Reference:Bitmap Modifiers

A bitmap modifier is a modifier used inside an image_map, bump_map or material_map to specify how the 2-D bitmap is to be applied to the 3-D surface. Several bitmap modifiers apply to specific kinds of maps and they are covered in the appropriate sections. The bitmap modifiers discussed in the following sections are applicable to all three types of bitmaps.

The once Option

Normally there are an infinite number of repeating image maps, bump maps or material maps created over every unit square of the x-y-plane like tiles. By adding the once keyword after a file name you can eliminate all other copies of the map except the one at (0,0) to (1,1). In image maps, areas outside this unit square are treated as fully transparent. In bump maps, areas outside this unit square are left flat with no normal modification. In material maps, areas outside this unit square are textured with the first texture of the texture list.

For example:

image_map {
  gif "mypic.gif"
  once
  }

The map_type Option

The default projection of the image onto the x-y-plane is called a planar map type. This option may be changed by adding the map_type keyword followed by an integer number specifying the way to wrap the image around the object.

A map_type 0 gives the default planar mapping already described.

A map_type 1 gives a spherical mapping. It assumes that the object is a sphere of any size sitting at the origin. The y-axis is the north/south pole of the spherical mapping. The top and bottom edges of the image just touch the pole regardless of any scaling. The left edge of the image begins at the positive x-axis and wraps the image around the sphere from west to east in a -y-rotation. The image covers the sphere exactly once. The once keyword has no meaning for this mapping type.

With map_type 2 you get a cylindrical mapping. It assumes that a cylinder of any diameter lies along the y-axis. The image wraps around the cylinder just like the spherical map but the image remains one unit tall from y=0 to y=1. This band of color is repeated at all heights unless the once keyword is applied.

Finally map_type 5 is a torus or donut shaped mapping. It assumes that a torus of major radius one sits at the origin in the x-z-plane. The image is wrapped around similar to spherical or cylindrical maps. However the top and bottom edges of the map wrap over and under the torus where they meet each other on the inner rim.

Types 3 and 4 are still under development.

Note: The map_type option may also be applied to bump_map and material_map statements.

For example:

sphere{<0,0,0>,1
  pigment{
    image_map {
      gif "world.gif"
      map_type 1
      }
    }
  }

The interpolate Option

Adding the interpolate keyword can smooth the jagged look of a bitmap. When POV-Ray checks a color for an image map or a bump amount for a bump map, it often checks a point that is not directly on top of one pixel but sort of between several differently colored pixels. Interpolations return an in-between value so that the steps between the pixels in the map will look smoother.

Although interpolate is legal in material maps, the color index is interpolated before the texture is chosen. It does not interpolate the final color as you might hope it would. In general, interpolation of material maps serves no useful purpose but this may be fixed in future versions.

There are currently three types of interpolation: interpolate 2 gives bilinear interpolation, interpolate 3 gives bicubic, and interpolate 4 gives normalized distance.

For example:

image_map {
  gif "mypic.gif"
  interpolate 2
  }

The default is no interpolation. Normalized distance is the slowest, bilinear does a better job of picking the between color, and arguably, bicubic interpolation is a slight improvement, however it is subject to over-sharpening at some color borders. Normally bilinear is used.

If your map looks jagged, try using interpolation instead of going to a higher resolution image. The results can be very good.