inkmap, a library for generating high resolution maps in the browser

Live demo here!

Introduction

inkmap is based on OpenLayers and will generate maps in PNG format based on a given JSON specification.

inkmap can handle long-running jobs (e.g. A0 format in 300 dpi) and provides an API for following a job progress. It uses a service worker in the background provided the user browser supports OffscreenCanvas, and falls back (almost) seamlessly to the main thread if not.

Please note that the first version of inkmap has been entirely funded and supported by the French Ministry of Ecology as part of their Descartes web mapping toolkit, hosted here: https://adullact.net/projects/descartes/

Usage

Basic

To include the library in your project:

$ npm install --save @camptocamp/inkmap

Then import the different methods from the inkmap package:

import { print, downloadBlob } from '@camptocamp/inkmap';

print({
  layers: [ ... ],
  projection: 'EPSG:4326',
  ...
}).then(downloadBlob);

Advanced

inkmap offers advanced job monitoring through the use of Observables provided by the rxjs library.

Observables are different from Promises in that they can emit multiple values instead of just one, and are a very good fit for progress reporting.

To use an Observable, simply call its subscribe() method with a function as argument. The function will be called anytime a new value is emitted, like so:

import { getJobStatus } from '@camptocamp/inkmap';

...

getJobStatus(jobId).subscribe((jobStatus) => {
  // do something with the status
});

Note that for long-lived Observables (i.e. Observables that never completes) it is important to call unsubscribe() when the emitted values are not needed anymore. Open subscriptions to Observables might create memory leaks.

Enabling the service worker

inkmap can and will use a dedicated service worker for running print jobs if given the chance. This offers the following advantages:

• Jobs run in a separate thread, meaning the user navigation will not be impacted at all by any CPU-intensive task
• The service worker isn't tied to a window or tab, so jobs will continue running when the tab is closed (and even when the browser is closed, depending on the platform)
• Push notifications might be sent to the user when a print job complete (not implemented yet)

To enable this, the inkmap-worker.js file located in the dist folder must be published on the same path as the application using inkmap.

The worker file can be published either using a symbolic link or by actually copying the file, for example in the application build pipeline.

If using Webpack to build the application, a solution is to use the CopyWebpackPlugin:

module.exports = {
  ...
  plugins: [
     new CopyWebpackPlugin([
       {
         from: 'node_modules/@camptocamp/inkmap/dist/inkmap-worker.js',
         to: 'dist'
       },
     ]),
  ],
  ...
}

API

Important note: all API functions are named exports from the inkmap package.

print(jsonSpec: PrintSpec): Promise<Blob>

Takes in a PrintSpec object and returns a Promise which resolves to a Blob containing the final image.

queuePrint(jsonSpec: PrintSpec): Promise<number>

Takes in a PrintSpec object and returns a Promise which resolves to a job id (number) immediately.

getJobStatus(id: number): Observable<PrintStatus>

Takes in a job id and returns an Observable which will regularly emit PrintStatus objects, and complete when the final image was generated.

cancelJob(id: number): void

Takes in a job id, cancel the job and cease all requests immediately.

getJobsStatus(): Observable<PrintStatus[]>

Returns a long-running observable which emits an array of print job status. Once a job is finished it will appear once in the array and then will not be part of subsequent emissions.

Note: This observable will never complete. Don't forget to unsubscribe!

registerProjection(definition: ProjectionDefinition): void

Takes in a projection definition and registers it with proj4.

getAttributionsText(spec: PrintSpec): string

Returns the full attributions text of the spec.

getNorthArrow(spec: PrintSpec, sizeHint?: [number, string]): PrintableImage

Returns a PrintableImage containing the north arrow for the given spec. The image will be square and have the same DPI as the spec. A sizeHint (e.g. [3, 'cm']) can be provided to request a larger or smaller arrow. Supported size units are the same as in the PrintSpec type.

getScaleBar(spec: PrintSpec, minimumWidth?: [number, string]): PrintableImage

Returns a PrintableImage containing the scale bar for the given spec. The image will have the same DPI as the spec. A minimumWidth (e.g. [80, 'mm']) can be provided to request a smaller or larger scale bar. Supported size units are the same as in the PrintSpec type.

Note that for this scale bar to be relevant in a printed document, its real-world size has to be respected.

createLegend(jsonSpec: PrintSpec): Promise<Blob>

Takes in a PrintSpec object and returns a Promise which resolves to a Blob containing the final legend image.

