expo-image
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A cross-platform, performant image component for React Native and Expo with Web support
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text/typescript
// Code copied and slightly modified from https://github.com/evanw/thumbhash
/**
* Encodes an RGBA image to a ThumbHash. RGB should not be premultiplied by A.
*
* @param w The width of the input image. Must be ≤100px.
* @param h The height of the input image. Must be ≤100px.
* @param rgba The pixels in the input image, row-by-row. Must have w*h*4 elements.
* @returns The ThumbHash as a Uint8Array.
*/
export function rgbaToThumbHash(w: number, h: number, rgba: Uint8Array) {
// Encoding an image larger than 100x100 is slow with no benefit
if (w > 100 || h > 100) throw new Error(`${w}x${h} doesn't fit in 100x100`);
const { PI, round, max, cos, abs } = Math;
// Determine the average color
let avg_r = 0,
avg_g = 0,
avg_b = 0,
avg_a = 0;
for (let i = 0, j = 0; i < w * h; i++, j += 4) {
const alpha = rgba[j + 3] / 255;
avg_r += (alpha / 255) * rgba[j];
avg_g += (alpha / 255) * rgba[j + 1];
avg_b += (alpha / 255) * rgba[j + 2];
avg_a += alpha;
}
if (avg_a) {
avg_r /= avg_a;
avg_g /= avg_a;
avg_b /= avg_a;
}
const hasAlpha = avg_a < w * h;
const l_limit = hasAlpha ? 5 : 7; // Use fewer luminance bits if there's alpha
const lx = max(1, round((l_limit * w) / max(w, h)));
const ly = max(1, round((l_limit * h) / max(w, h)));
const l: number[] = []; // luminance
const p: number[] = []; // yellow - blue
const q: number[] = []; // red - green
const a: number[] = []; // alpha
// Convert the image from RGBA to LPQA (composite atop the average color)
for (let i = 0, j = 0; i < w * h; i++, j += 4) {
const alpha = rgba[j + 3] / 255;
const r = avg_r * (1 - alpha) + (alpha / 255) * rgba[j];
const g = avg_g * (1 - alpha) + (alpha / 255) * rgba[j + 1];
const b = avg_b * (1 - alpha) + (alpha / 255) * rgba[j + 2];
l[i] = (r + g + b) / 3;
p[i] = (r + g) / 2 - b;
q[i] = r - g;
a[i] = alpha;
}
// Encode using the DCT into DC (constant) and normalized AC (varying) terms
const encodeChannel = (channel: number[], nx: number, ny: number) => {
let dc = 0;
const ac: number[] = [];
let scale = 0;
const fx: number[] = [];
for (let cy = 0; cy < ny; cy++) {
for (let cx = 0; cx * ny < nx * (ny - cy); cx++) {
let f = 0;
for (let x = 0; x < w; x++) fx[x] = cos((PI / w) * cx * (x + 0.5));
for (let y = 0; y < h; y++)
for (let x = 0, fy = cos((PI / h) * cy * (y + 0.5)); x < w; x++)
f += channel[x + y * w] * fx[x] * fy;
f /= w * h;
if (cx || cy) {
ac.push(f);
scale = max(scale, abs(f));
} else {
dc = f;
}
}
}
if (scale) for (let i = 0; i < ac.length; i++) ac[i] = 0.5 + (0.5 / scale) * ac[i];
return [dc, ac, scale];
};
const [l_dc, l_ac, l_scale] = encodeChannel(l, max(3, lx), max(3, ly));
const [p_dc, p_ac, p_scale] = encodeChannel(p, 3, 3);
const [q_dc, q_ac, q_scale] = encodeChannel(q, 3, 3);
const [a_dc, a_ac, a_scale] = hasAlpha ? encodeChannel(a, 5, 5) : [];
// Write the constants
const isLandscape = w > h;
const header24 =
round(63 * (l_dc as number)) |
(round(31.5 + 31.5 * (p_dc as number)) << 6) |
(round(31.5 + 31.5 * (q_dc as number)) << 12) |
(round(31 * (l_scale as number)) << 18) |
((hasAlpha ? 1 : 0) << 23);
const header16 =
(isLandscape ? ly : lx) |
(round(63 * (p_scale as number)) << 3) |
(round(63 * (q_scale as number)) << 9) |
((isLandscape ? 1 : 0) << 15);
const hash = [
header24 & 255,
(header24 >> 8) & 255,
header24 >> 16,
header16 & 255,
header16 >> 8,
];
const ac_start = hasAlpha ? 6 : 5;
let ac_index = 0;
if (hasAlpha) hash.push(round(15 * (a_dc as number)) | (round(15 * (a_scale as number)) << 4));
// Write the varying factors
for (const ac of hasAlpha ? [l_ac, p_ac, q_ac, a_ac] : [l_ac, p_ac, q_ac])
for (const f of ac as number[])
hash[ac_start + (ac_index >> 1)] |= round(15 * f) << ((ac_index++ & 1) << 2);
return new Uint8Array(hash);
}
/**
* Decodes a ThumbHash to an RGBA image. RGB is not be premultiplied by A.
