avfilter: add comments for duplicate line

comment about the looks like a duplicate line.
but that is used to reason x is expressed from y

Suggested-by: Paul B Mahol
Suggested-by: Michael Niedermayer
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/libavfilter/vf_overlay.c b/libavfilter/vf_overlay.c
index aa5835ae3a..c6a6ac82f3 100644
--- a/libavfilter/vf_overlay.c
+++ b/libavfilter/vf_overlay.c
@@ -149,6 +149,7 @@ static void eval_expr(AVFilterContext *ctx)
 
     s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, NULL);
     s->var_values[VAR_Y] = av_expr_eval(s->y_pexpr, s->var_values, NULL);
+    /* It is necessary if x is expressed from y  */
     s->var_values[VAR_X] = av_expr_eval(s->x_pexpr, s->var_values, NULL);
     s->x = normalize_xy(s->var_values[VAR_X], s->hsub);
     s->y = normalize_xy(s->var_values[VAR_Y], s->vsub);