Merge pull request #21337 from mansourmoufid:videocapture-get-property-return

Make cv::VideoCapture::get return cv::CAP_PROP_UNKNOWN (-1) for unsupported properties #21337

The return value indicating an unsupported property is not consistent across backends.

I stumbled on this issue because my code was determining if a property value is valid if it's non-zero (like the documentation says), which worked fine on macOS, but not on Android.

For example, auto-exposure is not supported on macOS, so get() returns 0. But it is supported on Android and 0 means auto-exposure is off.

I think -1 is the better return value to indicate unsupported properties.

I made changes to all the backends. I think I got every case. This breaks API compatibility for some backends, so I based this on branch 3.4.

- [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 other license that is incompatible with OpenCV
- [x] The PR is proposed to proper branch
- [ ] There is reference to original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/videoio/test/test_video_io.cpp

@@ -64,6 +64,14 @@ public:
             std::cout << "SKIP test: backend " << apiPref << " can't open the video: " << video_file << std::endl;
             return;
         }
+
+// TODO: Requires FFmpeg wrapper rebuild for Windows
+#ifdef _WIN32
+        if (apiPref != CAP_FFMPEG)
+            EXPECT_EQ(CAP_PROP_UNKNOWN, cap.get(CV__CAP_PROP_LATEST));
+#else
+        EXPECT_EQ(CAP_PROP_UNKNOWN, cap.get(CV__CAP_PROP_LATEST));
+#endif
         int n_frames = -1;
         EXPECT_NO_THROW(n_frames = (int)cap.get(CAP_PROP_FRAME_COUNT));
         if (n_frames > 0)
@@ -528,6 +536,15 @@ TEST_P(Videoio_Writer, write_nothing)
     VideoWriter writer;
     EXPECT_NO_THROW(writer.open(video_file, apiPref, fourcc, fps, frame_size, true));
     ASSERT_TRUE(writer.isOpened());
+
+// TODO: Requires FFmpeg wrapper rebuild for Windows
+#ifdef _WIN32
+    if (apiPref != CAP_FFMPEG)
+        EXPECT_EQ(CAP_PROP_UNKNOWN, writer.get(CV__CAP_PROP_LATEST));
+#else
+    EXPECT_EQ(cv::VIDEOWRITER_PROP_UNKNOWN, writer.get(CV__CAP_PROP_LATEST));
+#endif
+
 #if 0  // no frames
     cv::Mat m(frame_size, CV_8UC3, Scalar::all(127));
     writer << m;