tools (common): helping method to calculate luminance:

- a new `utils` module has been added for backend and frontends generic utilities
- per_luminance method calculate the perceived luminance of a colour (is it dark or
  light?). This is useful to automatically select the suitable colour in a theme.

sat/tools/common/utils.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+
+# SàT: a XMPP client
+# Copyright (C) 2009-2019 Jérôme Poisson (goffi@goffi.org)
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Misc utils for both backend and frontends"""
+
+
+def per_luminance(red, green, blue):
+    """Caculate the perceived luminance of a RGB color
+
+    @param red(int): 0-1 normalized value of red
+    @param green(int): 0-1 normalized value of green
+    @param blue(int): 0-1 normalized value of blue
+    @return (float): 0-1 value of luminance (<0.5 is dark, else it's light)
+    """
+    # cf. https://stackoverflow.com/a/1855903, thanks Gacek
+
+    return 0.299 * red + 0.587 * green + 0.114 * blue