tesseract.js/docs/api.md

# API

- [createWorker()](#create-worker)
  - [Worker.recognize](#worker-recognize)
  - [Worker.setParameters](#worker-set-parameters)
  - [Worker.reinitialize](#worker-reinitialize)
  - [Worker.detect](#worker-detect)
  - [Worker.terminate](#worker-terminate)
  - [Worker.writeText](#worker-writeText)
  - [Worker.readText](#worker-readText)
  - [Worker.removeFile](#worker-removeFile)
  - [Worker.FS](#worker-FS)
- [createScheduler()](#create-scheduler)
  - [Scheduler.addWorker](#scheduler-add-worker)
  - [Scheduler.addJob](#scheduler-add-job)
  - [Scheduler.getQueueLen](#scheduler-get-queue-len)
  - [Scheduler.getNumWorkers](#scheduler-get-num-workers)
- [setLogging()](#set-logging)
- [recognize()](#recognize)
- [detect()](#detect)
- [PSM](#psm)
- [OEM](#oem)

---

<a name="create-worker"></a>
## createWorker(options): Worker

`createWorker` is a function that creates a Tesseract.js worker.  A Tesseract.js worker is an object that creates and manages an instance of Tesseract running in a web worker (browser) or worker thread (Node.js).  Once created, OCR jobs are sent through the worker. 


**Arguments:**

- `langs` a string to indicate the languages traineddata to download, multiple languages are concated with **+**, ex: **eng+chi\_tra**
- `oem` a enum to indicate the OCR Engine Mode you use
- `options` an object of customized options
  - `corePath` path to a directory containing **both** `tesseract-core.wasm.js` and `tesseract-core-simd.wasm.js` from [Tesseract.js-core](https://www.npmjs.com/package/tesseract.js-core) package
     - Setting `corePath` to a specific `.js` file is **strongly discouraged.**  To provide the best performance on all devices, Tesseract.js needs to be able to pick between `tesseract-core.wasm.js` and `tesseract-core-simd.wasm.js`.  See [this issue](https://github.com/naptha/tesseract.js/issues/735) for more detail.
  - `langPath` path for downloading traineddata, do not include `/` at the end of the path
  - `workerPath` path for downloading worker script
  - `dataPath` path for saving traineddata in WebAssembly file system, not common to modify
  - `cachePath` path for the cached traineddata, more useful for Node, for browser it only changes the key in IndexDB
  - `cacheMethod` a string to indicate the method of cache management, should be one of the following options
    - write: read cache and write back (default method)
    - readOnly: read cache and not to write back
    - refresh: not to read cache and write back
    - none: not to read cache and not to write back
  - `legacyCore` set to `true` to ensure any code downloaded supports the Legacy model (in addition to LSTM model)
  - `legacyLang` set to `true` to ensure any language data downloaded supports the Legacy model (in addition to LSTM model)
  - `workerBlobURL` a boolean to define whether to use Blob URL for worker script, default: true
  - `gzip` a boolean to define whether the traineddata from the remote is gzipped, default: true
  - `logger` a function to log the progress, a quick example is `m => console.log(m)`
  - `errorHandler` a function to handle worker errors, a quick example is `err => console.error(err)`


**Examples:**

```javascript
const { createWorker } = Tesseract;
const worker = await createWorker('eng', 1, {
  langPath: '...',
  logger: m => console.log(m),
});
```

<a name="worker-recognize"></a>
### Worker.recognize(image, options, output, jobId): Promise

Worker.recognize() provides core function of Tesseract.js as it executes OCR

Figures out what words are in `image`, where the words are in `image`, etc.
> Note: `image` should be sufficiently high resolution.
> Often, the same image will get much better results if you upscale it before calling `recognize`.

**Arguments:**

- `image` see [Image Format](./image-format.md) for more details.
- `options` an object of customized options
  - `rectangle` an object to specify the regions you want to recognized in the image, should contain top, left, width and height, see example below.
- `output` an object specifying which output formats to return (by default `text`, `blocks`, `hocr`, and `tsv` are returned)
- `jobId` Please see details above

**Output:**

**Examples:**

```javascript
const { createWorker } = Tesseract;
(async () => {
  const worker = await createWorker('eng');
  const { data: { text } } = await worker.recognize(image);
  console.log(text);
})();
```

With rectangle

```javascript
const { createWorker } = Tesseract;
(async () => {
  const worker = await createWorker('eng');
  const { data: { text } } = await worker.recognize(image, {
    rectangle: { top: 0, left: 0, width: 100, height: 100 },
  });
  console.log(text);
})();
```

<a name="worker-set-parameters"></a>
### worker.setParameters(params, jobId): Promise

`worker.setParameters()` set parameters for Tesseract API (using SetVariable()), it changes the behavior of Tesseract and some parameters like tessedit\_char\_whitelist is very useful.

