[FFmpeg-devel] [PATCH] avfilter: Proof of Concept: enable out-of-tree filters

[Leandro Santiago]

This is a POC/prototype that aims to enable out of tree filters on
FFmpeg.

Here is how to test it, with an example filter:

```
mkdir -p ext/libavfilter
pushd ext/libavfilter
git clone https://gitlab.com/leandrosansilva/ffmpeg-extra-filter-example example
popd
```

Then compile ffmpeg as usual:

```
./configure && make && make install
```

Now you can use the new filters:

```
ffplay /path/to/file.webm -vf 'foo,bar'
```

What works:

- Building C based filters with no extra, or simple dependencies.
- Multiple filters in the same object file.

What was not implemented:

- I believe it would be useful to check if the license of the filter is
  compatible with the license used to build FFmpeg.

- Only extra filters written in C (maybe C++?) are supported for now.
  One of my goals is to enable Rust as well.

- Documentation

- There should be a way to have optional dependencies, for filters that
  can have multiple "backends", for instance, to be chosen at build time.

Signed-off-by: Leandro Santiago <leandrosansilva at gmail.com>
---
 .gitignore               |  3 ++
 configure                | 60 ++++++++++++++++++++++++++++++++++++++--
 libavfilter/Makefile     |  4 +++
 libavfilter/allfilters.c |  1 +
 4 files changed, 66 insertions(+), 2 deletions(-)

diff --git a/.gitignore b/.gitignore
index 430abaf91b..7d66dee516 100644
--- a/.gitignore
+++ b/.gitignore
@@ -44,3 +44,6 @@
 /tools/python/__pycache__/
 /libavcodec/vulkan/*.c
 /libavfilter/vulkan/*.c
+/ffbuild/external-filters.mak
+/libavfilter/external_filters_extern.h
+/ext
diff --git a/configure b/configure
index 2fdbe8cbbe..317862fa1a 100755
--- a/configure
+++ b/configure
@@ -1798,6 +1798,7 @@ AVDEVICE_COMPONENTS="
 
 AVFILTER_COMPONENTS="
     filters
+    external_filters
 "
 
 AVFORMAT_COMPONENTS="
@@ -4086,7 +4087,7 @@ swscale_deps="avutil"
 swscale_suggest="libm stdatomic"
 
 avcodec_extralibs="pthreads_extralibs iconv_extralibs dxva2_extralibs liblcevc_dec_extralibs lcms2_extralibs"
-avfilter_extralibs="pthreads_extralibs"
+avfilter_extralibs="pthreads_extralibs external_filters_extralibs"
 avutil_extralibs="d3d11va_extralibs d3d12va_extralibs mediacodec_extralibs nanosleep_extralibs pthreads_extralibs vaapi_drm_extralibs vaapi_x11_extralibs vaapi_win32_extralibs vdpau_x11_extralibs"
 
 # programs
@@ -4324,10 +4325,15 @@ ALL_COMPONENTS="
     $AVFORMAT_COMPONENTS_LIST
 "
 
-for n in $COMPONENT_LIST; do
+enable_component() {
+    n="$1"
     v=$(toupper ${n%s})_LIST
     eval enable \$$v
     eval ${n}_if_any="\$$v"
+}
+
+for n in $COMPONENT_LIST; do
+    enable_component "$n"
 done
 
 enable $ARCH_EXT_LIST
@@ -7463,6 +7469,56 @@ elif enabled iconv; then
     check_func_headers iconv.h iconv || check_lib iconv iconv.h iconv -liconv
 fi
 
+# Handle external filters from the directory ext/libavfilter
+
+echo "" > libavfilter/external_filters_extern.h
+echo "" > ffbuild/external-filters.mak
+
+EXTERNAL_FILTER_LIST=""
+
+# This function is used by the external filters' configure.sh script to register the filters
+register_external_filter_symbol() {
+    EXT_FILTER_SYMBOL="$1"
+    EXTERNAL_FILTER_LIST="$EXTERNAL_FILTER_LIST $(echo $EXT_FILTER_SYMBOL | sed 's/^ff_[avfsinkrc]\{2,5\}_\([[:alnum:]_]\{1,\}\)/\1_filter/')"
+
+    eval "$(echo "$EXT_FILTER_SYMBOL" | sed 's/^ff_\([avfsinkrc]\{2,5\}\)_\([[:alnum:]]\{1,\}\)$/full_filter_name_\2=\1_\2/')"
+
+    echo "extern const FFFilter $EXT_FILTER_SYMBOL;" >> libavfilter/external_filters_extern.h
+
+    echo "EXTERNAL_FILTER_$(\
+        echo $EXT_FILTER_SYMBOL \
+        | sed 's/^ff_[avfsinkrc]\{2,5\}_\([[:alnum:]]\{1,\}\)$/\1/' \
+        | tr a-z A-Z \
+        )_LOCATION = $CURRENT_EXT_FILTER_LOCATION" >> ffbuild/external-filters.mak
+}
+
+if [ -d "ext/libavfilter" ]; then
+    for f in ext/libavfilter/*; do
+        [ -f "$f/configure.sh" ] || die "Mandatory file $f/configure.sh not found"
+        [ -f "$f/Makefile" ] || die "Mandatory file $f/Makefile not found"
+        CURRENT_EXT_FILTER_LOCATION="$f"
+        . "$f/configure.sh" || die "Could not configure external filter at $f"
+        echo "include $f/Makefile" >> ffbuild/external-filters.mak
+    done
+
+    enable_component external_filters
+fi
+
+FILTER_LIST="
+    $FILTER_LIST
+    $EXTERNAL_FILTER_LIST
+"
+
+AVFILTER_COMPONENTS_LIST="
+    $AVFILTER_COMPONENTS_LIST
+    $EXTERNAL_FILTER_LIST
+"
+
+ALL_COMPONENTS="
+    $ALL_COMPONENTS
+    $EXTERNAL_FILTER_LIST
+"
+
 enabled debug && add_cflags -g"$debuglevel" && add_asflags -g"$debuglevel"
 
 # add some useful compiler flags if supported
diff --git a/libavfilter/Makefile b/libavfilter/Makefile
index 7c0d879ec9..877b24c30f 100644
--- a/libavfilter/Makefile
+++ b/libavfilter/Makefile
@@ -27,6 +27,10 @@ OBJS = allfilters.o                                                     \
 include $(SRC_PATH)/libavfilter/dnn/Makefile
 include $(SRC_PATH)/libavfilter/vulkan/Makefile
 
+# external filters handling
+include $(SRC_PATH)/ffbuild/external-filters.mak
+EXTERNAL_FILTER_FLAGS = -I$(PWD) -I$(PWD)/libavfilter
+
 OBJS-$(HAVE_LIBC_MSVCRT)                     += file_open.o
 OBJS-$(HAVE_THREADS)                         += pthread.o
 
diff --git a/libavfilter/allfilters.c b/libavfilter/allfilters.c
index 740d9ab265..c2d576e4be 100644
--- a/libavfilter/allfilters.c
+++ b/libavfilter/allfilters.c
@@ -621,6 +621,7 @@ extern  const FFFilter ff_vsrc_buffer;
 extern  const FFFilter ff_asink_abuffer;
 extern  const FFFilter ff_vsink_buffer;
 
+#include "libavfilter/external_filters_extern.h"
 #include "libavfilter/filter_list.c"
 
 
-- 
2.48.1