Add runtime option to render expired frames

Replace the compilation flag SKIP_FRAMES by a runtime flag to force
rendering of expired frames. By default, the expired frames are skipped.

README.md

@@ -271,6 +271,19 @@ Or by pressing `Ctrl`+`o` at any time.
 To turn it back on, press `POWER` (or `Ctrl`+`p`).
 
 
+### Render expired frames
+
+By default, to minimize latency, _scrcpy_ always renders the last decoded frame
+available, and drops any previous one.
+
+To force the rendering of all frames (at a cost of a possible increased
+latency), use:
+
+```bash
+scrcpy --render-expired-frames
+```
+
+
 ### Forward audio
 
 Audio is not forwarded by _scrcpy_.

app/meson.build

@@ -122,11 +122,6 @@ conf.set('DEFAULT_MAX_SIZE', '0')  # 0: unlimited
 # overridden by option --bit-rate
 conf.set('DEFAULT_BIT_RATE', '8000000')  # 8Mbps
 
-# whether the app should always display the most recent available frame, even
-# if the previous one has not been displayed
-# SKIP_FRAMES improves latency at the cost of framerate
-conf.set('SKIP_FRAMES', get_option('skip_frames'))
-
 # enable High DPI support
 conf.set('HIDPI_SUPPORT', get_option('hidpi_support'))
 

app/src/fps_counter.c

@@ -16,9 +16,7 @@ fps_counter_start(struct fps_counter *counter) {
     counter->started = true;
     counter->slice_start = SDL_GetTicks();
     counter->nr_rendered = 0;
-#ifdef SKIP_FRAMES
     counter->nr_skipped = 0;
-#endif
 }
 
 void
@@ -28,16 +26,12 @@ fps_counter_stop(struct fps_counter *counter) {
 
 static void
 display_fps(struct fps_counter *counter) {
-#ifdef SKIP_FRAMES
     if (counter->nr_skipped) {
         LOGI("%d fps (+%d frames skipped)", counter->nr_rendered,
                                             counter->nr_skipped);
     } else {
-#endif
-    LOGI("%d fps", counter->nr_rendered);
-#ifdef SKIP_FRAMES
+        LOGI("%d fps", counter->nr_rendered);
     }
-#endif
 }
 
 static void
@@ -49,9 +43,7 @@ check_expired(struct fps_counter *counter) {
         uint32_t elapsed_slices = (now - counter->slice_start) / 1000;
         counter->slice_start += 1000 * elapsed_slices;
         counter->nr_rendered = 0;
-#ifdef SKIP_FRAMES
         counter->nr_skipped = 0;
-#endif
     }
 }
 
@@ -61,10 +53,8 @@ fps_counter_add_rendered_frame(struct fps_counter *counter) {
     ++counter->nr_rendered;
 }
 
-#ifdef SKIP_FRAMES
 void
 fps_counter_add_skipped_frame(struct fps_counter *counter) {
     check_expired(counter);
     ++counter->nr_skipped;
 }
-#endif

app/src/fps_counter.h

@@ -4,15 +4,11 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-#include "config.h"
-
 struct fps_counter {
     bool started;
     uint32_t slice_start; // initialized by SDL_GetTicks()
     int nr_rendered;
-#ifdef SKIP_FRAMES
     int nr_skipped;
-#endif
 };
 
 void
@@ -27,9 +23,7 @@ fps_counter_stop(struct fps_counter *counter);
 void
 fps_counter_add_rendered_frame(struct fps_counter *counter);
 
-#ifdef SKIP_FRAMES
 void
 fps_counter_add_skipped_frame(struct fps_counter *counter);
-#endif
 
