Improve --vfilter docs. Describe VMAF usage & mention --reference-vfi…

…lter

Relates to #213

CHANGELOG.md

@@ -3,6 +3,7 @@
   - Don't allow many '\r'-ending updates to cause all other stored info to be truncated. 
   - Increase max heap storage of output ~4k->32k to allow more complete output in some cases.
 * Fix caching unaffected by `--reference-vfilter` usage.
+* Improve `--vfilter` docs. Describe VMAF usage & mention `--reference-vfilter`.
 
 # v0.7.16
 * Fix VMAF score parse failure of certain successful ffmpeg outputs.

src/command/args/encode.rs

@@ -26,10 +26,14 @@ pub struct Encode {
     #[arg(short, long, value_hint = ValueHint::FilePath)]
     pub input: PathBuf,
 
-    /// Ffmpeg video filter applied to the input before av1 encoding.
+    /// Ffmpeg video filter applied to the input before encoding.
     /// E.g. --vfilter "scale=1280:-1,fps=24".
     ///
     /// 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.
     #[arg(long)]
     pub vfilter: Option<String>,