Make SKIP_FRAMES a compilation flag

The skip_frames flag was a non-configurable runtime flag. Since it is
not exposed to the user, there is no need for a (possible) runtime cost.

For testing purpose, we still want it to be configurable, so make it a
compilation flag.

app/meson.build

@@ -44,6 +44,11 @@ conf.set('DEFAULT_MAX_SIZE', '0')  # 0: unlimited
 # overridden by option --bit-rate
 conf.set('DEFAULT_BIT_RATE', '4000000')  # 4Mbps
 
+# 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', true)
+
 configure_file(configuration: conf, output: 'config.h')
 
 executable('scrcpy', src, dependencies: dependencies)

app/src/decoder.c

@@ -6,6 +6,7 @@
 #include <SDL2/SDL_thread.h>
 #include <unistd.h>
 
+#include "config.h"
 #include "events.h"
 #include "frames.h"
 #include "lockutil.h"
@@ -22,13 +23,17 @@ static int read_packet(void *opaque, uint8_t *buf, int buf_size) {
 static void push_frame(struct decoder *decoder) {
     struct frames *frames = decoder->frames;
     mutex_lock(frames->mutex);
-    if (!decoder->skip_frames) {
+#ifndef SKIP_FRAMES
+    // if SKIP_FRAMES is disabled, then the decoder must wait for the current
+    // frame to be consumed
     while (!frames->rendering_frame_consumed) {
         cond_wait(frames->rendering_frame_consumed_cond, frames->mutex);
     }
-    } else if (!frames->rendering_frame_consumed) {
+#else
+    if (!frames->rendering_frame_consumed) {
         SDL_LogInfo(SDL_LOG_CATEGORY_RENDER, "Skip frame");
     }
+#endif
 
     frames_swap(frames);
     frames->rendering_frame_consumed = SDL_FALSE;

app/src/decoder.h

@@ -11,7 +11,6 @@ struct decoder {
     TCPsocket video_socket;
     SDL_Thread *thread;
     SDL_mutex *mutex;
-    SDL_bool skip_frames;
 };
 
 SDL_bool decoder_start(struct decoder *decoder);

app/src/frames.c

@@ -17,16 +17,17 @@ SDL_bool frames_init(struct frames *frames) {
         goto error_2;
     }
 
+#ifndef SKIP_FRAMES
     if (!(frames->rendering_frame_consumed_cond = SDL_CreateCond())) {
-        goto error_3;
+        SDL_DestroyMutex(frames->mutex);
+        goto error_2;
     }
+#endif
 
     frames->rendering_frame_consumed = SDL_TRUE;
 
     return SDL_TRUE;
 
-error_3:
-    SDL_DestroyMutex(frames->mutex);
 error_2:
     av_frame_free(&frames->rendering_frame);
 error_1:
@@ -36,7 +37,9 @@ error_0:
 }
 
 void frames_destroy(struct frames *frames) {
+#ifndef SKIP_FRAMES
     SDL_DestroyCond(frames->rendering_frame_consumed_cond);
+#endif
     SDL_DestroyMutex(frames->mutex);
     av_frame_free(&frames->rendering_frame);
     av_frame_free(&frames->decoding_frame);

app/src/frames.h

@@ -4,6 +4,8 @@
 #include <SDL2/SDL_mutex.h>
 #include <SDL2/SDL_stdinc.h>
 
+#include "config.h"
+
 // forward declarations
 typedef struct AVFrame AVFrame;
 
@@ -11,7 +13,9 @@ struct frames {
     AVFrame *decoding_frame;
     AVFrame *rendering_frame;
     SDL_mutex *mutex;
+#ifndef SKIP_FRAMES
     SDL_cond *rendering_frame_consumed_cond;
+#endif
     SDL_bool rendering_frame_consumed;
 };
 

app/src/scrcpy.c

@@ -336,9 +336,11 @@ static SDL_bool handle_new_frame(void) {
     mutex_lock(frames.mutex);
     AVFrame *frame = frames.rendering_frame;
     frames.rendering_frame_consumed = SDL_TRUE;
-    if (!decoder.skip_frames) {
+#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(frames.rendering_frame_consumed_cond);
-    }
+#endif
 
     struct size current_frame_size = {frame->width, frame->height};
     if (!prepare_for_frame(window, renderer, &texture, frame_size, current_frame_size)) {
@@ -583,7 +585,6 @@ SDL_bool scrcpy(const char *serial, Uint16 local_port, Uint16 max_size, Uint32 b
 
     decoder.frames = &frames;
     decoder.video_socket = device_socket;
-    decoder.skip_frames = SDL_TRUE;
 
     // now we consumed the header values, the socket receives the video stream
     // start the decoder