video: add --video-rotate option for controlling auto-rotation

DOCS/man/en/options.rst

@@ -2698,6 +2698,13 @@ OPTIONS
 
     This option is disabled if the ``--no-keepaspect`` option is used.
 
+``--video-rotate=<0-359|no>``
+    Rotate the video clockwise, in degrees. Currently supports 90° steps only.
+    If ``no`` is given, the video is never rotated, even if the file has
+    rotation metadata. (The rotation value is added to the rotation metadata,
+    which means the value ``0`` would rotate the video according to the
+    rotation metadata.)
+
 ``--video-unscaled``
     Disable scaling of the video. If the window is larger than the video,
     black bars are added. Otherwise, the video is cropped. The video still

options/options.c

@@ -498,6 +498,8 @@ const m_option_t mp_opts[] = {
                ({"auto", MP_CSP_LEVELS_AUTO},
                 {"limited", MP_CSP_LEVELS_TV},
                 {"full", MP_CSP_LEVELS_PC})),
+    OPT_CHOICE_OR_INT("video-rotate", video_rotate, 0, 0, 359,
+                      ({"no", -1})),
 
     OPT_CHOICE_OR_INT("cursor-autohide", cursor_autohide_delay, 0,
                       0, 30000, ({"no", -1}, {"always", -2})),

options/options.h

@@ -94,6 +94,8 @@ typedef struct MPOpts {
     int requested_input_range;
     int requested_output_range;
 
+    int video_rotate;
+
     char *audio_decoders;
     char *video_decoders;
 

video/decode/vd_lavc.c

@@ -420,6 +420,7 @@ static void update_image_params(struct dec_video *vd, AVFrame *frame,
                                 struct mp_image_params *out_params)
 {
     vd_ffmpeg_ctx *ctx = vd->priv;
+    struct MPOpts *opts = ctx->opts;
     int width = frame->width;
     int height = frame->height;
     float aspect = av_q2d(frame->sample_aspect_ratio) * width / height;
@@ -448,6 +449,12 @@ static void update_image_params(struct dec_video *vd, AVFrame *frame,
             avchroma_location_to_mp(ctx->avctx->chroma_sample_location),
         .rotate = vd->header->video->rotate,
     };
+
+    if (opts->video_rotate < 0) {
+        out_params->rotate = 0;
+    } else {
+        out_params->rotate = (out_params->rotate + opts->video_rotate) % 360;
+    }
 }
 
 static enum AVPixelFormat get_format_hwdec(struct AVCodecContext *avctx,