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idct.js
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idct.js
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/**
* Constant used in IDCT
*/
const INV_SQRT2 = 1 / Math.SQRT2;
/**
* Precalcuated values for speeding up IDCT
*/
let idctCache;
/**
* 'C' function defined in the IDCT operation
*
* @param {*} val
*/
function C(val) {
return (val === 0 ? INV_SQRT2 : 1);
}
/**
* Implement the Inverse Discrete Cosine Transformation defined in the JPEG spec.
* Naïve implementation that does not use caching, so it's slow.
*
* Returns an array of the IDCT'ed coefficients
*
* @param {*} coeffs Array of 64 coefficients to be processed
*/
function idct(coeffs) {
let ret = new Array(coeffs.length);
for (let y = 0, ystart = 0; y < DATA_UNIT_SIZE; y++, ystart += DATA_UNIT_SIZE) {
for (let x = 0; x < DATA_UNIT_SIZE; x++) {
let sum = 0;
for (let v = 0, vstart = 0; v < DATA_UNIT_SIZE; v++, vstart += DATA_UNIT_SIZE) {
for (let u = 0; u < DATA_UNIT_SIZE; u++) {
sum += C(u) * C(v) * coeffs[vstart + u]
* Math.cos((((2*x)+1) * u * Math.PI) / 16)
* Math.cos((((2*y)+1) * v * Math.PI) / 16);
}
}
ret[ystart + x] = Math.round(sum / 4);
}
}
return ret;
}
/**
* Initialized cache of precomputed values for IDCT
*/
function initIDCTCache() {
idctCache = [
[], [], [], [], [], [], [], [],
];
for (let j = 0; j < DATA_UNIT_SIZE; j++) {
for (let i = 0; i < DATA_UNIT_SIZE; i++) {
idctCache[i][j] = Math.cos((((2*i)+1) * j * Math.PI) / 16);
}
}
}
/**
* Implement the Inverse Discrete Cosine Transformation defined in the JPEG spec.
*
* Returns an array of the IDCT'ed coefficients
*
* @param {*} coeffs Array of 64 coefficients to be processed
*/
function idctCached(coeffs) {
if (idctCache === undefined) {
initIDCTCache();
}
let ret = new Array(coeffs.length);
for (let y = 0, ystart = 0; y < DATA_UNIT_SIZE; y++, ystart += DATA_UNIT_SIZE) {
for (let x = 0; x < DATA_UNIT_SIZE; x++) {
let sum = 0;
for (let v = 0, vstart = 0; v < DATA_UNIT_SIZE; v++, vstart += DATA_UNIT_SIZE) {
for (let u = 0; u < DATA_UNIT_SIZE; u++) {
sum += C(u) * C(v) * coeffs[vstart + u] * idctCache[x][u] * idctCache[y][v];
}
}
ret[ystart + x] = Math.round(sum / 4);
}
}
return ret;
}
/**
* Integer IDCT decoder ported from https://web.ece.ucsb.edu/EXPRESS/benchmark/mpeg2enc/idctcol.c
* with help from NanoJPEG: http://svn.emphy.de/nanojpeg/trunk/nanojpeg/nanojpeg.c
*
* Returns an array of the IDCT'ed coefficients
*
* @param {*} coeffs Array of 64 coefficients to be processed
*/
function idctChenWang(coeffs) {
for (let row = 0; row < 64; row += 8) {
idctrow(coeffs, row);
}
for (let col = 0; col < 8; col++) {
idctcol(coeffs, col)
}
return coeffs;
}
const W1 = 2841 // 2048*sqrt(2)*cos(1*pi/16)
const W2 = 2676 // 2048*sqrt(2)*cos(2*pi/16)
