Don't use group -1 for luts_png

README.rst

@@ -51,6 +51,36 @@ Usage
 For running omero-web in production with NGINX, see See: `OMERO.web install`_ documentation.
 To run in development mode, see below.
 
+LUTs caching
+------------
+
+The OMERO server ships with a set of look-up tables (LUTs) for rendering images. Admins can also
+add their own LUTs to the server. The LUTs available on the server can be retrieved from the
+``/webgateway/luts/`` endpoint as JSON data.
+
+To include the ``rgb`` values of each LUT in the JSON response, append ``?rgb=true`` to the URL.
+
+A copy of the JSON output from ``/webgateway/luts/?rgb=true`` is included in the ``omero-web`` package
+as a static file at
+https://github.com/ome/omero-web/blob/master/omeroweb/webgateway/static/webgateway/json/luts.json.
+
+This is used to generate a LUT preview png at ``/webgateway/luts_png/`` that can be used by clients
+such as OMERO.ivewer and OMERO.figure to display the LUTs in a user-friendly way.
+By using the static ``luts.json``, the ``/luts_png/`` can be quickly generated without needing to load LUTs from
+the server.
+
+The sequence of LUTs in ``/webgateway/luts_png/`` corresponds to the dynamic list from ``/webgateway/luts/``.
+If there are new LUTs on the server that are not found in the static ``luts.json``, the ``/luts_png/`` will
+show a blank placeholder for that LUT.
+
+If you have Django caching enabled on your server (e.g. with Redis) then you can cache the png *with* new
+LUTs by visiting ``/webgateway/luts_png/?cached=false``. This will then be read by other users when
+they visit ``/webgateway/luts_png/``.
+
+If Django caching isn't enabled, the JSON response from ``/webgateway/luts/?rgb=true``
+can instead be manually updated in the static ``luts.json`` file in the ``omero-web`` package.
+
+
 Contributing
 ------------