*
* @param hash The bytes of the ThumbHash.
* @returns The width, height, and pixels of the rendered placeholder image.
*/
export function thumbHashToRGBA(hash: Uint8Array) {
const { PI, min, max, cos, round } = Math;
// Read the constants
const header24 = hash[0] | (hash[1] << 8) | (hash[2] << 16);
const header16 = hash[3] | (hash[4] << 8);
const l_dc = (header24 & 63) / 63;
const p_dc = ((header24 >> 6) & 63) / 31.5 - 1;
const q_dc = ((header24 >> 12) & 63) / 31.5 - 1;
const l_scale = ((header24 >> 18) & 31) / 31;
const hasAlpha = header24 >> 23;
const p_scale = ((header16 >> 3) & 63) / 63;
const q_scale = ((header16 >> 9) & 63) / 63;
const isLandscape = header16 >> 15;
const lx = max(3, isLandscape ? (hasAlpha ? 5 : 7) : header16 & 7);
const ly = max(3, isLandscape ? header16 & 7 : hasAlpha ? 5 : 7);
const a_dc = hasAlpha ? (hash[5] & 15) / 15 : 1;
const a_scale = (hash[5] >> 4) / 15;
// Read the varying factors (boost saturation by 1.25x to compensate for quantization)
const ac_start = hasAlpha ? 6 : 5;
let ac_index = 0;
const decodeChannel = (nx: number, ny: number, scale: number) => {
const ac: number[] = [];
for (let cy = 0; cy < ny; cy++)
for (let cx = cy ? 0 : 1; cx * ny < nx * (ny - cy); cx++)
ac.push(
(((hash[ac_start + (ac_index >> 1)] >> ((ac_index++ & 1) << 2)) & 15) / 7.5 - 1) * scale
);
return ac;
};
const l_ac = decodeChannel(lx, ly, l_scale);
const p_ac = decodeChannel(3, 3, p_scale * 1.25);
const q_ac = decodeChannel(3, 3, q_scale * 1.25);
const a_ac = hasAlpha ? decodeChannel(5, 5, a_scale) : null;
// Decode using the DCT into RGB
const ratio = thumbHashToApproximateAspectRatio(hash);
const w = round(ratio > 1 ? 32 : 32 * ratio);
const h = round(ratio > 1 ? 32 / ratio : 32);
const rgba = new Uint8Array(w * h * 4),
fx: number[] = [],
fy: number[] = [];
for (let y = 0, i = 0; y < h; y++) {
for (let x = 0; x < w; x++, i += 4) {
let l = l_dc,
p = p_dc,
q = q_dc,
a = a_dc;
// Precompute the coefficients
for (let cx = 0, n = max(lx, hasAlpha ? 5 : 3); cx < n; cx++)
fx[cx] = cos((PI / w) * (x + 0.5) * cx);
for (let cy = 0, n = max(ly, hasAlpha ? 5 : 3); cy < n; cy++)
fy[cy] = cos((PI / h) * (y + 0.5) * cy);
// Decode L
for (let cy = 0, j = 0; cy < ly; cy++)
for (let cx = cy ? 0 : 1, fy2 = fy[cy] * 2; cx * ly < lx * (ly - cy); cx++, j++)
l += l_ac[j] * fx[cx] * fy2;
// Decode P and Q
for (let cy = 0, j = 0; cy < 3; cy++) {
for (let cx = cy ? 0 : 1, fy2 = fy[cy] * 2; cx < 3 - cy; cx++, j++) {
const f = fx[cx] * fy2;
p += p_ac[j] * f;
q += q_ac[j] * f;
}
}
// Decode A
if (hasAlpha)
for (let cy = 0, j = 0; cy < 5; cy++)
for (let cx = cy ? 0 : 1, fy2 = fy[cy] * 2; cx < 5 - cy; cx++, j++)
a += a_ac![j] * fx[cx] * fy2;
// Convert to RGB
const b = l - (2 / 3) * p;
const r = (3 * l - b + q) / 2;
const g = r - q;
rgba[i] = max(0, 255 * min(1, r));
rgba[i + 1] = max(0, 255 * min(1, g));
rgba[i + 2] = max(0, 255 * min(1, b));
rgba[i + 3] = max(0, 255 * min(1, a));
}
}
return { w, h, rgba };
}
/**
* Extracts the average color from a ThumbHash. RGB is not be premultiplied by A.
*
* @param hash The bytes of the ThumbHash.
