Global settings

### 4.2.4 Global settings

    #declare ImageWidth = 160;
    #declare ImageHeight = 120;
    #declare MaxRecLev = 5;
    #declare AmbientLight = <.2,.2,.2>;
    #declare BGColor = <0,0,0>;

These lines just declare some identifiers defining some general values which will be used later in the code. The keyword we use here is `#declare` and it means that we are declaring a global identifier, which will be seen in the whole code.

As you can see, we declare some identifiers to be of float type and others to be of vector type. The vector type identifiers are, in fact, used later for color definition (as their name implies).

The `ImageWidth` and `ImageHeight` define the resolution of the image we are going to render.

Note: this only defines the resolution of the image we are going to render in our SDL (ie. into the array we will define later); it doesn't set the resolution of the image which POV-Ray will render.

The `MaxRecLev` limits the maximum number of recursive reflections the code will calculate. It's equivalent to the `max_trace_level` value in `global_settings` which POV-Ray uses to raytrace.

The `AmbientLight` defines a color which is added to all surfaces. This is used to "lighten up" shadowed parts so that they are not completely dark. It's equivalent to the `ambient_light` value in `global_settings`.

Finally, `BGColor` defines the color of the rays which do not hit anything. It's equivalent to the `background` block of POV-Ray.