 #endif

app/src/main.c

@@ -29,6 +29,7 @@ struct args {
     uint32_t bit_rate;
     bool always_on_top;
     bool turn_screen_off;
+    bool render_expired_frames;
 };
 
 static void usage(const char *arg0) {
@@ -79,6 +80,12 @@ static void usage(const char *arg0) {
         "        The format is determined by the -F/--record-format option if\n"
         "        set, or by the file extension (.mp4 or .mkv).\n"
         "\n"
+        "    --render-expired-frames\n"
+        "        By default, to minimize latency, scrcpy always renders the\n"
+        "        last available decoded frame, and drops any previous ones.\n"
+        "        This flag forces to render all frames, at a cost of a\n"
+        "        possible increased latency.\n"
+        "\n"
         "    -s, --serial\n"
         "        The device serial number. Mandatory only if several devices\n"
         "        are connected to adb.\n"
@@ -287,6 +294,8 @@ guess_record_format(const char *filename) {
     return 0;
 }
 
+#define OPT_RENDER_EXPIRED_FRAMES 1000
+
 static bool
 parse_args(struct args *args, int argc, char *argv[]) {
     static const struct option long_options[] = {
@@ -301,6 +310,8 @@ parse_args(struct args *args, int argc, char *argv[]) {
         {"port",                  required_argument, NULL, 'p'},
         {"record",                required_argument, NULL, 'r'},
         {"record-format",         required_argument, NULL, 'f'},
+        {"render-expired-frames", no_argument,       NULL,
+                                                 OPT_RENDER_EXPIRED_FRAMES},
         {"serial",                required_argument, NULL, 's'},
         {"show-touches",          no_argument,       NULL, 't'},
         {"turn-screen-off",       no_argument,       NULL, 'S'},
@@ -364,6 +375,9 @@ parse_args(struct args *args, int argc, char *argv[]) {
             case 'v':
                 args->version = true;
                 break;
+            case OPT_RENDER_EXPIRED_FRAMES:
+                args->render_expired_frames = true;
+                break;
             default:
                 // getopt prints the error message on stderr
                 return false;
@@ -426,6 +440,7 @@ main(int argc, char *argv[]) {
         .no_control = false,
         .no_display = false,
         .turn_screen_off = false,
+        .render_expired_frames = false,
     };
     if (!parse_args(&args, argc, argv)) {
         return 1;
@@ -467,6 +482,7 @@ main(int argc, char *argv[]) {
         .control = !args.no_control,
         .display = !args.no_display,
         .turn_screen_off = args.turn_screen_off,
+        .render_expired_frames = args.render_expired_frames,
     };
     int res = scrcpy(&options) ? 0 : 1;
 

app/src/scrcpy.c

@@ -320,7 +320,7 @@ scrcpy(const struct scrcpy_options *options) {
 
     struct decoder *dec = NULL;
     if (options->display) {
-        if (!video_buffer_init(&video_buffer)) {
+        if (!video_buffer_init(&video_buffer, options->render_expired_frames)) {
             goto end;
         }
         video_buffer_initialized = true;

app/src/scrcpy.h

@@ -19,6 +19,7 @@ struct scrcpy_options {
     bool control;
     bool display;
     bool turn_screen_off;
+    bool render_expired_frames;
 };
 
 bool

app/src/video_buffer.c

@@ -10,7 +10,7 @@
 #include "log.h"
 
 bool
-video_buffer_init(struct video_buffer *vb) {
+video_buffer_init(struct video_buffer *vb, bool render_expired_frames) {
     if (!(vb->decoding_frame = av_frame_alloc())) {
         goto error_0;
     }
@@ -23,13 +23,16 @@ video_buffer_init(struct video_buffer *vb) {
         goto error_2;
     }
 
-#ifndef SKIP_FRAMES
-    if (!(vb->rendering_frame_consumed_cond = SDL_CreateCond())) {
-        SDL_DestroyMutex(vb->mutex);
-        goto error_2;
+    vb->render_expired_frames = render_expired_frames;
+    if (render_expired_frames) {
+        if (!(vb->rendering_frame_consumed_cond = SDL_CreateCond())) {
+            SDL_DestroyMutex(vb->mutex);
+            goto error_2;
+        }
+        // interrupted is not used if expired frames are not rendered
+        // since offering a frame will never block
+        vb->interrupted = false;
     }
-    vb->interrupted = false;
-#endif
 