* @returns The RGBA values for the average color. Each value ranges from 0 to 1.
*/
export function thumbHashToAverageRGBA(hash: Uint8Array) {
const { min, max } = Math;
const header = hash[0] | (hash[1] << 8) | (hash[2] << 16);
const l = (header & 63) / 63;
const p = ((header >> 6) & 63) / 31.5 - 1;
const q = ((header >> 12) & 63) / 31.5 - 1;
const hasAlpha = header >> 23;
const a = hasAlpha ? (hash[5] & 15) / 15 : 1;
const b = l - (2 / 3) * p;
const r = (3 * l - b + q) / 2;
const g = r - q;
return {
r: max(0, min(1, r)),
g: max(0, min(1, g)),
b: max(0, min(1, b)),
a,
};
}
/**
* Extracts the approximate aspect ratio of the original image.
*
* @param hash The bytes of the ThumbHash.
* @returns The approximate aspect ratio (i.e. width / height).
*/
export function thumbHashToApproximateAspectRatio(hash: Uint8Array) {
const header = hash[3];
const hasAlpha = hash[2] & 0x80;
const isLandscape = hash[4] & 0x80;
const lx = isLandscape ? (hasAlpha ? 5 : 7) : header & 7;
const ly = isLandscape ? header & 7 : hasAlpha ? 5 : 7;
return lx / ly;
}
/**
* Encodes an RGBA image to a PNG data URL. RGB should not be premultiplied by
* A. This is optimized for speed and simplicity and does not optimize for size
* at all. This doesn't do any compression (all values are stored uncompressed).
*
* @param w The width of the input image. Must be ≤100px.
* @param h The height of the input image. Must be ≤100px.
* @param rgba The pixels in the input image, row-by-row. Must have w*h*4 elements.
* @returns A data URL containing a PNG for the input image.
*/
export function rgbaToDataURL(w: number, h: number, rgba: Uint8Array) {
const row = w * 4 + 1;
const idat = 6 + h * (5 + row);
const bytes = [
137,
80,
78,
71,
13,
10,
26,
10,
0,
0,
0,
13,
73,
72,
68,
82,
0,
0,
w >> 8,
w & 255,
0,
0,
h >> 8,
h & 255,
8,
6,
0,
0,
0,
0,
0,
0,
0,
idat >>> 24,
(idat >> 16) & 255,
(idat >> 8) & 255,
idat & 255,
73,
68,
65,
84,
120,
1,
];
const table = [
0, 498536548, 997073096, 651767980, 1994146192, 1802195444, 1303535960, 1342533948, -306674912,
-267414716, -690576408, -882789492, -1687895376, -2032938284, -1609899400, -1111625188,
];
let a = 1,
b = 0;
for (let y = 0, i = 0, end = row - 1; y < h; y++, end += row - 1) {
bytes.push(y + 1 < h ? 0 : 1, row & 255, row >> 8, ~row & 255, (row >> 8) ^ 255, 0);
for (b = (b + a) % 65521; i < end; i++) {
const u = rgba[i] & 255;
bytes.push(u);
a = (a + u) % 65521;
b = (b + a) % 65521;
}
}
bytes.push(
b >> 8,
b & 255,
a >> 8,
a & 255,
0,
0,
0,
0,
0,
0,
0,
0,
73,
69,
78,
68,
174,
66,
96,
130
);
for (let [start, end] of [
[12, 29],
[37, 41 + idat],
]) {
let c = ~0;
for (let i = start; i < end; i++) {
c ^= bytes[i];
c = (c >>> 4) ^ table[c & 15];
c = (c >>> 4) ^ table[c & 15];
}
c = ~c;
bytes[end++] = c >>> 24;
bytes[end++] = (c >> 16) & 255;
bytes[end++] = (c >> 8) & 255;
bytes[end++] = c & 255;
}
return 'data:image/png;base64,' + btoa(String.fromCharCode(...bytes));
}
/**
* Decodes a ThumbHash to a PNG data URL. This is a convenience function that
* just calls "thumbHashToRGBA" followed by "rgbaToDataURL".
*
* @param hash The bytes of the ThumbHash.
* @returns A data URL containing a PNG for the rendered ThumbHash.
*/
export function thumbHashToDataURL(hash: Uint8Array): string {
const image = thumbHashToRGBA(hash);
return rgbaToDataURL(image.w, image.h, image.rgba);
}
/**
* Convenience function added to the original thumbhash code, allows generating a thumbhash image directly from
* thumbhash string.
* @param thumbhash string from which thumbhashDataURL should be generated
* @returns A data URL containing a PNG for the rendered ThumbHash
*/
export function thumbHashStringToDataURL(thumbhash: string): string {
const hash = Uint8Array.from(atob(thumbhash), (c) => c.charCodeAt(0));
return thumbHashToDataURL(hash);
}