Graphics Reference
In-Depth Information
Although computing noise in the fragment shader is prohibitively
expensive, we can work around the problem using a 3D texture. It is
possible to easily produce acceptable-quality noise by precomputing
the noise and placing the results in a 3D texture. A number of
algorithms can be used to generate noise. The list of references and
links described at the end of this chapter can be used to obtain more
information about the various noise algorithms. Here, we discuss a
specific algorithm that generates a lattice-based gradient noise. Ken
Perlin's noise function (Perlin, 1985) is a lattice-based gradient noise and
a widely used method for generating noise. For example, a lattice-based
gradient noise is implemented by the
noise
function in the Renderman
shading language.
The gradient noise algorithm takes a 3D coordinate as input and returns
a floating-point noise value. To generate this noise value given an input
(
x
,
y
, z), we map the
x
,
y
, and
z
values to appropriate integer locations
in a lattice. The number of cells in a lattice is programmable and for our
implementation is set to 256 cells. For each cell in the lattice, we need
to generate and store a pseudorandom gradient vector. Example 14-15
describes how these gradient vectors are generated.
Example 14-15
Generating Gradient Vectors
// permTable describes a random permutation of
// 8−bit values from 0 to 255
static unsigned char permTable[256] = {
0xE1, 0x9B, 0xD2, 0x6C, 0xAF, 0xC7, 0xDD, 0x90,
0xCB, 0x74, 0x46, 0xD5, 0x45, 0x9E, 0x21, 0xFC,
0x05, 0x52, 0xAD, 0x85, 0xDE, 0x8B, 0xAE, 0x1B,
0x09, 0x47, 0x5A, 0xF6, 0x4B, 0x82, 0x5B, 0xBF,
0xA9, 0x8A, 0x02, 0x97, 0xC2, 0xEB, 0x51, 0x07,
0x19, 0x71, 0xE4, 0x9F, 0xCD, 0xFD, 0x86, 0x8E,
0xF8, 0x41, 0xE0, 0xD9, 0x16, 0x79, 0xE5, 0x3F,
0x59, 0x67, 0x60, 0x68, 0x9C, 0x11, 0xC9, 0x81,
0x24, 0x08, 0xA5, 0x6E, 0xED, 0x75, 0xE7, 0x38,
0x84, 0xD3, 0x98, 0x14, 0xB5, 0x6F, 0xEF, 0xDA,
0xAA, 0xA3, 0x33, 0xAC, 0x9D, 0x2F, 0x50, 0xD4,
0xB0, 0xFA, 0x57, 0x31, 0x63, 0xF2, 0x88, 0xBD,
0xA2, 0x73, 0x2C, 0x2B, 0x7C, 0x5E, 0x96, 0x10,
0x8D, 0xF7, 0x20, 0x0A, 0xC6, 0xDF, 0xFF, 0x48,
0x35, 0x83, 0x54, 0x39, 0xDC, 0xC5, 0x3A, 0x32,
0xD0, 0x0B, 0xF1, 0x1C, 0x03, 0xC0, 0x3E, 0xCA,
0x12, 0xD7, 0x99, 0x18, 0x4C, 0x29, 0x0F, 0xB3,
0x27, 0x2E, 0x37, 0x06, 0x80, 0xA7, 0x17, 0xBC,
0x6A, 0x22, 0xBB, 0x8C, 0xA4, 0x49, 0x70, 0xB6,
