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Decoder for webp #50

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Jun 14, 2018
Merged

Decoder for webp #50

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Decoder for webp
surma Jun 13, 2018
91945da
Add documentation
surma Jun 13, 2018
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2 changes: 2 additions & 0 deletions codecs/webp_dec/.gitignore
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/*.wasm
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Weren't we going to start checking the wasm in?

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Yeah, that’s currently only in the testing branch. I’ll move that to master once this lands.

/*.js
42 changes: 42 additions & 0 deletions codecs/webp_dec/README.md
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# WebP decoder

- Source: <https://github.com/webmproject/libwebp>
- Version: v0.6.1

## Example

See `example.html`

## API

### `int version()`

Returns the version of libwebp as a number. va.b.c is encoded as 0x0a0b0c

### `uint8_t* create_buffer(int size)`

Allocates an buffer for the file data.
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"a buffer"


### `void destroy_buffer(uint8_t* p)`

Frees a buffer created with `create_buffer`.

### `void decode(uint8_t* img_in, int size)`

Decodes the given webp file into raw RGBA. The result is implicitly stored and can be accessed using the `get_result_*()` functions.

### `void free_result()`

Frees the result created by `decode()`.

### `int get_result_pointer()`

Returns the pointer to the start of the buffer holding the encoded data. Length is width x height x 4 bytes.

### `int get_result_width()`

Returns the width of the image.

### `int get_result_height()`

Returns the height of the image.
44 changes: 44 additions & 0 deletions codecs/webp_dec/example.html
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<!doctype html>
<script src='webp_dec.js'></script>
<script>
const Module = webp_dec();
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I'm not going to review this file in terms of project types, but isn't this not-a-constructor, so should start with a lower-case?

The reason I ask, is I saw @developit doing a similar thing in the options PR. I'm worried I've missed out on some change to consensus I always thought it was:

ThisIsAClass
THIS_IS_A_PRIMITIVE_CONSTANT
thisIsEverythingElse

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It’s mostly this way ’cause that’s how all the Emscripten code examples are written. By default, Emscripten glue creates a global called Module. We set a flag so that instead it registers a function that returns that Module, hence the naming. It’s nbd to change that, tho

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I don't mind so much for this example. I'd want to rename it if it was part of the main JS though. Was mostly just curious.


async function loadFile(src) {
const resp = await fetch(src);
return await resp.arrayBuffer();
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Just return here is fine. The await is redundant.

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Oh cool. I thought it would make the stack trace look different, but it doesn’t. TIL.

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https://jakearchibald.com/2017/await-vs-return-vs-return-await/ 😄

}

Module.onRuntimeInitialized = async _ => {
const api = {
version: Module.cwrap('version', 'number', []),
create_buffer: Module.cwrap('create_buffer', 'number', ['number', 'number']),
destroy_buffer: Module.cwrap('destroy_buffer', '', ['number']),
decode: Module.cwrap('decode', '', ['number', 'number']),
free_result: Module.cwrap('free_result', '', ['number']),
get_result_pointer: Module.cwrap('get_result_pointer', 'number', []),
get_result_width: Module.cwrap('get_result_width', 'number', []),
get_result_height: Module.cwrap('get_result_height', 'number', []),
};
const image = await loadFile('../example.webp');
const p = api.create_buffer(image.byteLength);
Module.HEAP8.set(new Uint8Array(image), p);
api.decode(p, image.byteLength);
const resultPointer = api.get_result_pointer();
if(resultPointer === 0) {
throw new Error("Could not decode image");
}
const resultWidth = api.get_result_width();
const resultHeight = api.get_result_height();
const resultView = new Uint8Array(Module.HEAP8.buffer, resultPointer, resultWidth * resultHeight * 4);
const result = new Uint8ClampedArray(resultView);
const imageData = new ImageData(result, resultWidth, resultHeight);
api.free_result(resultPointer);
api.destroy_buffer(p);
const canvas = document.createElement('canvas');
canvas.width = resultWidth;
canvas.height = resultHeight;
document.body.appendChild(canvas);
const ctx = canvas.getContext('2d');
ctx.putImageData(imageData, 0, 0);
};
</script>
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