**Arguments:**

- `params` an object with key and value of the parameters
- `jobId` Please see details above

Note:  `worker.setParameters` cannot be used to change the `oem`, as this value is set at initialization.  `oem` is initially set using an argument of `createWorker`.  After a worker already exists, changing `oem` requires running `worker.reinitialize`.

**Useful Parameters:**

| name                        | type   | default value     | description                                                                                                                     |
| --------------------------- | ------ | ----------------- | ------------------------------------------------------------------------------------------------------------------------------- |
| tessedit\_pageseg\_mode     | enum   | PSM.SINGLE\_BLOCK | Check [HERE](https://github.com/tesseract-ocr/tesseract/blob/4.0.0/src/ccstruct/publictypes.h#L163) for definition of each mode |
| tessedit\_char\_whitelist   | string | ''                | setting white list characters makes the result only contains these characters, useful if content in image is limited           |
| preserve\_interword\_spaces | string | '0'               | '0' or '1', keeps the space between words                                                                                       |
| user\_defined\_dpi          | string | ''                | Define custom dpi, use to fix **Warning: Invalid resolution 0 dpi. Using 70 instead.**                                          |

This list is incomplete.  As Tesseract.js passes parameters to the Tesseract engine, all parameters supported by the underlying version of Tesseract should also be supported by Tesseract.js.  (Note that parameters marked as “init only” in Tesseract documentation cannot be set by `setParameters` or `recognize`.) 

**Examples:**

```javascript
(async () => {
  await worker.setParameters({
    tessedit_char_whitelist: '0123456789',
  });
})
```

<a name="worker-reinitialize"></a>
### worker.reinitialize(langs, oem, jobId): Promise

`worker.reinitialize()` re-initializes an existing Tesseract.js worker with different `langs` and `oem` arguments.  

**Arguments:**

- `langs` a string to indicate the languages traineddata to download, multiple languages are concated with **+**, ex: **eng+chi\_tra**
- `oem` a enum to indicate the OCR Engine Mode you use
- `jobId` Please see details above

Note: to switch from Tesseract LSTM (`oem` value `1`) to Tesseract Legacy (`oem` value `0`) using `worker.reinitialize()`, the worker must already contain the code required to run the Tesseract Legacy model.  Setting `legacyCore: true` and `legacyLang: true` in `createWorker` options ensures this is the case.

**Examples:**

```javascript
await worker.reinitialize('eng', 1);
```

<a name="worker-detect"></a>
### Worker.detect(image, jobId): Promise

Worker.detect() does OSD (Orientation and Script Detection) to the image instead of OCR.

Note: Running `worker.detect` requires a worker with code and language data that supports Tesseract Legacy (this is not enabled by default).  If you want to run `worker.detect`, set `legacyCore` and `legacyLang` to `true` in the `createWorker` options. 

**Arguments:**

- `image` see [Image Format](./image-format.md) for more details.
- `jobId` Please see details above

**Examples:**

```javascript
const { createWorker } = Tesseract;
(async () => {
  const worker = await createWorker('eng', 1, {legacyCore: true, legacyLang: true});
  const { data } = await worker.detect(image);
  console.log(data);
})();
```

<a name="worker-terminate"></a>
### Worker.terminate(jobId): Promise

Worker.terminate() terminates the worker and cleans up

```javascript
(async () => {
  await worker.terminate();
})();
```


<a name="worker-writeText"></a>
### Worker.writeText(path, text, jobId): Promise

Worker.writeText() writes a text file to the path specified in MEMFS, it is useful when you want to use some features that requires tesseract.js
to read file from file system.

**Arguments:**

- `path` text file path
- `text` content of the text file
- `jobId` Please see details above

**Examples:**

```javascript
(async () => {
  await worker.writeText('tmp.txt', 'Hi\nTesseract.js\n');
})();
```

<a name="worker-readText"></a>
### Worker.readText(path, jobId): Promise

Worker.readText() reads a text file to the path specified in MEMFS, it is useful when you want to check the content.

