9.5.2

964 • CHAPTER 18
TEXTURE MAPPING • 965
To get the effect shown in Figure 1, we apply these materials to our object so that the order of the
Texture tags in the Object manager from left to right is: bricks, poster, grafti. See Figure 4 below.
Figure 2.
The grafti is the top material layer; it uses an alpha channel to remove the non-grafti parts, thereby
exposing the next layer down, the poster material. Even though the poster material is on a layer above
the bricks, the bricks can still be seen because the poster material has been scaled down and is not
tiled (i.e. the poster does not cover the entire surface).
The brick material was made using a color channel and a bump channel. The bump channel creates
the illusion that the bricks have joints. Now we want to add a second bump map to the wall without
changing the texture itself. How can this be done?
The powerful texture tag option that makes this possible is called Mix Textures and can be found on
the Tag page in the Attribute manager, for a selected Texture tag.
An additive texture must be to the right of the texture to which it should be
added in the Object manager. All textures to the left of the rst additive texture
— up to but not including the next non-additive texture — are added.
By added, we mean that the material properties are added together, hence the term additive textures.
For example, let’s add two textures that are different colors: red and green; the sum of the colors RGB
100,0,0 (red) and 0,100,0 (green) is 100,100,0 (yellow). If the green color has a brightness of 50%,
then only 50% of the color is added. The result in this case would be 100,50,0 (orange). However,
the result cannot exceed the maximum color values. Adding 100,0,0 to 100,100,0 does not produce
200,100,0, but 100,100,0.
You can add together as many textures as you wish. Only active properties
are evaluated.
Some channels cannot be mixed meaningfully. For example, mixing two materials each with a
refractive index of 1 would result in a material with a refractive index of 2, which is probably not
what you intended. In such cases, the value of the additive (right-most) material is used (provided
that the channel is active).
Only the following are additive: Color, Transparency, Reection, Bump,
Displacement, Luminance.
You can mix multiple textures within the same Texture tag using the Layer
shader or Fusion shader. These shaders include a rich variety of blending
modes, making many effects possible.