Add variable frame rate (VFR) option for cores.

It is enabled only for N64.
Disabling it for everything else may lead to a much smoother video playback (less stuttering).

configs/config.yaml

@@ -174,6 +174,13 @@ emulator:
       #   - isGlAllowed (bool)
       #   - usesLibCo (bool)
       #   - hasMultitap (bool)
+      #   - vfr (bool)
+      #    (experimental)
+      #       Enable variable frame rate only for cores that can't produce a constant frame rate.
+      #       By default, we assume that cores output frames at a constant rate which equals
+      #       their tick rate (1/system FPS), but OpenGL cores like N64 may have significant
+      #       frame rendering time inconsistencies. In general, VFR for CFR cores leads to
+      #       noticeable video stutter (with the current frame rendering time calculations).
       list:
         gba:
           lib: mgba_libretro
@@ -203,6 +210,7 @@ emulator:
           roms: [ "n64", "v64", "z64" ]
           isGlAllowed: true
           usesLibCo: true
+          vfr: true
 
 encoder:
   audio:

pkg/config/emulator/config.go

@@ -59,19 +59,18 @@ type LibretroRepoConfig struct {
 }
 
 type LibretroCoreConfig struct {
-	Lib         string
-	Config      string
-	Roms        []string
-	Folder      string
-	Width       int
-	Height      int
-	IsGlAllowed bool
-	UsesLibCo   bool
-	HasMultitap bool
-	AltRepo     bool
-
-	// hack: keep it here to pass it down the emulator
-	AutoGlContext bool
+	AltRepo       bool
+	AutoGlContext bool // hack: keep it here to pass it down the emulator
+	Config        string
+	Folder        string
+	HasMultitap   bool
+	Height        int
+	IsGlAllowed   bool
+	Lib           string
+	Roms          []string
+	UsesLibCo     bool
+	VFR           bool
+	Width         int
 }
 
 type CoreInfo struct {

pkg/worker/emulator/emulator.go

@@ -54,6 +54,7 @@ type Metadata struct {
 	UsesLibCo       bool
 	AutoGlContext   bool
 	HasMultitap     bool
+	HasVFR          bool
 }
 
 type (

pkg/worker/emulator/libretro/frontend.go

@@ -108,12 +108,13 @@ func (f *Frontend) LoadMetadata(emu string) {
 	config := f.conf.GetLibretroCoreConfig(emu)
 	f.mu.Lock()
 	coreLoad(emulator.Metadata{
-		LibPath:       config.Lib,
-		ConfigPath:    config.Config,
-		IsGlAllowed:   config.IsGlAllowed,
-		UsesLibCo:     config.UsesLibCo,
-		HasMultitap:   config.HasMultitap,
 		AutoGlContext: config.AutoGlContext,
+		ConfigPath:    config.Config,
+		HasMultitap:   config.HasMultitap,
+		HasVFR:        config.VFR,
+		IsGlAllowed:   config.IsGlAllowed,
+		LibPath:       config.Lib,
+		UsesLibCo:     config.UsesLibCo,
 	})
 	f.mu.Unlock()
 }

pkg/worker/emulator/libretro/nanoarch.go

@@ -69,6 +69,8 @@ var (
 	libretroLogger   = logger.Default()
 	sdlCtx           *graphics.SDL
 	usesLibCo        bool
+	hasVFR           bool
+	tickTime         int64
 	cSaveDirectory   *C.char
 	cSystemDirectory *C.char
 	cUserName        *C.char
@@ -113,7 +115,11 @@ func coreVideoRefresh(data unsafe.Pointer, width, height uint, packed uint) {
 	// this is useful only for cores with variable framerate, for the fixed framerate cores this adds stutter
 	// !to find docs on Libretro refresh sync and frame times
 	t := time.Now().UnixNano()
-	dt := t - lastFrameTime
+	dt := tickTime
+	// override frame rendering with dynamic frame times
+	if hasVFR {
+		dt = t - lastFrameTime
+	}
 	lastFrameTime = t
 
 	// some cores can return nothing
@@ -414,6 +420,7 @@ func coreLoad(meta emulator.Metadata) {
 	nano.v.isGl = meta.IsGlAllowed
 	usesLibCo = meta.UsesLibCo
 	nano.v.autoGlContext = meta.AutoGlContext
+	hasVFR = meta.HasVFR
 	coreConfig, err = ReadProperties(meta.ConfigPath)
 	if err != nil {
 		libretroLogger.Warn().Err(err).Msg("config scan has been failed")
@@ -510,6 +517,11 @@ func LoadGame(path string) error {
 		nano.sysAvInfo.timing.fps, nano.sysAvInfo.geometry.aspect_ratio, nano.sysAvInfo.timing.sample_rate,
 	)
 
+	tickTime = int64(time.Second / time.Duration(nano.sysAvInfo.timing.fps))
+	if hasVFR {
+		libretroLogger.Info().Msgf("variable framerate (VFR) is enabled")
+	}
+
 	if nano.v.isGl {
 		// flip Y coordinates of OpenGL
 		setRotation(uint(image.Flip180))