lavfi/tinterlace: add tinterlace mode 6

This new mode is useful for generating frames for interlaced video
displays. Typically interlaced video displays have no form of field
synchronisation. This new mode guarantees correct field order without
any requirement for field synchronisation.
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/filters.texi

@@ -2801,6 +2801,13 @@ even frames, generating a frame with unchanged height at half framerate.
 @item 5
 Interleave the lower field from odd frames with the upper field from
 even frames, generating a frame with unchanged height at half framerate.
+
+@item 6
+Double frame rate with unchanged height. Frames are inserted each
+containing the second temporal field from the previous input frame and
+the first temporal field from the next input frame. This mode relies on
+the top_field_first flag. Useful for interlaced video displays with no
+field synchronisation.
 @end table
 
 Default mode is 0.

libavfilter/version.h

@@ -30,7 +30,7 @@
 
 #define LIBAVFILTER_VERSION_MAJOR  2
 #define LIBAVFILTER_VERSION_MINOR 72
-#define LIBAVFILTER_VERSION_MICRO 100
+#define LIBAVFILTER_VERSION_MICRO 101
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \

libavfilter/vf_tinterlace.c

@@ -68,9 +68,9 @@ static av_cold int init(AVFilterContext *ctx, const char *args, void *opaque)
     if (args) {
         n = sscanf(args, "%d", &tinterlace->mode);
 
-        if (n != 1 || tinterlace->mode < 0 || tinterlace->mode > 5) {
+        if (n != 1 || tinterlace->mode < 0 || tinterlace->mode > 6) {
             av_log(ctx, AV_LOG_ERROR,
-                   "Invalid mode '%s', use an integer between 0 and 5\n", args);
+                   "Invalid mode '%s', use an integer between 0 and 6\n", args);
             return AVERROR(EINVAL);
         }
     }
@@ -253,6 +253,31 @@ static void end_frame(AVFilterLink *inlink)
                            tff ? FIELD_LOWER : FIELD_UPPER, 1, tff ? FIELD_LOWER : FIELD_UPPER);
         avfilter_unref_bufferp(&tinterlace->next);
         break;
+    case 6: /* re-interlace preserving image height, double frame rate */
+        /* output current frame first */
+        out = avfilter_ref_buffer(cur, AV_PERM_READ);
+
+        avfilter_start_frame(outlink, out);
+        avfilter_draw_slice(outlink, 0, outlink->h, 1);
+        avfilter_end_frame(outlink);
+
+        /* output mix of current and next frame */
+        tff = next->video->top_field_first;
+        out = avfilter_get_video_buffer(outlink, AV_PERM_WRITE, outlink->w, outlink->h);
+        avfilter_copy_buffer_ref_props(out, next);
+        out->video->interlaced = 1;
+
+        /* write current frame second field lines into the second field of the new frame */
+        copy_picture_field(out->data, out->linesize,
+                           cur->data, cur->linesize,
+                           inlink->format, inlink->w, inlink->h,
+                           tff ? FIELD_LOWER : FIELD_UPPER, 1, tff ? FIELD_LOWER : FIELD_UPPER);
+        /* write next frame first field lines into the first field of the new frame */
+        copy_picture_field(out->data, out->linesize,
+                           next->data, next->linesize,
+                           inlink->format, inlink->w, inlink->h,
+                           tff ? FIELD_UPPER : FIELD_LOWER, 1, tff ? FIELD_UPPER : FIELD_LOWER);
+        break;
     }
 
     avfilter_start_frame(outlink, out);