complete documentation

docs/source/color.rst

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+.. default-domain:: C
+
+color
+================================================================================
+
+Header: cglm/color.h
+
+Table of contents (click to go):
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Functions:
+
+1. :c:func:`glm_luminance`
+
+Functions documentation
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. c:function:: float  glm_luminance(vec3 rgb)
+
+    | averages the color channels into one value
+
+    This function uses formula in COLLADA 1.5 spec which is
+
+    .. code-block:: text
+
+       luminance = (color.r * 0.212671) +
+                   (color.g * 0.715160) +
+                   (color.b * 0.072169)
+
+    It is based on the ISO/CIE color standards (see ITU-R Recommendation BT.709-4),
+    that averages the color channels into one value
+
+    Parameters:
+      | *[in]*  **rgb**   RGB color