From 4c024c35fbc7f0610501e087a9ef20c336a75e2b Mon Sep 17 00:00:00 2001
From: Sachin Shah <39803835+inventshah@users.noreply.github.com>
Date: Mon, 14 Jul 2025 04:59:44 -0400
Subject: [PATCH] Merge pull request #27523 from
 inventshah:fix-set-then-get-pos

FIX: CvCapture_FFMPEG::setProperty(CAP_PROP_POS_*) followed by getProperty #27523

Partially fixes #23088 and #23472. This PR fixes `get(CAP_PROP_POS_MSEC)` calls after a `set(CAP_PROP_POS_*)` without calling `read` first.

Since `seek` calls `grabFrame` which already sets `picture_pts`, manually setting `picture_pts` anywhere else in the call stack should not be necessary (except for the special case of seeking to frame 0).

Minimal example from #23088
```cpp
for(int i = 0; i < 3; i++) cap.read(img);
printf("at: %f frames, %f msec\n", cap.get(CAP_PROP_POS_FRAMES), cap.get(CAP_PROP_POS_MSEC));

cap.set(CAP_PROP_POS_FRAMES, 3);
printf("at: %f frames, %f msec\n", cap.get(CAP_PROP_POS_FRAMES), cap.get(CAP_PROP_POS_MSEC));
```

Current
```txt
at: 3.000000 frames, 80.000000 msec
at: 3.000000 frames, 0.234375 msec
```

PR
```txt
at: 3.000000 frames, 80.000000 msec
at: 3.000000 frames, 80.000000 msec
```

It similarly helps with `CAP_PROP_POS_MSEC`:

Current
```txt
at: 3.000000 frames, 80.000000 msec
at: 2.000000 frames, 6.250000 msec
```

PR
```txt
at: 3.000000 frames, 80.000000 msec
at: 2.000000 frames, 40.000000 msec
```

Note the seek operation is still inconsistent between the `CAP_PROP_POS_*` options as mentioned by #23088, and VFR video seeking has issues discussed in #9053. For fixed-frame rate video, we could change 0.5 to 1 in `void CvCapture_FFMPEG::seek(double sec);` to align `CAP_PROP_POS_MSEC` with `CAP_PROP_POS_FRAMES`, but `CAP_POS_AVI_RATIO` and VFR video would still be broken.

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake
---
 modules/videoio/src/cap_ffmpeg_impl.hpp | 26 +++++++-----------------
 modules/videoio/test/test_ffmpeg.cpp    | 27 +++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/modules/videoio/src/cap_ffmpeg_impl.hpp b/modules/videoio/src/cap_ffmpeg_impl.hpp
index 9cd20fbf77..489dbe565d 100644
--- a/modules/videoio/src/cap_ffmpeg_impl.hpp
+++ b/modules/videoio/src/cap_ffmpeg_impl.hpp
@@ -2080,6 +2080,7 @@ void CvCapture_FFMPEG::seek(int64_t _frame_number)
         else
         {
             frame_number = 0;
+            picture_pts = AV_NOPTS_VALUE_;
             break;
         }
     }
@@ -2097,27 +2098,14 @@ bool CvCapture_FFMPEG::setProperty( int property_id, double value )
     switch( property_id )
     {
     case CAP_PROP_POS_MSEC:
+        seek(value/1000.0);
+        return true;
     case CAP_PROP_POS_FRAMES:
+        seek((int64_t)value);
+        return true;
     case CAP_PROP_POS_AVI_RATIO:
-        {
-            switch( property_id )
-            {
-            case CAP_PROP_POS_FRAMES:
-                seek((int64_t)value);
-                break;
-
-            case CAP_PROP_POS_MSEC:
-                seek(value/1000.0);
-                break;
-
-            case CAP_PROP_POS_AVI_RATIO:
-                seek((int64_t)(value*ic->duration));
-                break;
-            }
-
-            picture_pts=(int64_t)value;
-        }
-        break;
+        seek((int64_t)(value*ic->duration));
+        return true;
     case CAP_PROP_FORMAT:
         if (value == -1)
             return setRaw();
diff --git a/modules/videoio/test/test_ffmpeg.cpp b/modules/videoio/test/test_ffmpeg.cpp
index eeb0835078..3021bdfa4e 100644
--- a/modules/videoio/test/test_ffmpeg.cpp
+++ b/modules/videoio/test/test_ffmpeg.cpp
@@ -1030,4 +1030,31 @@ inline static std::string videoio_ffmpeg_mismatch_name_printer(const testing::Te
 
 INSTANTIATE_TEST_CASE_P(/**/, videoio_ffmpeg_channel_mismatch, testing::ValuesIn(mismatch_cases), videoio_ffmpeg_mismatch_name_printer);
 
+// PR: https://github.com/opencv/opencv/pull/27523
+// TODO: Enable the tests back on Windows after FFmpeg plugin rebuild
+#ifndef _WIN32
+
+// related issue: https://github.com/opencv/opencv/issues/23088
+TEST(ffmpeg_cap_properties, set_pos_get_msec)
+{
+    if (!videoio_registry::hasBackend(CAP_FFMPEG))
+        throw SkipTestException("FFmpeg backend was not found");
+
+    string video_file = findDataFile("video/big_buck_bunny.mp4");
+    VideoCapture cap;
+    EXPECT_NO_THROW(cap.open(video_file, CAP_FFMPEG));
+    ASSERT_TRUE(cap.isOpened()) << "Can't open the video";
+
+    cap.set(CAP_PROP_POS_FRAMES, 25);
+    EXPECT_EQ(cap.get(CAP_PROP_POS_MSEC), 1000.0);
+
+    cap.set(CAP_PROP_POS_MSEC, 525);
+    EXPECT_EQ(cap.get(CAP_PROP_POS_MSEC), 500.0);
+
+    cap.set(CAP_PROP_POS_AVI_RATIO, 0);
+    EXPECT_EQ(cap.get(CAP_PROP_POS_MSEC), 0.0);
+}
+
+#endif // WIN32
+
 }} // namespace