     // there is initially no rendering frame, so consider it has already been
     // consumed
@@ -48,9 +51,9 @@ video_buffer_init(struct video_buffer *vb) {
 
 void
 video_buffer_destroy(struct video_buffer *vb) {
-#ifndef SKIP_FRAMES
-    SDL_DestroyCond(vb->rendering_frame_consumed_cond);
-#endif
+    if (vb->render_expired_frames) {
+        SDL_DestroyCond(vb->rendering_frame_consumed_cond);
+    }
     SDL_DestroyMutex(vb->mutex);
     av_frame_free(&vb->rendering_frame);
     av_frame_free(&vb->decoding_frame);
@@ -67,17 +70,16 @@ void
 video_buffer_offer_decoded_frame(struct video_buffer *vb,
                                  bool *previous_frame_skipped) {
     mutex_lock(vb->mutex);
-#ifndef SKIP_FRAMES
-    // if SKIP_FRAMES is disabled, then the decoder must wait for the current
-    // frame to be consumed
-    while (!vb->rendering_frame_consumed && !vb->interrupted) {
-        cond_wait(vb->rendering_frame_consumed_cond, vb->mutex);
+    if (vb->render_expired_frames) {
+        // wait for the current (expired) frame to be consumed
+        while (!vb->rendering_frame_consumed && !vb->interrupted) {
+            cond_wait(vb->rendering_frame_consumed_cond, vb->mutex);
+        }
+    } else {
+        if (vb->fps_counter.started && !vb->rendering_frame_consumed) {
+            fps_counter_add_skipped_frame(&vb->fps_counter);
+        }
     }
-#else
-    if (vb->fps_counter.started && !vb->rendering_frame_consumed) {
-        fps_counter_add_skipped_frame(&vb->fps_counter);
-    }
-#endif
 
     video_buffer_swap_frames(vb);
 
@@ -94,23 +96,20 @@ video_buffer_consume_rendered_frame(struct video_buffer *vb) {
     if (vb->fps_counter.started) {
         fps_counter_add_rendered_frame(&vb->fps_counter);
     }
-#ifndef SKIP_FRAMES
-    // if SKIP_FRAMES is disabled, then notify the decoder the current frame is
-    // consumed, so that it may push a new one
-    cond_signal(vb->rendering_frame_consumed_cond);
-#endif
+    if (vb->render_expired_frames) {
+        // unblock video_buffer_offer_decoded_frame()
+        cond_signal(vb->rendering_frame_consumed_cond);
+    }
     return vb->rendering_frame;
 }
 
 void
 video_buffer_interrupt(struct video_buffer *vb) {
-#ifdef SKIP_FRAMES
-    (void) vb; // unused
-#else
-    mutex_lock(vb->mutex);
-    vb->interrupted = true;
-    mutex_unlock(vb->mutex);
-    // wake up blocking wait
-    cond_signal(vb->rendering_frame_consumed_cond);
-#endif
+    if (vb->render_expired_frames) {
+        mutex_lock(vb->mutex);
+        vb->interrupted = true;
+        mutex_unlock(vb->mutex);
+        // wake up blocking wait
+        cond_signal(vb->rendering_frame_consumed_cond);
+    }
 }

app/src/video_buffer.h

@@ -4,7 +4,6 @@
 #include <stdbool.h>
 #include <SDL2/SDL_mutex.h>
 
-#include "config.h"
 #include "fps_counter.h"
 
 // forward declarations
@@ -14,16 +13,15 @@ struct video_buffer {
     AVFrame *decoding_frame;
     AVFrame *rendering_frame;
     SDL_mutex *mutex;
-#ifndef SKIP_FRAMES
+    bool render_expired_frames;
     bool interrupted;
     SDL_cond *rendering_frame_consumed_cond;
-#endif
     bool rendering_frame_consumed;
     struct fps_counter fps_counter;
 };
 
 bool
-video_buffer_init(struct video_buffer *vb);
+video_buffer_init(struct video_buffer *vb, bool render_expired_frames);
 
 void
 video_buffer_destroy(struct video_buffer *vb);