144 lines
7.2 KiB
Markdown
144 lines
7.2 KiB
Markdown
<h1>
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upng
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<br />
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<!-- NPM Version -->
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<a href="https://www.npmjs.com/package/@pdf-lib/upng">
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<img
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src="https://img.shields.io/npm/v/png-ts.svg?style=flat-square"
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alt="NPM Version"
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/>
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</a>
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</h1>
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> A small, fast and advanced PNG / APNG encoder and decoder
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This project is a fork of [`UPNG.js`](https://github.com/photopea/UPNG.js) and was created for use in [`pdf-lib`](https://github.com/Hopding/pdf-lib). The maintainer of the original repo does not publish it to NPM. That is the primary purpose of this fork. In addition, an `index.d.ts` file has been added (copied directly from [`@types/upng-js`](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/upng-js/index.d.ts)) to makes TypeScript definitions available without requiring additional packages to be installed.
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## Example of `UPNG.toRGBA8`
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```javascript
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// Import the UPNG class
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import UPNG from '@pdf-lib/upng';
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// Create a UPNG object
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const pngImage = UPNG.decode(/* Uint8Array containing bytes of PNG image */);
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// `pixels` is a 1D array (in rgba order) of decoded pixel data
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const pixels = pngImage.UPNG.toRGBA8();
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```
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## Installation
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### NPM Module
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To install the latest stable version:
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```bash
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# With npm
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npm install --save @pdf-lib/upng
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# With yarn
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yarn add @pdf-lib/upng
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```
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This assumes you're using [npm](https://www.npmjs.com/) or [yarn](https://yarnpkg.com/lang/en/) as your package manager.
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### UMD Module
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You can also download `@pdf-lib/upng` as a UMD module from [unpkg](https://unpkg.com/#/). The UMD builds have been compiled to ES5, so they should work [in any modern browser](https://caniuse.com/#feat=es5). UMD builds are useful if you aren't using a package manager or module bundler. For example, you can use them directly in the `<script>` tag of an HTML page.
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The following builds are available:
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* https://unpkg.com/@pdf-lib/upng/dist/UPNG.js
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* https://unpkg.com/@pdf-lib/upng/dist/UPNG.min.js
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When using a UMD build, you will have access to a global `window.UPNG` variable. This variable contains the `UPNG` class exported by `@pdf-lib/upng`. For example:
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```javascript
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// NPM module
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import UPNG from '@pdf-lib/upng';
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const pngImage = UPNG.decode(/* ... */)
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// UMD module
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var pngImage = window.UPNG.decode(/* ... */)
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```
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## Encoder
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UPNG.js supports APNG and the interface expects "frames". Regular PNG is just a single-frame animation (single-item array).
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#### `UPNG.encode(imgs, w, h, cnum, [dels])`
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* `imgs`: array of frames. A frame is an ArrayBuffer containing the pixel data (RGBA, 8 bits per channel)
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* `w`, `h` : width and height of the image
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* `cnum`: number of colors in the result; 0: all colors (lossless PNG)
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* `dels`: array of millisecond delays for each frame (only when 2 or more frames)
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* returns an ArrayBuffer with binary data of a PNG file
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UPNG.js can do a lossy minification of PNG files, similar to [TinyPNG](https://tinypng.com/) and other tools. It performed quantization with [k-means algorithm](https://en.wikipedia.org/wiki/K-means_clustering) in the past, but now we use [K-d trees](https://en.wikipedia.org/wiki/K-d_tree).
