add FPS fallback

plugins/mac-avcapture/OBSAVCapture.h

@@ -25,6 +25,7 @@ typedef struct obs_source_frame OBSAVCaptureVideoFrame;
 typedef struct obs_source_audio OBSAVCaptureAudioFrame;
 typedef struct gs_texture OBSAVCaptureTexture;
 typedef struct gs_effect OBSAVCaptureEffect;
+typedef struct media_frames_per_second OBSAVCaptureMediaFPS;
 
 /// C struct for errors encountered in capture callback
 typedef enum : NSUInteger {
@@ -152,6 +153,17 @@ typedef struct av_capture_info {
 /// - Returns: [CMVideoDimensions](https://developer.apple.com/documentation/coremedia/cmvideodimensions?language=objc) struct with resolution from user settings
 + (CMVideoDimensions)legacyDimensionsFromSettings:(void *)settings;
 
+/// Generates an appropriate frame rate value to fall back to (based on OBS's configured output framerate) when the user selects a format that does not support their previously configured frame rate.
+///
+/// This function fetches OBS's configured output frame rate and uses it to determine the appropriate default frame rate supported by the format. It will return:
+///
+/// * The frame rate nearest up to and including OBS's configured output frame rate.
+/// * If that does not exist on the format, the frame rate nearest above OBS's configured output frame rate.
+/// * If that does not exist, a struct representing an invalid frame rate.
+/// - Parameter format: [AVCaptureDeviceFormat](https://developer.apple.com/documentation/avfoundation/avcapturedevice/format?language=objc) instance that we are determining a fallback FPS for.
+/// - Returns: Struct representing a frames per second value as defined in ``libobs``.
++ (OBSAVCaptureMediaFPS)fallbackFrameRateForFormat:(AVCaptureDeviceFormat *)format;
+
 /// Generates a new [NSString](https://developer.apple.com/documentation/foundation/nsstring?language=objc) instance containing a human-readable aspect ratio for a given pixel width and height.
 /// - Parameter dimensions: [CMVideoDimensions](https://developer.apple.com/documentation/coremedia/cmvideodimensions?language=objc) struct containing the width and height in pixels.
 /// - Returns: New [NSString](https://developer.apple.com/documentation/foundation/nsstring?language=objc) instance containing the aspect ratio description.

plugins/mac-avcapture/OBSAVCapture.m

@@ -411,7 +411,7 @@ - (BOOL)configureSessionWithPreset:(AVCaptureSessionPreset)preset withError:(NSE
 
 - (BOOL)configureSession:(NSError *__autoreleasing *)error
 {
-    struct media_frames_per_second fps;
+    OBSAVCaptureMediaFPS fps;
     if (!obs_data_get_frames_per_second(self.captureInfo->settings, "frame_rate", &fps, NULL)) {
         [self AVCaptureLog:LOG_DEBUG withFormat:@"No valid framerate found in settings"];
         return NO;
@@ -477,9 +477,23 @@ - (BOOL)configureSession:(NSError *__autoreleasing *)error
     }
 
     if (!fpsSupported) {
-        [self AVCaptureLog:LOG_WARNING withFormat:@"Frame rate is not supported: %g FPS (%u/%u)",
-                                                  media_frames_per_second_to_fps(fps), fps.numerator, fps.denominator];
-        return NO;
+        OBSAVCaptureMediaFPS fallbackFPS = [OBSAVCapture fallbackFrameRateForFormat:format];
+        if (fallbackFPS.denominator > 0 && fallbackFPS.numerator > 0) {
+            [self AVCaptureLog:LOG_WARNING withFormat:@"Frame rate is not supported: %g FPS (%u/%u), \n"
+                                                       " falling back to value supported by device: %G FPS (%u/%u)",
+                                                      media_frames_per_second_to_fps(fps), fps.numerator,
+                                                      fps.denominator, media_frames_per_second_to_fps(fallbackFPS),
+                                                      fallbackFPS.numerator, fallbackFPS.denominator];
+            obs_data_set_frames_per_second(self.captureInfo->settings, "frame_rate", fallbackFPS, NULL);
+            time.value = fallbackFPS.denominator;
+            time.timescale = fallbackFPS.numerator;
+        } else {
+            [self AVCaptureLog:LOG_WARNING
+                    withFormat:@"Frame rate is not supported: %g FPS (%u/%u), \n"
+                                " no supported fallback FPS found",
+                               media_frames_per_second_to_fps(fps), fps.numerator, fps.denominator];
+            return NO;
+        }
     }
 
     [self.session beginConfiguration];
@@ -643,6 +657,37 @@ + (CMVideoDimensions)legacyDimensionsFromSettings:(void *)settings
     return dimensions;
 }
 
++ (OBSAVCaptureMediaFPS)fallbackFrameRateForFormat:(AVCaptureDeviceFormat *)format
+{
+    struct obs_video_info video_info;
+    bool result = obs_get_video_info(&video_info);
+
+    double outputFPS = result ? ((double) video_info.fps_num / (double) video_info.fps_den) : 0;
+
+    double closestUpTo = 0;
+    double closestAbove = DBL_MAX;
+    OBSAVCaptureMediaFPS closestUpToMFPS = {};
+    OBSAVCaptureMediaFPS closestAboveMFPS = {};
+
+    for (AVFrameRateRange *range in format.videoSupportedFrameRateRanges) {
+        if (range.maxFrameRate > closestUpTo && range.maxFrameRate <= outputFPS) {
+            closestUpTo = range.maxFrameRate;
+            closestUpToMFPS.numerator = (uint32_t) clamp_Uint(range.minFrameDuration.timescale, 0, UINT32_MAX);
+            closestUpToMFPS.denominator = (uint32_t) clamp_Uint(range.minFrameDuration.value, 0, UINT32_MAX);
+        }
+        if (range.minFrameRate > outputFPS && range.minFrameRate < closestAbove) {
+            closestAbove = range.minFrameRate;
+            closestAboveMFPS.numerator = (uint32_t) clamp_Uint(range.maxFrameDuration.timescale, 0, UINT32_MAX);
+            closestAboveMFPS.denominator = (uint32_t) clamp_Uint(range.maxFrameDuration.value, 0, UINT32_MAX);
+        }
+    }
+    if (closestUpTo > 0) {
+        return closestUpToMFPS;
+    } else {
+        return closestAboveMFPS;
+    }
+}
+
 + (NSString *)aspectRatioStringFromDimensions:(CMVideoDimensions)dimensions
 {
     if (dimensions.width <= 0 || dimensions.height <= 0) {