Reference:Gradient Pattern

One of the simplest patterns is the gradient pattern. It is specified as

pigment {
  gradient <Orientation>
  [PIGMENT_MODIFIERS...]
  }

where <Orientation> is a vector pointing in the direction that the colors blend. For example

pigment { gradient x } // bands of color vary as you move
                       // along the "x" direction.

produces a series of smooth bands of color that look like layers of colors next to each other. Points at x=0 are the first color in the color map. As the x location increases it smoothly turns to the last color at x=1. Then it starts over with the first again and gradually turns into the last color at x=2. In POV-Ray versions older than 3.5 the pattern reverses for negative values of x. As per POV-Ray 3.5 this is not the case anymore. Using gradient y or gradient z makes the colors blend along the y- or z-axis. Any vector may be used but x, y and z are most common.

As a normal pattern, gradient generates a saw-tooth or ramped wave appearance. The syntax is

normal {
  gradient <Orientation> [, Bump_Size]
  [NORMAL_MODIFIERS...]
  }

where the vector <Orientation> is a required parameter but the float Bump_Size which follows is optional.

Note: The comma is required especially if Bump_Size is negative.

If only the range -1 to 1 was used of the old gradient, for example in a sky_sphere, it can be replaced by the planar or marble pattern and revert the color_map. Also rotate the pattern for other orientations than y. A more general solution is to use function{abs(x)} as a pattern instead of gradient x and similar for gradient y and gradient z.