[FFmpeg-devel] [PR] vf_mpdecimate: Fix keep option logic for keep > 0 (PR #22261)

[FFmpeg-devel] [PR] vf_mpdecimate: Fix keep option logic for keep > 0 (PR #22261)

[dana-feng via ffmpeg-devel]

PR #22261 opened by dana-feng
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/22261
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/22261.patch

Fix the following issues with the keep option:

- Add similarity check during keep period. Previously, the code
  returned early during the keep period without checking if the
  frame is actually similar to the reference.

- Reset keep_count on different frames. Previously, the counter
  could accumulate across non-consecutive similar frames, causing
  frames to be dropped earlier than expected.

- Keep the same frame reference if appropriate. Previously, the
  code made similar frames the new reference, causing reference
  drift and gradual scene changes.

Signed-off-by: Dana Feng <danaf@twosigma.com>


>From dbe69d8ed9b5e4027b6789de3cc7ea3902f5ef13 Mon Sep 17 00:00:00 2001
From: Dana Feng <danaf@twosigma.com>
Date: Sun, 22 Feb 2026 21:33:55 -0500
Subject: [PATCH] vf_mpdecimate: Fix keep option logic for keep > 0

Fix the following issues with the keep option:

- Add similarity check during keep period. Previously, the code
  returned early during the keep period without checking if the
  frame is actually similar to the reference.

- Reset keep_count on different frames. Previously, the counter
  could accumulate across non-consecutive similar frames, causing
  frames to be dropped earlier than expected.

- Keep the same frame reference if appropriate. Previously, the
  code made similar frames the new reference, causing reference
  drift and gradual scene changes.

Signed-off-by: Dana Feng <danaf@twosigma.com>
---
 libavfilter/vf_mpdecimate.c | 50 +++++++++++++++++++++++++------------
 1 file changed, 34 insertions(+), 16 deletions(-)

diff --git a/libavfilter/vf_mpdecimate.c b/libavfilter/vf_mpdecimate.c
index 0fc95556b7..f91bd6cd29 100644
--- a/libavfilter/vf_mpdecimate.c
+++ b/libavfilter/vf_mpdecimate.c
@@ -109,19 +109,18 @@ static int diff_planes(AVFilterContext *ctx,
 /**
  * Tell if the frame should be decimated, for example if it is no much
  * different with respect to the reference frame ref.
+
+ @return 1 if frame should be dropped (similar),
+ 0 if frame should be kept (different),
+ -1 if frame should be kept (similar, under keep_count).
  */
 static int decimate_frame(AVFilterContext *ctx,
                           AVFrame *cur, AVFrame *ref)
 {
     DecimateContext *decimate = ctx->priv;
     int plane;
+    int is_similar;
 
-    if (decimate->max_keep_count > 0 &&
-        decimate->keep_count > -1 &&
-        decimate->keep_count < decimate->max_keep_count) {
-        decimate->keep_count++;
-        return 0;
-    }
     if (decimate->max_drop_count > 0 &&
         decimate->drop_count >= decimate->max_drop_count)
         return 0;
@@ -129,6 +128,8 @@ static int decimate_frame(AVFilterContext *ctx,
         (decimate->drop_count-1) > decimate->max_drop_count)
         return 0;
 
+    is_similar = 1;
+
     for (plane = 0; ref->data[plane] && ref->linesize[plane]; plane++) {
         /* use 8x8 SAD even on subsampled planes.  The blocks won't match up with
          * luma blocks, but hopefully nobody is depending on this to catch
@@ -141,10 +142,21 @@ static int decimate_frame(AVFilterContext *ctx,
                         cur->data[plane], cur->linesize[plane],
                         ref->data[plane], ref->linesize[plane],
                         AV_CEIL_RSHIFT(ref->width,  hsub),
-                        AV_CEIL_RSHIFT(ref->height, vsub)))
-            return 0;
+                        AV_CEIL_RSHIFT(ref->height, vsub))) {
+            is_similar = 0;
+            break;
+        }
+    }
+    if (!is_similar) {
+        return 0;
+    }
+    /* Frame is similar - apply keep_count logic */
+    if (decimate->max_keep_count > 0 &&
+        decimate->keep_count > -1 &&
+        decimate->keep_count < decimate->max_keep_count) {
+        decimate->keep_count++;
+        return -1; /* Similar but under keep threshold - keep without updating ref*/
     }
-
     return 1;
 }
 
@@ -200,29 +212,35 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *cur)
     DecimateContext *decimate = inlink->dst->priv;
     AVFilterLink *outlink = inlink->dst->outputs[0];
     int ret;
+    int result = decimate->ref ? decimate_frame(inlink->dst, cur, decimate->ref) : 0;
 
-    if (decimate->ref && decimate_frame(inlink->dst, cur, decimate->ref)) {
+    if (result == 1){
+        /* Drop: similar frame, over keep threshold */
         decimate->drop_count = FFMAX(1, decimate->drop_count+1);
-        decimate->keep_count = -1; // do not keep any more frames until non-similar frames are detected
+        decimate->keep_count = -1;
+    } else if (result == -1) {
+        /* Keep: similar frame, under keep threshold - don't update ref */
+        decimate->drop_count = FFMIN(-1, decimate->drop_count-1);
+        if ((ret = ff_filter_frame(outlink, cur)) < 0)
+            return ret;
     } else {
+        /* Keep: different frame - update ref and reset keep_count */
         av_frame_free(&decimate->ref);
         decimate->ref = cur;
         decimate->drop_count = FFMIN(-1, decimate->drop_count-1);
-        if (decimate->keep_count < 0) // re-enable counting similar frames to ignore before dropping
-            decimate->keep_count = 0;
-
+        decimate->keep_count = 0;
         if ((ret = ff_filter_frame(outlink, av_frame_clone(cur))) < 0)
             return ret;
     }
 
     av_log(inlink->dst, AV_LOG_DEBUG,
            "%s pts:%s pts_time:%s drop_count:%d keep_count:%d\n",
-           decimate->drop_count > 0 ? "drop" : "keep",
+           result > 0 ? "drop" : "keep",
            av_ts2str(cur->pts), av_ts2timestr(cur->pts, &inlink->time_base),
            decimate->drop_count,
            decimate->keep_count);
 
-    if (decimate->drop_count > 0)
+    if (result > 0)
         av_frame_free(&cur);
 
     return 0;
-- 
2.52.0

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