AVFilterContext.extra_hw_frames functions identically to the field of
the same name in AVCodecContext.
---
Implemented in such a way that an init_hw_frames callback could be added here 
as well, if anyone had a use-case for it.


 doc/APIchanges         |  3 +++
 libavfilter/avfilter.c | 23 +++++++++++++++++++++++
 libavfilter/avfilter.h | 13 +++++++++++++
 libavfilter/internal.h | 16 ++++++++++++++++
 4 files changed, 55 insertions(+)

diff --git a/doc/APIchanges b/doc/APIchanges
index c111d08ad..5b8143561 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -13,6 +13,9 @@ libavutil:     2017-03-23
 
 API changes, most recent first:
 
+2017-xx-xx - xxxxxxx - lavfi 7.x+1.0 - avfilter.h
+  Add AVFilterContext.extra_hw_frames.
+
 2017-xx-xx - xxxxxxx - lavc 58.x+1.0 - avcodec.h
   Add AVCodecContext.init_hw_frames and AVCodecContext.extra_hw_frames.
 
diff --git a/libavfilter/avfilter.c b/libavfilter/avfilter.c
index 83c1a7c20..1161f4771 100644
--- a/libavfilter/avfilter.c
+++ b/libavfilter/avfilter.c
@@ -368,6 +368,8 @@ static const AVOption avfilter_options[] = {
     { "thread_type", "Allowed thread types", OFFSET(thread_type), 
AV_OPT_TYPE_FLAGS,
         { .i64 = AVFILTER_THREAD_SLICE }, 0, INT_MAX, FLAGS, "thread_type" },
         { "slice", NULL, 0, AV_OPT_TYPE_CONST, { .i64 = AVFILTER_THREAD_SLICE 
}, .unit = "thread_type" },
+    { "extra_hw_frames", "Number of extra hardware frames to allocate for the 
user",
+        OFFSET(extra_hw_frames), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, INT_MAX, 
FLAGS },
     { NULL },
 };
 
@@ -707,3 +709,24 @@ const AVClass *avfilter_get_class(void)
 {
     return &avfilter_class;
 }
+
+int ff_filter_init_hw_frames(AVFilterContext *avctx, AVFilterLink *link,
+                             int default_pool_size)
+{
+    AVHWFramesContext *frames;
+
+    // Must already be set by caller.
+    av_assert0(link->hw_frames_ctx);
+
+    frames = (AVHWFramesContext*)link->hw_frames_ctx->data;
+
+    if (frames->initial_pool_size == 0) {
+        // Dynamically allocation is necessarily supported.
+    } else if (avctx->extra_hw_frames >= 0) {
+        frames->initial_pool_size += avctx->extra_hw_frames;
+    } else {
+        frames->initial_pool_size = default_pool_size;
+    }
+
+    return 0;
+}
diff --git a/libavfilter/avfilter.h b/libavfilter/avfilter.h
index 6df69dbbb..c8f861076 100644
--- a/libavfilter/avfilter.h
+++ b/libavfilter/avfilter.h
@@ -311,6 +311,19 @@ struct AVFilterContext {
      * hardware context information.
      */
     AVBufferRef *hw_device_ctx;
+
+    /**
+     * Sets the number of extra hardware frames which the filter will
+     * allocate on its output links for use in following filters or by
+     * the caller.
+     *
+     * Some hardware filters require all frames that they will use for
+     * output to be defined in advance before filtering starts.  For such
+     * filters, any hardware frame pools used for output must therefore be
+     * of fixed size.  The extra frames set here are on top of any number
+     * that the filter needs internally in order to operate normally.
+     */
+    int extra_hw_frames;
 };
 
 /**
diff --git a/libavfilter/internal.h b/libavfilter/internal.h
index a377f9b2b..d17f917c6 100644
--- a/libavfilter/internal.h
+++ b/libavfilter/internal.h
@@ -226,4 +226,20 @@ void ff_filter_graph_remove_filter(AVFilterGraph *graph, 
AVFilterContext *filter
  */
 #define FF_FILTER_FLAG_HWFRAME_AWARE (1 << 0)
 
+/**
+ * Perform any additional setup required for hardware frames.
+ *
+ * link->hw_frames_ctx must be set before calling this function.
+ * Inside link->hw_frames_ctx, the fields format, sw_format, width and
+ * height must be set.  If dynamically allocated pools are not supported,
+ * then initial_pool_size must also be set, to the minimum hardware frame
+ * pool size necessary for decode (taking into account reference frames
+ * and delay as appropriate).  If default_pool_size is nonzero, then it
+ * will be used as the pool size if no other modification takes place
+ * (this can be used to preserve compatibility).
+ */
+int ff_filter_init_hw_frames(AVFilterContext *avctx, AVFilterLink *link,
+                             int default_pool_size);
+
+
 #endif /* AVFILTER_INTERNAL_H */
-- 
2.11.0

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