**Arguments:**

- `path` text file path
- `jobId` Please see details above

**Examples:**

```javascript
(async () => {
  const { data } = await worker.readText('tmp.txt');
  console.log(data);
})();
```

<a name="worker-removeFile"></a>
### Worker.removeFile(path, jobId): Promise

Worker.removeFile() remove a file in MEMFS, it is useful when you want to free the memory.

**Arguments:**

- `path` file path
- `jobId` Please see details above

**Examples:**

```javascript
(async () => {
  await worker.removeFile('tmp.txt');
})();
```

<a name="worker-FS"></a>
### Worker.FS(method, args, jobId): Promise

Worker.FS() is a generic FS function to do anything you want, you can check [HERE](https://emscripten.org/docs/api_reference/Filesystem-API.html) for all functions.

**Arguments:**

- `method` method name
- `args` array of arguments to pass  
- `jobId` Please see details above

**Examples:**

```javascript
(async () => {
  await worker.FS('writeFile', ['tmp.txt', 'Hi\nTesseract.js\n']);
  // equal to:
  // await worker.writeText('tmp.txt', 'Hi\nTesseract.js\n');
})();
```

<a name="create-scheduler"></a>
## createScheduler(): Scheduler

createScheduler() is a factory function to create a scheduler, a scheduler manages a job queue and workers to enable multiple workers to work together, it is useful when you want to speed up your performance.

**Examples:**

```javascript
const { createScheduler } = Tesseract;
const scheduler = createScheduler();
```

### Scheduler

<a name="scheduler-add-worker"></a>
### Scheduler.addWorker(worker): string

Scheduler.addWorker() adds a worker into the worker pool inside scheduler, it is suggested to add one worker to only one scheduler.

**Arguments:**

- `worker` see Worker above

**Examples:**

```javascript
const { createWorker, createScheduler } = Tesseract;
const scheduler = createScheduler();
const worker = await createWorker();
scheduler.addWorker(worker);
```

<a name="scheduler-add-job"></a>
### Scheduler.addJob(action, ...payload): Promise

Scheduler.addJob() adds a job to the job queue and scheduler waits and finds an idle worker to take the job.

**Arguments:**

- `action` a string to indicate the action you want to do, right now only **recognize** and **detect** are supported
- `payload` a arbitrary number of args depending on the action you called.

**Examples:**

```javascript
(async () => {
 const { data: { text } } = await scheduler.addJob('recognize', image, options);
 const { data } = await scheduler.addJob('detect', image);
})();
```

<a name="scheduler-get-queue-len"></a>
### Scheduler.getQueueLen(): number

Scheduler.getNumWorkers() returns the length of job queue.

<a name="scheduler-get-num-workers"></a>
### Scheduler.getNumWorkers(): number

Scheduler.getNumWorkers() returns number of workers added into the scheduler

<a name="scheduler-terminate"></a>
### Scheduler.terminate(): Promise

Scheduler.terminate() terminates all workers added, useful to do quick clean up.

**Examples:**

```javascript
(async () => {
  await scheduler.terminate();
})();
```

<a name="set-logging"></a>
## setLogging(logging: boolean)

setLogging() sets the logging flag, you can `setLogging(true)` to see detailed information, useful for debugging.

**Arguments:**

- `logging` boolean to define whether to see detailed logs, default: false

**Examples:**

```javascript
const { setLogging } = Tesseract;
setLogging(true);
```

<a name="recognize"></a>
## recognize(image, langs, options): Promise

This function is depreciated and should be replaced with `worker.recognize` (see above). 

`recognize` works the same as `worker.recognize`, except that a new worker is created, loaded, and destroyed every time the function is called.

See [Tesseract.js](../src/Tesseract.js)

<a name="detect"></a>
## detect(image, options): Promise

This function is depreciated and should be replaced with `worker.detect` (see above). 

Same background as recognize(), but it does detect instead.

See [Tesseract.js](../src/Tesseract.js)

<a name="psm"></a>
## PSM

See [PSM.js](../src/constants/PSM.js)

<a name="oem"></a>
## OEM

See [OEM.js](../src/constants/OEM.js)