Clarify --vfilter doc further (#223)

* Clarify --vfilter doc further

* clarify

CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased (v0.7.18)
+* Improve `--vfilter` docs, clarify VMAF usage.
+
 # v0.7.17
 * Improve failing ffmpeg stderr printing: 
   - Don't allow many '\r'-ending updates to cause all other stored info to be truncated. 

src/command/args/encode.rs

@@ -32,8 +32,15 @@ pub struct Encode {
     /// See https://ffmpeg.org/ffmpeg-filters.html#Video-Filters
     ///
     /// For VMAF calculations this is also applied to the reference video meaning VMAF
-    /// scores represent the quality of original+vfilters compared to the encoded result.
-    /// To override this behaviour set --reference-vfilter.
+    /// scores represent the quality of input stream *after* applying filters compared
+    /// to the encoded result.
+    /// This allows filters like cropping to work with VMAF, as it would be the
+    /// cropped stream that is VMAF compared to a cropped-then-encoded stream. Such filters
+    /// would not otherwise generally be comparable.
+    ///
+    /// A consequence is the VMAF score will not reflect any quality lost
+    /// by the vfilter itself, only the encode.
+    /// To override the VMAF vfilter set --reference-vfilter.
     #[arg(long)]
     pub vfilter: Option<String>,