avfilter/zoompan: add in_time variable

Currently, the zoompan filter exposes a 'time' variable (missing from docs) for use in
the 'zoom', 'x', and 'y' expressions. This variable is perhaps better named
'out_time' as it represents the timestamp in seconds of each output frame
produced by zoompan. This patch adds aliases 'out_time' and 'ot' for 'time'.

This patch also adds an 'in_time' (alias 'it') variable that provides access
to the timestamp in seconds of each input frame to the zoompan filter.
This helps to design zoompan filters that depend on the input video timestamps.
For example, it makes it easy to zoom in instantly for only some portion of a video.
Both the 'out_time' and 'in_time' variables have been added in the documentation
for zoompan.

Example usage of 'in_time' in the zoompan filter to zoom in 2x for the
first second of the input video and 1x for the rest:
    zoompan=z='if(between(in_time,0,1),2,1):d=1'

V2: Fix zoompan filter documentation stating that the time variable
would be NAN if the input timestamp is unknown.

V3: Add 'it' alias for 'in_time. Add 'out_time' and 'ot' aliases for 'time'.
Minor corrections to zoompan docs.

Signed-off-by: exwm <thighsman@protonmail.com>

doc/filters.texi

@@ -20782,6 +20782,12 @@ Input frame count.
 @item on
 Output frame count.
 
+@item in_time, it
+The input timestamp expressed in seconds. It's NAN if the input timestamp is unknown.
+
+@item out_time, time, ot
+The output timestamp expressed in seconds.
+
 @item x
 @item y
 Last calculated 'x' and 'y' position from 'x' and 'y' expression
@@ -20820,13 +20826,13 @@ display aspect ratio
 
 @itemize
 @item
-Zoom-in up to 1.5 and pan at same time to some spot near center of picture:
+Zoom in up to 1.5x and pan at same time to some spot near center of picture:
 @example
 zoompan=z='min(zoom+0.0015,1.5)':d=700:x='if(gte(zoom,1.5),x,x+1/a)':y='if(gte(zoom,1.5),y,y+1)':s=640x360
 @end example
 
 @item
-Zoom-in up to 1.5 and pan always at center of picture:
+Zoom in up to 1.5x and pan always at center of picture:
 @example
 zoompan=z='min(zoom+0.0015,1.5)':d=700:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'
 @end example
@@ -20836,6 +20842,13 @@ Same as above but without pausing:
 @example
 zoompan=z='min(max(zoom,pzoom)+0.0015,1.5)':d=1:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'
 @end example
+
+@item
+Zoom in 2x into center of picture only for the first second of the input video:
+@example
+zoompan=z='if(between(in_time,0,1),2,1)':d=1:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'
+@end example
+
 @end itemize
 
 @anchor{zscale}

libavfilter/vf_zoompan.c

@@ -38,7 +38,8 @@ static const char *const var_names[] = {
     "on",
     "duration",
     "pduration",
-    "time",
+    "in_time", "it",
+    "out_time", "time", "ot",
     "frame",
     "zoom",
     "pzoom",
@@ -61,7 +62,8 @@ enum var_name {
     VAR_ON,
     VAR_DURATION,
     VAR_PDURATION,
-    VAR_TIME,
+    VAR_IN_TIME, VAR_IT,
+    VAR_TIME, VAR_OUT_TIME, VAR_OT,
     VAR_FRAME,
     VAR_ZOOM,
     VAR_PZOOM,
@@ -155,6 +157,7 @@ static int output_single_frame(AVFilterContext *ctx, AVFrame *in, double *var_va
 {
     ZPContext *s = ctx->priv;
     AVFilterLink *outlink = ctx->outputs[0];
+    AVFilterLink *inlink = ctx->inputs[0];
     int64_t pts = s->frame_count;
     int k, x, y, w, h, ret = 0;
     uint8_t *input[4];
@@ -165,7 +168,10 @@ static int output_single_frame(AVFilterContext *ctx, AVFrame *in, double *var_va
     var_values[VAR_PY]    = s->y;
     var_values[VAR_PZOOM] = s->prev_zoom;
     var_values[VAR_PDURATION] = s->prev_nb_frames;
-    var_values[VAR_TIME] = pts * av_q2d(outlink->time_base);
+    var_values[VAR_IN_TIME] = var_values[VAR_IT]  = in->pts == AV_NOPTS_VALUE ?
+        NAN : in->pts * av_q2d(inlink->time_base);
+    var_values[VAR_OUT_TIME] = pts * av_q2d(outlink->time_base);
+    var_values[VAR_TIME] = var_values[VAR_OT] = var_values[VAR_OUT_TIME];
     var_values[VAR_FRAME] = i;
     var_values[VAR_ON] = outlink->frame_count_in;