Add --render-fit feature

Add an option to configure how the rendering fits the window.

The default, `--render-fit=letterbox`, preserves the aspect ratio
and fits the window as best as possible, adding black bars at
the top/bottom or left/right if needed. This has been the only behavior
scrcpy supported so far.

Another mode, `--render-fit=disabled`, renders the display at the
top-left corner without scaling. This mode will be useful for virtual
display resizing.

app/src/cli.c

@@ -105,6 +105,7 @@ enum {
     OPT_CAMERA_ZOOM,
     OPT_MIN_SIZE_ALIGNMENT,
     OPT_NO_WINDOW_ASPECT_RATIO_LOCK,
+    OPT_RENDER_FIT,
 };
 
 struct sc_option {
@@ -777,6 +778,20 @@ static const struct sc_option options[] = {
                 "\"opengles2\", \"opengles\", \"metal\" and \"software\".\n"
                 "<https://wiki.libsdl.org/SDL_HINT_RENDER_DRIVER>",
     },
+    {
+        .longopt_id = OPT_RENDER_FIT,
+        .longopt = "render-fit",
+        .argdesc = "mode",
+        .text = "Set the render-fit mode to configure how the rendering fits "
+                "the window.\n"
+                "Possible values are \"letterbox\" and \"disabled\".\n"
+                "\"letterbox\": preserve the aspect ratio and fit the window "
+                "as best as possible (black bars are added either at the top "
+                "and bottom or at the sides if needed).\n"
+                "\"disabled\": render the display at the top-left corner, "
+                "without scaling.\n"
+                "Default is \"letterbox\".",
+    },
     {
         .longopt_id = OPT_REQUIRE_AUDIO,
         .longopt = "require-audio",
@@ -2322,6 +2337,22 @@ parse_mouse_bindings(const char *s, struct sc_mouse_bindings *mb) {
     return true;
 }
 
+static bool
+parse_render_fit(const char *optarg, enum sc_render_fit *mode) {
+    if (!strcmp(optarg, "letterbox")) {
+        *mode = SC_RENDER_FIT_LETTERBOX;
+        return true;
+    }
+
+    if (!strcmp(optarg, "disabled")) {
+        *mode = SC_RENDER_FIT_DISABLED;
+        return true;
+    }
+
+    LOGE("Unsupported render-fit: %s (expected letterbox or disabled)", optarg);
+    return false;
+}
+
 static bool
 parse_args_with_getopt(struct scrcpy_cli_args *args, int argc, char *argv[],
                        const char *optstring, const struct option *longopts) {
@@ -2755,6 +2786,11 @@ parse_args_with_getopt(struct scrcpy_cli_args *args, int argc, char *argv[],
             case OPT_NO_WINDOW_ASPECT_RATIO_LOCK:
                 opts->window_aspect_ratio_lock = false;
                 break;
+            case OPT_RENDER_FIT:
+                if (!parse_render_fit(optarg, &opts->render_fit)) {
+                    return false;
+                }
+                break;
             default:
                 // getopt prints the error message on stderr
                 return false;

app/src/options.c

@@ -59,6 +59,7 @@ const struct scrcpy_options scrcpy_options_default = {
     .display_orientation = SC_ORIENTATION_0,
     .record_orientation = SC_ORIENTATION_0,
     .display_ime_policy = SC_DISPLAY_IME_POLICY_UNDEFINED,
+    .render_fit = SC_RENDER_FIT_LETTERBOX,
     .window_x = SC_WINDOW_POSITION_UNDEFINED,
     .window_y = SC_WINDOW_POSITION_UNDEFINED,
     .window_width = 0,

app/src/options.h

@@ -220,6 +220,11 @@ enum sc_shortcut_mod {
     SC_SHORTCUT_MOD_RSUPER = 1 << 5,
 };
 
+enum sc_render_fit {
+    SC_RENDER_FIT_LETTERBOX,
+    SC_RENDER_FIT_DISABLED,
+};
+
 struct sc_port_range {
     uint16_t first;
     uint16_t last;
@@ -269,6 +274,7 @@ struct scrcpy_options {
     enum sc_orientation display_orientation;
     enum sc_orientation record_orientation;
     enum sc_display_ime_policy display_ime_policy;
+    enum sc_render_fit render_fit;
     int16_t window_x; // SC_WINDOW_POSITION_UNDEFINED for "auto"
     int16_t window_y; // SC_WINDOW_POSITION_UNDEFINED for "auto"
     uint16_t window_width;

app/src/scrcpy.c

@@ -814,6 +814,7 @@ aoa_complete:
             .window_height = options->window_height,
             .window_aspect_ratio_lock = options->window_aspect_ratio_lock,
             .window_borderless = options->window_borderless,
+            .render_fit = options->render_fit,
             .orientation = options->display_orientation,
             .mipmaps = options->mipmaps,
             .fullscreen = options->fullscreen,

app/src/screen.c

@@ -158,7 +158,16 @@ sc_screen_is_relative_mode(struct sc_screen *screen) {
 
 static void
 compute_content_rect(struct sc_size render_size, struct sc_size content_size,
-                     bool can_upscale, SDL_FRect *rect) {
+                     bool can_upscale, enum sc_render_fit render_fit,
+                     SDL_FRect *rect) {
+    if (render_fit == SC_RENDER_FIT_DISABLED) {
+        rect->x = 0;
+        rect->y = 0;
+        rect->w = content_size.width;
+        rect->h = content_size.height;
+        return;
+    }
+
     if (is_optimal_size(render_size, content_size)) {
         rect->x = 0;
         rect->y = 0;
@@ -202,7 +211,7 @@ sc_screen_update_content_rect(struct sc_screen *screen) {
     struct sc_size render_size =
         sc_sdl_get_render_output_size(screen->renderer);
     compute_content_rect(render_size, screen->content_size, can_upscale,
-                         &screen->rect);
+                         screen->render_fit, &screen->rect);
 }
 
 // render the texture to the renderer
@@ -406,6 +415,7 @@ sc_screen_init(struct sc_screen *screen,
     screen->video = params->video;
     screen->camera = params->camera;
     screen->window_aspect_ratio_lock = params->window_aspect_ratio_lock;
+    screen->render_fit = params->render_fit;
 
     screen->req.x = params->window_x;
     screen->req.y = params->window_y;

app/src/screen.h

@@ -65,6 +65,8 @@ struct sc_screen {
     SDL_GLContext gl_context;
 #endif
 
+    enum sc_render_fit render_fit;
+
     struct sc_size frame_size;
     struct sc_size content_size; // rotated frame_size
 
@@ -118,6 +120,7 @@ struct sc_screen_params {
     bool window_aspect_ratio_lock;
     bool window_borderless;
 
+    enum sc_render_fit render_fit;
     enum sc_orientation orientation;
     bool mipmaps;