avfilter/vf_tinterlace: Favor using standard timebases for the output

Reported-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Inspired by discussion with Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/tinterlace.h

@@ -44,6 +44,7 @@ enum TInterlaceMode {
 typedef struct {
     const AVClass *class;
     enum TInterlaceMode mode;   ///< interlace mode selected
+    AVRational preout_time_base;
     int flags;                  ///< flags affecting interlacing algorithm
     int frame;                  ///< number of the output frame
     int vsub;                   ///< chroma vertical subsampling

libavfilter/vf_tinterlace.c

@@ -35,6 +35,7 @@
 #define OFFSET(x) offsetof(TInterlaceContext, x)
 #define FLAGS AV_OPT_FLAG_FILTERING_PARAM|AV_OPT_FLAG_VIDEO_PARAM
 #define TINTERLACE_FLAG_VLPF 01
+#define TINTERLACE_FLAG_EXACT_TB 2
 
 static const AVOption tinterlace_options[] = {
     {"mode",              "select interlace mode", OFFSET(mode), AV_OPT_TYPE_INT, {.i64=MODE_MERGE}, 0, MODE_NB-1, FLAGS, "mode"},
@@ -49,6 +50,7 @@ static const AVOption tinterlace_options[] = {
     {"flags",             "set flags", OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = 0}, 0, INT_MAX, 0, "flags" },
     {"low_pass_filter",   "enable vertical low-pass filter",              0, AV_OPT_TYPE_CONST, {.i64 = TINTERLACE_FLAG_VLPF}, INT_MIN, INT_MAX, FLAGS, "flags" },
     {"vlpf",              "enable vertical low-pass filter",              0, AV_OPT_TYPE_CONST, {.i64 = TINTERLACE_FLAG_VLPF}, INT_MIN, INT_MAX, FLAGS, "flags" },
+    {"exact_tb",          "force a timebase which can represent timestamps exactly", 0, AV_OPT_TYPE_CONST, {.i64 = TINTERLACE_FLAG_EXACT_TB}, INT_MIN, INT_MAX, FLAGS, "flags" },
 
     {NULL}
 };
@@ -62,6 +64,12 @@ static const enum AVPixelFormat full_scale_yuvj_pix_fmts[] = {
     FULL_SCALE_YUVJ_FORMATS, AV_PIX_FMT_NONE
 };
 
+static const AVRational standard_tbs[] = {
+    {1, 25},
+    {1, 30},
+    {1001, 30000},
+};
+
 static int query_formats(AVFilterContext *ctx)
 {
     static const enum AVPixelFormat pix_fmts[] = {
@@ -104,6 +112,7 @@ static int config_out_props(AVFilterLink *outlink)
     AVFilterLink *inlink = outlink->src->inputs[0];
     const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(outlink->format);
     TInterlaceContext *tinterlace = ctx->priv;
+    int i;
 
     tinterlace->vsub = desc->log2_chroma_h;
     outlink->flags |= FF_LINK_FLAG_REQUEST_LOOP;
@@ -135,13 +144,23 @@ static int config_out_props(AVFilterLink *outlink)
                 tinterlace->mode);
         tinterlace->flags &= ~TINTERLACE_FLAG_VLPF;
     }
+    tinterlace->preout_time_base = inlink->time_base;
     if (tinterlace->mode == MODE_INTERLACEX2) {
-        outlink->time_base.num = inlink->time_base.num;
-        outlink->time_base.den = inlink->time_base.den * 2;
+        tinterlace->preout_time_base.den *= 2;
         outlink->frame_rate = av_mul_q(inlink->frame_rate, (AVRational){2,1});
+        outlink->time_base  = av_mul_q(inlink->time_base , (AVRational){1,2});
     } else if (tinterlace->mode != MODE_PAD) {
         outlink->frame_rate = av_mul_q(inlink->frame_rate, (AVRational){1,2});
+        outlink->time_base  = av_mul_q(inlink->time_base , (AVRational){2,1});
+    }
+
+    for (i = 0; i<FF_ARRAY_ELEMS(standard_tbs); i++){
+        if (!av_cmp_q(standard_tbs[i], outlink->time_base))
+            break;
     }
+    if (i == FF_ARRAY_ELEMS(standard_tbs) ||
+        (tinterlace->flags & TINTERLACE_FLAG_EXACT_TB))
+        outlink->time_base = tinterlace->preout_time_base;
 
     if (tinterlace->flags & TINTERLACE_FLAG_VLPF) {
         tinterlace->lowpass_line = lowpass_line_c;
@@ -326,6 +345,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *picref)
         if (cur->pts != AV_NOPTS_VALUE)
             out->pts = cur->pts*2;
 
+        out->pts = av_rescale_q(out->pts, tinterlace->preout_time_base, outlink->time_base);
         if ((ret = ff_filter_frame(outlink, out)) < 0)
             return ret;
 
@@ -359,6 +379,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *picref)
         av_assert0(0);
     }
 
+    out->pts = av_rescale_q(out->pts, tinterlace->preout_time_base, outlink->time_base);
     ret = ff_filter_frame(outlink, out);
     tinterlace->frame++;