Image Offset Convert

java

//offset to (x, y)
int y = i / width;
int x = i - width*y;

//(x, y) to offset
int i = x + (y * w);

js:

var x = i % width,
    y = ~~( i / width );

int value = pixmap.getPixel(x, y);
int R = ((value & 0xff000000) >>> 24);
int G = ((value & 0x00ff0000) >>> 16);
int B = ((value & 0x0000ff00) >>> 8);
int A = ((value & 0x000000ff));

common stuff:

//remap [0.0 .. 1.0] to [-1.0 .. 1.0]
N * 2 - 1

//remap [-1.0 .. 1.0] to [0.0 .. 1.0]
N / 2 + 0.5

//remap [0.0 .. 1.0] to [1.0 .. 0.0 .. 1.0] 
abs(N * 2 - 1)

// index to (x, y)
const x = Math.floor(index % columns);
const y = Math.floor(index / columns);

//(x, y) to offset
const index = x + (y * columns);