diff --git a/doc/filters.texi b/doc/filters.texi
index 1a8843fe4f..a8797e5dc3 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -14008,6 +14008,8 @@ ffmpeg -i input.mov -vf lensfun=make=Canon:model="Canon EOS 100D":lens_model="Ca
 Obtain the VMAF (Video Multi-Method Assessment Fusion)
 score between two input videos.
 
+The first input is the encoded video, and the second input is the reference video.
+
 The obtained VMAF score is printed through the logging system.
 
 It requires Netflix's vmaf library (libvmaf) as a pre-requisite.