Improve lighting doc.

Fixed some minor issues and tried to improve the description of directional and omni light.

(cherry picked from commit 4ab77cf6aeda39ba6a6a8cc79dd40d8eedaf746b)

tutorials/3d/lighting.rst

@@ -16,14 +16,14 @@ result. Light can come from several types of sources in a scene:
    :ref:`Environment <class_Environment>`.
 -  Baked Light (read :ref:`doc_light_baking`).
 
-The emission color is a material property, as seen in the previous
-tutorials about materials (go read them if you didn't at this point!).
+The emission color is a material property. You can read more about it
+in the :ref:`doc_fixed_materials` tutorial.
 
 Light nodes
 -----------
 
 As mentioned before, there are three types of light nodes: Directional,
-Ambient and Spot. Each has different uses and will be described in
+Omni and Spot. Each has different uses and will be described in
 detail below, but first let's take a look at the common parameters for
 lights:
 
@@ -31,17 +31,17 @@ lights:
 
 Each one has a specific function:
 
--  **Enabled**: Lights can be disabled at any time.
+-  **Enabled**: Light is emitted only if this flag is set.
 -  **Bake Mode**: When using the light baker, the role of this light can
    be defined in this enumerator. The role will be followed even if the
    light is disabled, which allows to configure a light and then disable
    it for baking.
 -  **Energy**: This value is a multiplier for the light, it's specially
    useful for :ref:`doc_high_dynamic_range` and for Spot and Omni lights, because it can
-   create very bright spots near the emissor.
+   create very bright spots near the emitter.
 -  **Diffuse and Specular**: These light values get multiplied by the
-   material light and diffuse colors, so a white value does not mean
-   that light will be white, but that the original color will be kept.
+   material light and diffuse colors. A white value does not mean
+   that light will be white, but that the material color will be kept.
 -  **Operator**: It is possible to make some lights negative for a
    darkening effect.
 -  **Projector**: Lights can project a texture for the diffuse light
@@ -50,35 +50,46 @@ Each one has a specific function:
 Directional light
 ~~~~~~~~~~~~~~~~~
 
-This is the most common type of light and represents the sun. It is also
+This is the most common type of light and represents a light source 
+very far away (such as the sun). It is also
 the cheapest light to compute and should be used whenever possible
 (although it's not the cheapest shadow-map to compute, but more on that
-later). Directional light nodes are represented by a big arrow, which
-represent the direction of the light, however the position of the node
+later). 
+
+Directional light models an infinite number of parallel light rays
+covering the whole scene. The directional light node is represented by a big arrow, which
+indicates the direction of the light rays. However, the position of the node
 does not affect the lighting at all, and can be anywhere.
 
 .. image:: /img/light_directional.png
 
-Basically what faces the light is lit, what doesn't is dark. Most lights
+Every face whose front-side is hit by the light rays is lit, the others stay dark.
+Most light types
 have specific parameters but directional lights are pretty simple in
 nature so they don't.
 
 Omni light
 ~~~~~~~~~~
 
-Omni light is a point that throws light all around it up to a given
-radius (distance) that can be controlled by the user. The light
-attenuates with the distance and reaches 0 at the edge. It represents
-lamps or any other light source that comes from a point.
+Omni light is a point source that emits light spherically in all directions up to a given
+radius (distance from the node's position). The radius is a parameter of the light and
+can be controlled by the user. Just as in real life, the intensity of omni light
+decreases with the distance and vanishes at the defined radius. Omni light sources
+should be used to represent lamps or bulbs or any other light source that originates
+approximately in a point.
 
 .. image:: /img/light_omni.png
 
-The attenuation curve for these kind of lights in nature is computed
-with an inverse-quadratic function that never reaches zero and has
-almost infinitely large values near the emissor.
+In reality, the attenuation of omni light is proportional to the squared distance
+from the point source. This can be easily understood if you imagine a sqhere around
+the omni light with a certain radius. No matter how large the sphere is, the number
+of rays passing through it is always the same. If the radius of the sphere is doubled,
+the area of the sphere increases by four. In other words, the density of rays
+(the number of rays per squere area) decreases quadratically with the distance.
 
-This makes them considerably inconvenient to tweak for artists, so
-Godot simulates them with an artist-controlled exponential curve
+Inverse-quadratic attenuation curves are inconvenient for artists: they
+never reach zero and have almost intinitely large values near the emitter.
+So Godot simulates omni light with an artist-controlled exponential curve
 instead.
 
 .. image:: /img/light_attenuation.png
@@ -86,8 +97,8 @@ instead.
 Spot light
 ~~~~~~~~~~
 
-Spot lights are similar to Omni lights, except they only operate between
-a given angle (or "cutoff"). They are useful to simulate flashlights,
+Spot lights are similar to omni lights, except they emit light only into a cone
+(or "cutoff"). They are useful to simulate flashlights,
 car lights, etc. This kind of light is also attenuated towards the
 opposite direction it points to.
 
@@ -106,6 +117,6 @@ Baked light
 -----------
 
 Baked light stands for pre-computed ambient light. It can serve multiple
-purposes, such as baking light emissors that are not going to be used in
+purposes, such as baking light emitters that are not going to be used in
 real-time, and baking light bounces from real-time lights to add more
 realism to a scene (see Baked Light]] tutorial for more information).