videoio: add routines to query information about backends API

into cv::videoio_registry namespace

modules/videoio/src/videoio_registry.cpp

@@ -6,6 +6,8 @@
 
 #include "videoio_registry.hpp"
 
+#include "opencv2/videoio/registry.hpp"
+
 #include "cap_intelperc.hpp"
 #include "cap_dshow.hpp"
 
@@ -247,6 +249,8 @@ public:
         return g_instance;
     }
 
+    inline std::vector<VideoBackendInfo> getEnabledBackends() const { return enabledBackends; }
+
     inline std::vector<VideoBackendInfo> getAvailableBackends_CaptureByIndex() const
     {
         std::vector<VideoBackendInfo> result;
@@ -302,6 +306,58 @@ std::vector<VideoBackendInfo> getAvailableBackends_Writer()
     return result;
 }
 
+cv::String getBackendName(VideoCaptureAPIs api)
+{
+    if (api == CAP_ANY)
+        return "CAP_ANY";  // special case, not a part of backends list
+    const int N = sizeof(builtin_backends)/sizeof(builtin_backends[0]);
+    for (size_t i = 0; i < N; i++)
+    {
+        const VideoBackendInfo& backend = builtin_backends[i];
+        if (backend.id == api)
+            return backend.name;
+    }
+    return cv::format("UnknownVideoAPI(%d)", (int)api);
+}
+
+std::vector<VideoCaptureAPIs> getBackends()
+{
+    std::vector<VideoBackendInfo> backends = VideoBackendRegistry::getInstance().getEnabledBackends();
+    std::vector<VideoCaptureAPIs> result;
+    for (size_t i = 0; i < backends.size(); i++)
+        result.push_back((VideoCaptureAPIs)backends[i].id);
+    return result;
+}
+
+std::vector<VideoCaptureAPIs> getCameraBackends()
+{
+    const std::vector<VideoBackendInfo> backends = VideoBackendRegistry::getInstance().getAvailableBackends_CaptureByIndex();
+    std::vector<VideoCaptureAPIs> result;
+    for (size_t i = 0; i < backends.size(); i++)
+        result.push_back((VideoCaptureAPIs)backends[i].id);
+    return result;
+
+}
+
+std::vector<VideoCaptureAPIs> getStreamBackends()
+{
+    const std::vector<VideoBackendInfo> backends = VideoBackendRegistry::getInstance().getAvailableBackends_CaptureByFilename();
+    std::vector<VideoCaptureAPIs> result;
+    for (size_t i = 0; i < backends.size(); i++)
+        result.push_back((VideoCaptureAPIs)backends[i].id);
+    return result;
+
+}
+
+std::vector<VideoCaptureAPIs> getWriterBackends()
+{
+    const std::vector<VideoBackendInfo> backends = VideoBackendRegistry::getInstance().getAvailableBackends_Writer();
+    std::vector<VideoCaptureAPIs> result;
+    for (size_t i = 0; i < backends.size(); i++)
+        result.push_back((VideoCaptureAPIs)backends[i].id);
+    return result;
+}
+
 } // namespace registry
 
 #define TRY_OPEN(backend_func) \