const W3 = 2408 // 2048*sqrt(2)*cos(3*pi/16)
const W5 = 1609 // 2048*sqrt(2)*cos(5*pi/16)
const W6 = 1108 // 2048*sqrt(2)*cos(6*pi/16)
const W7 = 565 // 2048*sqrt(2)*cos(7*pi/16)
/**
* Implement 1-D IDCT on each row of the block
*
* @param {*} blk
* @param {*} idx
*/
function idctrow(blk, idx) {
let x0, x1, x2, x3, x4, x5, x6, x7, x8;
// Shortcut
if (!((x1 = blk[4 + idx] << 11)
| (x2 = blk[6 + idx])
| (x3 = blk[2 + idx])
| (x4 = blk[1 + idx])
| (x5 = blk[7 + idx])
| (x6 = blk[5 + idx])
| (x7 = blk[3 + idx]))) {
let tmp = blk[0 + idx] << 3;
let rowend = idx + 8;
for (let i = idx; i < rowend; i++) {
blk[i] = tmp;
}
return;
}
x0 = (blk[0 + idx] << 11) + 128;
/* first stage */
x8 = W7*(x4+x5);
x4 = x8 + (W1-W7)*x4;
x5 = x8 - (W1+W7)*x5;
x8 = W3*(x6+x7);
x6 = x8 - (W3-W5)*x6;
x7 = x8 - (W3+W5)*x7;
/* second stage */
x8 = x0 + x1;
x0 -= x1;
x1 = W6*(x3+x2);
x2 = x1 - (W2+W6)*x2;
x3 = x1 + (W2-W6)*x3;
x1 = x4 + x6;
x4 -= x6;
x6 = x5 + x7;
x5 -= x7;
/* third stage */
x7 = x8 + x3;
x8 -= x3;
x3 = x0 + x2;
x0 -= x2;
x2 = (181*(x4+x5)+128)>>8;
x4 = (181*(x4-x5)+128)>>8;
/* fourth stage */
blk[0 + idx] = (x7+x1)>>8;
blk[1 + idx] = (x3+x2)>>8;
blk[2 + idx] = (x0+x4)>>8;
blk[3 + idx] = (x8+x6)>>8;
blk[4 + idx] = (x8-x6)>>8;
blk[5 + idx] = (x0-x4)>>8;
blk[6 + idx] = (x3-x2)>>8;
blk[7 + idx] = (x7-x1)>>8;
}
/**
* Implement 1-D IDCT on each column of the block
*
* @param {*} blk
* @param {*} idx
*/
function idctcol(blk, idx) {
let x0, x1, x2, x3, x4, x5, x6, x7, x8;
/* shortcut */
if (!((x1 = blk[idx+8*4]<<8)
| (x2 = blk[idx+8*6])
| (x3 = blk[idx+8*2])
| (x4 = blk[idx+8*1])
| (x5 = blk[idx+8*7])
| (x6 = blk[idx+8*5])
| (x7 = blk[idx+8*3]))) {
let tmp = clampIdct((blk[idx+8*0]+32)>>6);
for (let i = 0; i < 8; i++) {
blk[idx+8*i] = tmp;
}
return;
}
x0 = (blk[idx+8*0]<<8) + 8192;
/* first stage */
x8 = W7*(x4+x5) + 4;
x4 = (x8+(W1-W7)*x4)>>3;
x5 = (x8-(W1+W7)*x5)>>3;
x8 = W3*(x6+x7) + 4;
x6 = (x8-(W3-W5)*x6)>>3;
x7 = (x8-(W3+W5)*x7)>>3;
/* second stage */
x8 = x0 + x1;
x0 -= x1;
x1 = W6*(x3+x2) + 4;
x2 = (x1-(W2+W6)*x2)>>3;
x3 = (x1+(W2-W6)*x3)>>3;
x1 = x4 + x6;
x4 -= x6;
x6 = x5 + x7;
x5 -= x7;
/* third stage */
x7 = x8 + x3;
x8 -= x3;
x3 = x0 + x2;
x0 -= x2;
x2 = (181*(x4+x5)+128)>>8;
x4 = (181*(x4-x5)+128)>>8;
/* fourth stage */
blk[idx+8*0] = clampIdct((x7+x1)>>14);
blk[idx+8*1] = clampIdct((x3+x2)>>14);
blk[idx+8*2] = clampIdct((x0+x4)>>14);
blk[idx+8*3] = clampIdct((x8+x6)>>14);
blk[idx+8*4] = clampIdct((x8-x6)>>14);
blk[idx+8*5] = clampIdct((x0-x4)>>14);
blk[idx+8*6] = clampIdct((x3-x2)>>14);
blk[idx+8*7] = clampIdct((x7-x1)>>14);
}
/**
* Clamp an integer value to the range [-128 - 127]
*
* @param {*} val
*/
function clampIdct(val) {
return (val > 127 ? 127 : (val < -128 ? -128 : val));
}