avutil/eval: add linear interpolation helper

doc/utils.texi

@@ -864,6 +864,9 @@ Load the value of the internal variable with number
 @var{var}, which was previously stored with st(@var{var}, @var{expr}).
 The function returns the loaded value.
 
+@item lerp(x, y, z)
+Return linear interpolation between @var{x} and @var{y} by amount of @var{z}.
+
 @item log(x)
 Compute natural logarithm of @var{x}.
 

libavutil/eval.c

@@ -155,7 +155,7 @@ struct AVExpr {
         e_pow, e_mul, e_div, e_add,
         e_last, e_st, e_while, e_taylor, e_root, e_floor, e_ceil, e_trunc, e_round,
         e_sqrt, e_not, e_random, e_hypot, e_gcd,
-        e_if, e_ifnot, e_print, e_bitand, e_bitor, e_between, e_clip, e_atan2
+        e_if, e_ifnot, e_print, e_bitand, e_bitor, e_between, e_clip, e_atan2, e_lerp,
     } type;
     double value; // is sign in other types
     union {
@@ -208,6 +208,12 @@ static double eval_expr(Parser *p, AVExpr *e)
             return e->value * (d >= eval_expr(p, e->param[1]) &&
                                d <= eval_expr(p, e->param[2]));
         }
+        case e_lerp: {
+            double v0 = eval_expr(p, e->param[0]);
+            double v1 = eval_expr(p, e->param[1]);
+            double f  = eval_expr(p, e->param[2]);
+            return v0 + (v1 - v0) * f;
+        }
         case e_print: {
             double x = eval_expr(p, e->param[0]);
             int level = e->param[1] ? av_clip(eval_expr(p, e->param[1]), INT_MIN, INT_MAX) : AV_LOG_INFO;
@@ -456,6 +462,7 @@ static int parse_primary(AVExpr **e, Parser *p)
     else if (strmatch(next, "between"))d->type = e_between;
     else if (strmatch(next, "clip"  )) d->type = e_clip;
     else if (strmatch(next, "atan2" )) d->type = e_atan2;
+    else if (strmatch(next, "lerp"  )) d->type = e_lerp;
     else {
         for (i=0; p->func1_names && p->func1_names[i]; i++) {
             if (strmatch(next, p->func1_names[i])) {
@@ -654,6 +661,7 @@ static int verify_expr(AVExpr *e)
                    && (!e->param[2] || verify_expr(e->param[2]));
         case e_between:
         case e_clip:
+        case e_lerp:
             return verify_expr(e->param[0]) &&
                    verify_expr(e->param[1]) &&
                    verify_expr(e->param[2]);