Add documentation for constants.py (#572)

* Add documentation for constants.py

* Update documentation for constants.py

With suggestions from eulertour

docs/source/constants.rst

@@ -0,0 +1,107 @@
+Manim Constants
+===============
+
+The ``constants.py`` under ``manimlib/`` contains variables that are used
+during setup and running manim. Some variables are not documented here as they are
+only used internally by manim.
+
+Directories
+-----------
+
+    MEDIA_DIR
+              The root directory for media files. First the environment variable
+              ``MEDIA_DIR`` is queried. If the env is not found, use the path
+              as specified in ``manimlib/media_dir.txt``. If media_dir.txt is empty,
+              a directory is created under users home. If ``MEDIA_DIR`` is not a
+              directory, ``media`` will be created under current working directory.
+              Defaults to ``manimlib/media``
+    VIDEO_DIR
+              Created under ``MEDIA_DIR``. Used to store rendered manim scenes.
+              When scene finished rendering, it will be stored under ``media/project_name/quality/scene_name.mp4``
+    RASTER_IMAGE_DIR
+              Created under ``media/designs/raster_images``. For storing
+              raster images like jpg, png, etc.
+    SVG_IMAGE_DIR
+              Created under ``media/designs/svg_images``. For storing svg files.
+    SOUND_DIR
+              Created under ``media/designs/sounds``. For storing sound files.
+    THIS_DIR
+              Path to the directory which ``constants.py`` resides.
+    FILE_DIR
+              The root directory for misc files. Specified by the environment variable
+              ``FILE_DIR``. If the variable is not found, a directory ``files`` will be
+              created under current working directory.
+    TEX_DIR
+              Created under ``FILE_DIR/files``. TexMObject and TextMobject are rendered
+              here for temporary storage.
+
+Those directories are created if they don't exist.
+
+.. note::
+    ``MOBJECT_DIR`` and ``IMAGE_MOBJECT_DIR`` is deprecated.
+
+Tex
+---
+    TEX_USE_CTEX
+              A boolean value. Change it to True if you need to use Chinese typesetting.
+    TEX_TEXT_TO_REPLACE
+              Placeholder text used by manim when generating tex files
+    TEMPLATE_TEX_FILE
+              By default ``manimlib/tex_template.tex`` is used. If ``TEX_USE_CTEX``
+              is set to True then ``manimlib/ctex_template.tex`` is used.
+
+Numerical Constants
+-------------------
+
+    PI
+            alias to ``numpy.pi``
+    TAU
+            PI * 2
+
+    DEGREES
+            TAU / 360
+
+Camera Configuration
+--------------------
+
+Render setting presets
+
+    PRODUCTION_QUALITY_CAMERA_CONFIG
+            2560x1440 @ 60fps # This is the default when rendering a scene
+    HIGH_QUALITY_CAMERA_CONFIG
+            1920x1080 @ 60fps.
+    MEDIUM_QUALITY_CAMERA_CONFIG
+            1280x720 @ 30fps # Used when ``-m`` flag is passed
+    LOW_QUALITY_CAMERA_CONFIG
+            854x480 @ 15fps. # Used when ``-l`` flag is passed
+
+Coordinates
+-----------
+
+Used for 2d/3d animations and placements::
+
+    ORIGIN
+    UP
+    DOWN
+    RIGHT
+    LEFT
+    IN # 3d camera only, away from camera
+    OUT # 3d camera only, close to camera
+
+    UL = UP + LEFT # diagonal abbreviations. You can use either one
+    UR = UP + RIGHT
+    DL = DOWN + LEFT
+    DR = DOWN + RIGHT
+
+    TOP
+    BOTTOM
+    LEFT_SIDE
+    RIGHT_SIDE``
+
+Colors
+------
+
+    COLOR_MAP
+            A predefined color maps
+    PALETTE
+            A list of color hex strings, derived from COLOR_MAP

docs/source/index.rst

@@ -17,7 +17,8 @@ a pull request there.
     about
     installation/index
     getting_started/index
-
+    constants
+    
 
 Indices and tables
 ==================