Blender provides a number of very interesting settings to complete your renderings with a nice background, and some interesting 'depth' effects. These are accessible via this icon which brings up the World Buttons shown in Figure 1.
The simplest use of the World Buttons is to provide a nice gradient background for images.
The background buttons (Figure 2) allow you to define the colour at the horizon (HoR, HoG, HoB buttons) and the colour at the zenith (ZeR, ZeG, ZeB buttons).
These colours are then interpreted differently, according to which Buttons at the top of Figure 2 are selected:
Blend - The background colour is blended from Horizon to Zenith. If only this button is pressed, then the gradient occurs from bottom to top of the rendered image regardless of camera orientation.
Real - If this button is also pressed the blending is dependent on camera orientation. The Horizon colour is there exactly at the horizon, that is on the x-y plane, and the zenith colour is used for points vertically above and below the camera.
Paper - If this button is pressed the gradient occurs on Zenith-Horizon-Zenith colours, hence there are two transitions, not one, on the image taking into account camera rotation but keeping the horizon colour to the centre and the zenith colour to the extremes.
The World Buttons also provide some texture buttons. Their basic usage is analogous to Materials textures, except for a couple of differences (Figure 3):
There are only 6 texture channels.
Texture mapping - Only has the Object and View options, View being the default orientation.
Affect - Texture just affects colour, but in four different ways: it can affect the Blend channel, making the Horizon colour appear where the texture is non-zero; it can affect the colour of the Horizon, or the colour of the Zenith, up or down (Zen Up, Zen Down)