# yulicai/xDaysOfMaking

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# day 008

Move mouse to slightly change the color and movement.

icy cloud,dark sky; single boat, floating away, hilly-gully passing by, to the heart of river, down the roaring of waves, wind coming around, merchants greetings to each other, thousands of boats. lonely bird, ethereal singing, hollow nightfall .

## learned today

Jan 26th 2017

Fractal Brownian Motion(fBm) Section in the book of shaders

waves adding up, either sin waves or noise waves
this technique is usually used for describing natural mountain, landscapes, cloud etc.
"This technique is commonly used to construct procedural landscapes. The self-similarity of the fBm is perfect for mountains, because the erosion processes that create mountains work in a manner that yields this kind of self-similarity across a large range of scales."

### main constructors

1. octaves : iterations of waves
2. lacunarity: frequency in regular steps, 地表凹凸起伏纹理指数
3. gain: amplitude of the noise
``````
// Properties
const int octaves = 8;
float lacunarity = 4.0;
float gain = 0.5;
//
// Initial values
float amplitude = 0.5;
float frequency = 1.;
//
// Loop of octaves
for (int i = 0; i < octaves; i++) {
y += amplitude * noise ( frequency * x );
frequency * = lacunarity;
amplitude * = gain;
}
``````
1. turbulence: It's essentially an fBm, but constructed from the absolute value of a signed noise to create sharp valleys in the function. ```for (int i = 0; i < OCTAVES; i++) { value += amplitude * abs(snoise(st)); st * = 2.; amplitud * = .5; } ```

### Domain Warping

mainly based on the function by Inigo Quiles to to use fBm to warp a space of a fBm, so thus it has cloud or space warping effect

``````
#define NUM_OCTAVES 23
float fbm ( in vec2 _st) {
float v = 0.0;
float a = 0.5;
vec2 shift = vec2(100.0);
// Rotate to reduce axial bias
mat2 rot = mat2(cos(0.5), sin(0.5),
-sin(0.5), cos(0.50));
for (int i = 0; i < NUM_OCTAVES; ++i) {
v += a * noise(_st);
_st = rot * _st * 2.0 + shift;
a *= 0.5;
}
return v;
}
``````

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