From 28026fe2cf3f668448f02bffa0f8c9829847582b Mon Sep 17 00:00:00 2001
From: Lazizbek Ergashev <lazerg2@gmail.com>
Date: Wed, 12 Aug 2026 15:07:57 +0500
Subject: [PATCH] videoio: fix FFmpeg VideoCapture ignoring mid-stream pixel
 format change

(cherry picked from commit f3c9241611cfb46dd17de2b3e3f25f6fe36b71f8)
---
 modules/videoio/src/cap_ffmpeg_impl.hpp |  8 +++-
 modules/videoio/test/test_ffmpeg.cpp    | 62 +++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 1 deletion(-)

diff --git a/modules/videoio/src/cap_ffmpeg_impl.hpp b/modules/videoio/src/cap_ffmpeg_impl.hpp
index fc4479e07c..6a8543999a 100644
--- a/modules/videoio/src/cap_ffmpeg_impl.hpp
+++ b/modules/videoio/src/cap_ffmpeg_impl.hpp
@@ -582,6 +582,7 @@ struct CvCapture_FFMPEG
     AVPacket          packet;
     Image_FFMPEG      frame;
     struct SwsContext *img_convert_ctx;
+    AVPixelFormat     img_convert_ctx_format;
 
     int64_t frame_number, first_frame_number;
 
@@ -648,6 +649,7 @@ void CvCapture_FFMPEG::init()
     memset(&packet, 0, sizeof(packet));
     av_init_packet(&packet);
     img_convert_ctx = 0;
+    img_convert_ctx_format = AV_PIX_FMT_NONE;
 
     avcodec = 0;
     context = 0;
@@ -689,6 +691,7 @@ void CvCapture_FFMPEG::close()
     {
         sws_freeContext(img_convert_ctx);
         img_convert_ctx = 0;
+        img_convert_ctx_format = AV_PIX_FMT_NONE;
     }
 
     if( picture )
@@ -1922,7 +1925,8 @@ bool CvCapture_FFMPEG::retrieveFrame(int flag, unsigned char** data, int* step,
     if( img_convert_ctx == NULL ||
         frame.width != video_st->CV_FFMPEG_CODEC_FIELD->width ||
         frame.height != video_st->CV_FFMPEG_CODEC_FIELD->height ||
-        frame.data == NULL )
+        frame.data == NULL ||
+        (AVPixelFormat)sw_picture->format != img_convert_ctx_format )
     {
 #if LIBSWSCALE_BUILD >= CALC_FFMPEG_VERSION(6, 4, 100)
         int buffer_width = video_st->CV_FFMPEG_CODEC_FIELD->width;
@@ -2000,6 +2004,8 @@ bool CvCapture_FFMPEG::retrieveFrame(int flag, unsigned char** data, int* step,
             return false;
         }
 
+        img_convert_ctx_format = (AVPixelFormat)sw_picture->format;
+
 #if USE_AV_FRAME_GET_BUFFER
         av_frame_unref(&rgb_picture);
         rgb_picture.format = result_format;
diff --git a/modules/videoio/test/test_ffmpeg.cpp b/modules/videoio/test/test_ffmpeg.cpp
index a6ae33a6b3..e42901fe2d 100644
--- a/modules/videoio/test/test_ffmpeg.cpp
+++ b/modules/videoio/test/test_ffmpeg.cpp
@@ -3,6 +3,7 @@
 // of this distribution and at http://opencv.org/license.html.
 
 #include "test_precomp.hpp"
+#include "opencv2/imgcodecs.hpp"
 
 using namespace std;
 
@@ -1188,4 +1189,65 @@ TEST(videoio_ffmpeg, seek_with_negative_dts)
     }
 }
 
+// MJPEG stream whose chroma subsampling changes mid-stream at constant
+// frame size must not reuse a conversion context built for the previous frame.
+// related issue: https://github.com/opencv/opencv/issues/29699
+TEST(videoio_ffmpeg, mjpeg_pixel_format_change)
+{
+    if (!videoio_registry::hasBackend(CAP_FFMPEG))
+        throw SkipTestException("FFmpeg backend was not found");
+
+    Mat frame0(64, 64, CV_8UC3, Scalar(0, 0, 255));
+    Mat frame1(64, 64, CV_8UC3);
+    for (int y = 0; y < frame1.rows; y++)
+        for (int x = 0; x < frame1.cols; x++)
+            frame1.at<Vec3b>(y, x) = Vec3b((uchar)(x * 4), (uchar)(y * 4), (uchar)((x + y) * 2));
+
+    vector<uchar> buf0, buf1;
+    ASSERT_TRUE(imencode(".jpg", frame0, buf0,
+        {IMWRITE_JPEG_SAMPLING_FACTOR, IMWRITE_JPEG_SAMPLING_FACTOR_422, IMWRITE_JPEG_QUALITY, 100}));
+    ASSERT_TRUE(imencode(".jpg", frame1, buf1,
+        {IMWRITE_JPEG_SAMPLING_FACTOR, IMWRITE_JPEG_SAMPLING_FACTOR_420, IMWRITE_JPEG_QUALITY, 100}));
+
+    const string filename = tempfile("test_pixfmt_change.mjpeg");
+    std::ofstream file(filename.c_str(), ios::out | ios::trunc | std::ios::binary);
+    file.write(reinterpret_cast<char*>(buf0.data()), buf0.size());
+    file.write(reinterpret_cast<char*>(buf1.data()), buf1.size());
+    file.close();
+
+    VideoCapture cap(filename, CAP_FFMPEG);
+    ASSERT_TRUE(cap.isOpened());
+
+    Mat read0, read1;
+    ASSERT_TRUE(cap.read(read0));
+    ASSERT_TRUE(cap.read(read1));
+
+    Mat decodedRef0 = imdecode(buf0, IMREAD_COLOR);
+    Mat decodedRef1 = imdecode(buf1, IMREAD_COLOR);
+    ASSERT_FALSE(decodedRef0.empty());
+    ASSERT_FALSE(decodedRef1.empty());
+    ASSERT_EQ(read0.size(), decodedRef0.size());
+    ASSERT_EQ(read1.size(), decodedRef1.size());
+
+    // JPEG re-encoding is not bit exact, but a correctly decoded frame stays
+    // within a couple of intensity levels per channel; a stale conversion
+    // context (the bug) produces gross corruption of tens of levels.
+    const double maeThreshold = 5.0;
+
+    Mat diff0, diff1;
+    absdiff(read0, decodedRef0, diff0);
+    absdiff(read1, decodedRef1, diff1);
+    Scalar mae0 = mean(diff0);
+    Scalar mae1 = mean(diff1);
+    for (int c = 0; c < 3; c++)
+    {
+        EXPECT_LE(mae0[c], maeThreshold)
+            << "First frame (channel " << c << ") decoded incorrectly";
+        EXPECT_LE(mae1[c], maeThreshold)
+            << "Second frame (channel " << c << ") decoded incorrectly after pixel format change mid-stream";
+    }
+
+    remove(filename.c_str());
+}
+
 }} // namespace