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Lossy compression is allowed by the last parameter `cnum`. Set it to zero for a lossless compression, or write the number of allowed colors in the image. Smaller values produce smaller files. **Or just use 0 for lossless / 256 for lossy.**
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// Read RGBA from canvas and encode with UPNG
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var dta = ctx.getImageData(0,0,200,300).data; // ctx is Context2D of a Canvas
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// dta = new Uint8Array(200 * 300 * 4); // or generate pixels manually
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var png = UPNG.encode([dta.buffer], 200, 300, 0); console.log(new Uint8Array(png));
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#### `UPNG.encodeLL(imgs, w, h, cc, ac, depth, [dels])` - low-level encode
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* `imgs`: array of frames. A frame is an ArrayBuffer containing the pixel data (corresponding to following parameters)
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* `w`, `h` : width and height of the image
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* `cc`, `ac`: number of color channels (1 or 3) and alpha channels (0 or 1)
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* `depth`: bit depth of pixel data (1, 2, 4, 8, 16)
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* `dels`: array of millisecond delays for each frame (only when 2 or more frames)
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* returns an ArrayBuffer with binary data of a PNG file
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This function does not do any optimizations, it just stores what you give it. There are two cases when it is useful:
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* saving 16-bit colors (note, that PNG is big-endian, unlike Uint16Array in JS)
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* your image is too large, and "expanding" to 8-bit RGBA would use too much memory (e.g. 4-bit grayscale 50,000 x 50,000 = 1.25 GB, 8-bit RGBA would be 10 GB)
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## Decoder
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Supports all color types (including Grayscale and Palettes), all channel depths (1, 2, 4, 8, 16), interlaced images etc. Opens PNGs which other libraries can not open (tested with [PngSuite](http://www.schaik.com/pngsuite/)).
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#### `UPNG.decode(buffer)`
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* `buffer`: ArrayBuffer containing the PNG file
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* returns an image object with following properties:
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* * `width`: the width of the image
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* * `height`: the height of the image
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* * `depth`: number of bits per channel
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* * `ctype`: color type of the file (Truecolor, Grayscale, Palette ...)
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* * `frames`: additional info about frames (frame delays etc.)
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* * `tabs`: additional chunks of the PNG file
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* * `data`: pixel data of the image
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PNG files may have a various number of channels and a various color depth. The interpretation of `data` depends on the current color type and color depth (see the [PNG specification](https://www.w3.org/TR/PNG/)).
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#### `UPNG.toRGBA8(img)`
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* `img`: PNG image object (returned by UPNG.decode())
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* returns an array of frames. A frame is ArrayBuffer of the image in RGBA format, 8 bits per channel.
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### Example
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var img = UPNG.decode(buff); // put ArrayBuffer of the PNG file into UPNG.decode
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var rgba = UPNG.toRGBA8(img)[0]; // UPNG.toRGBA8 returns array of frames, size: width * height * 4 bytes.
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PNG format uses the Inflate algorithm. Right now, UPNG.js calls [Pako.js](https://github.com/nodeca/pako) for the Inflate and Deflate method.
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## Quantizer
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UPNG.js contains a very good Quantizer of 4-component 8-bit vectors (i.e. pixels). It can be used to generate nice color palettes (e.g. Photopea uses UPNG.js to make palettes for GIF images).
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Quantization consists of two important steps: Finding a nice palette and Finding the closest color in the palette for each sample (non-trivial for large palettes). UPNG perfroms both steps.
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var res = UPNG.quantize(data, psize);
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* `data`: ArrayBuffer of samples (byte length is a multiple of four)
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* `psize` : Palette size (how many colors you want to have)
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The result object "res" has following properties:
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* `abuf`: ArrayBuffer corresponding to `data`, where colors are remapped by a palette
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* `inds`: Uint8Array : the index of a color for each sample (only when `psize`<=256)
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* `plte`: Array : the Palette - a list of colors, `plte[i].est.q` and `plte[i].est.rgba` is the color value
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### FAQ
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- To get one common palette for multiple images (e.g. frames of the animation), concatenate them into one array `data`.
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- When working with less than four components, set the remaining components to a constant value (e.g. to zero)
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- When working with transparency, premultiply color components by transparency (otherwise, rgba(1,1,1,0) would be closer to rgba(1,1,1,1) than to rgba(0,0,0,0) - transparent mapped to white instead of transparent)
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