subplugin```.
+With this interface, subplugin writers are supposed to write their own concrete class based on ```nnstreamer::tensor_filter_subplugin``` by filling up virtual methods (methods marked ```These should be filled/implemented by subplugin authores```).
+Mandatory methods to be filled are declared as pure virtual function and optional methods are declared as regular virtual function (```eventHandler```).
+
+
+As in C subplugin, the derived (concrete) class should be registered at init and unregistered at exit.
+Subplugin writers are supposed to use the static methods of the base class, ```register_subplugin()``` and ```unregister_subplugin()```; refer to the function ```init_filter_snap()``` and ```fini_filter_snap()``` in the reference example.
+
+
+
+Note that C++ subplugin is simpler and easy-to-maintain compared to C subplugin. Unless you really need to write your subplugin in C, we recommend to use the ```nnstreamer::tensor_filter_subplugin``` base class.