Docs: clarify JPEG quality/chromaSubsampling interdependence

lovell · Mar 26, 2020 · ecd01af · ecd01af
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Showing 2 changed files with 2 additions and 2 deletions.
diff --git a/docs/api-output.md b/docs/api-output.md
@@ -143,7 +143,7 @@ Use these JPEG options for output image.
 -   `options` **[Object][6]?** output options
     -   `options.quality` **[Number][9]** quality, integer 1-100 (optional, default `80`)
     -   `options.progressive` **[Boolean][7]** use progressive (interlace) scan (optional, default `false`)
-    -   `options.chromaSubsampling` **[String][2]** set to '4:4:4' to prevent chroma subsampling when quality &lt;= 90 (optional, default `'4:2:0'`)
+    -   `options.chromaSubsampling` **[String][2]** for quality &lt; 90, set to '4:4:4' to prevent chroma subsampling otherwise defaults to '4:2:0' (use chroma subsampling); for quality >= 90 chroma is never subsampled (optional, default `'4:2:0'`)
     -   `options.trellisQuantisation` **[Boolean][7]** apply trellis quantisation, requires libvips compiled with support for mozjpeg (optional, default `false`)
     -   `options.overshootDeringing` **[Boolean][7]** apply overshoot deringing, requires libvips compiled with support for mozjpeg (optional, default `false`)
     -   `options.optimiseScans` **[Boolean][7]** optimise progressive scans, forces progressive, requires libvips compiled with support for mozjpeg (optional, default `false`)

diff --git a/lib/output.js b/lib/output.js
@@ -183,7 +183,7 @@ function toFormat (format, options) {
  * @param {Object} [options] - output options
  * @param {Number} [options.quality=80] - quality, integer 1-100
  * @param {Boolean} [options.progressive=false] - use progressive (interlace) scan
- * @param {String} [options.chromaSubsampling='4:2:0'] - set to '4:4:4' to prevent chroma subsampling when quality <= 90
+ * @param {String} [options.chromaSubsampling='4:2:0'] - for quality < 90, set to '4:4:4' to prevent chroma subsampling otherwise defaults to '4:2:0' (use chroma subsampling); for quality >= 90 chroma is never subsampled
  * @param {Boolean} [options.trellisQuantisation=false] - apply trellis quantisation, requires libvips compiled with support for mozjpeg
  * @param {Boolean} [options.overshootDeringing=false] - apply overshoot deringing, requires libvips compiled with support for mozjpeg
  * @param {Boolean} [options.optimiseScans=false] - optimise progressive scans, forces progressive, requires libvips compiled with support for mozjpeg