PrintSpec type

A PrintSpec object describes the content and aspect of the map to be printed.

field	type	description
layers	Layer[]	Array of Layer objects that will be rendered in the map; last layers will be rendered on top of first layers.
size	[number, number] or [number, number, string]	Width and height in pixels, or in the specified unit in 3rd place; valid units are px, mm, cm, m and in.
center	[number, number]	Longitude and latitude of the map center.
dpi	number	Dot-per-inch, usually 96 for a computer screen and 300 for a detailed print.
scale	number	Scale denominator.
scaleBar	boolean | ScaleBarSpec	Indicates whether scalebar should be printed (true). Also allows to pass options object: {"position": "bottom-left", "units": "metric" } (default values). Possible values are: position: "bottom-left" | "bottom-right" units: "degrees" | "imperial" | "metric" | "nautical" | "us" (same as ol.control.ScaleLine)
northArrow	boolean | string	North arrow position; either 'top-left', 'bottom-left', 'bottom-right' or 'top-right'; true defaults to 'top-right'; absent or false means not to print the north arrow.
projection	string	Projection name. If starting with EPSG:, and other than EPSG:3857 or EPSG:4326, definition will be downloaded on [https://epsg.io/].
projectionDefinitions	[projectionDefinition]	Optional. Registers new projections from the definitions.
attributions	boolean | string	Position of the attributions to be printed on the map; either 'top-left', 'bottom-left', 'bottom-right' or 'top-right'; if not defined, attributions will not be printed.

Layer type

A Layer object describes a layer in the printed map.

field	type	description
type	string	Either XYZ, WMTS, WMS, WFS or GeoJSON.
url	string	URL or URL template for the layer; for XYZ layers, a URL can contain the following tokens: {a-d} for randomly choosing a letter, {x}, {y} and {z}.
opacity	number	Opacity, from 0 (hidden) to 1 (visible).
attribution	string	Attribution for the data present in the layer.
legend	boolean	Include this layer in the map legend. Defaults is false.

WMS layer type

Additional options for WMS layer type.

field	type	description
layer	string	Layer name.
version	string	Version of WMS protocol used: 1.1.1 or 1.3.0 (default).
tiled	boolean	Indicates whether the WMS layer should be requested as tiles. Defaults to false.
customParams	Object	Additional params used in all GetMap requests (optional).

WMTS layer type

Additional options for WMTS layer to define the layer source. See https://openlayers.org/en/latest/apidoc/module-ol_source_WMTS-WMTS.html for the full list of options. The following table introduces the common options to use.

field	type	description
requestEncoding	string	Request encoding: KVP, REST.
format	string	Image format. Only used when requestEncoding is 'KVP'. eg image/png.
layer	string	Layer name as advertised in the WMTS capabilities.
style	number	Style name as advertised in the WMTS capabilities.
projection	string	Projection.
matrixSet	string	Matrix set.
tileGrid	TileGrid	TileGrid object, see https://openlayers.org/en/latest/apidoc/module-ol_tilegrid_TileGrid-TileGrid.html for options

WFS layer type

Additional options for WFS layer type.

field	type	description
layer	string	Layer name.
version	string	Version of WFS protocol used: 1.0.0, 1.1.0 (default) or 2.0.0.
format	string	Format used when querying WFS, gml (default) or geojson. inkmap determines the GML parser based on the WFS version used.
style	object	JSON object in geostyler notation, defining the layer style.

GeoJSON layer type

Additional options for GeoJSON layer type.

field	type	description
geojson	object	Feature collection in GeoJSON format; coordinates are expected to be in the print job reference system.
style	object	JSON object in geostyler notation, defining the layer style.

ImageArcGISRest layer type

Additional options for ImageArcGISRest layer type.

field	type	description
hidpi	boolean	Use the ol/Map#pixelRatio value when requesting the image from the remote server.
interpolate	boolean	Use interpolated values when resampling. Set to false to use the nearest neighbor instead.
tiled	boolean	Indicates whether the WMS layer should be requested as tiles. Defaults to false.
params	object	ArcGIS Rest parameters. This field is optional.

projectionDefinition type

A projectionDefinition object describes a projection to be registered in proj4.

field	type	description
name	string	Name of the projection, written as prefix:code.
bbox	[number, number, number, number]	Extent of the projection, written as [maxlat, minlon, minlat, maxlon].
proj4	string	Proj4 definition string.

PrintStatus type

A PrintStatus object describes the status of a print job.

field	type	description
id	number	Job id.
progress	number	Job progress, from 0 to 1.
status	string	Either 'pending', 'ongoing', 'finished' or 'canceled'.
imageBlob	`Blob	null`
spec	PrintSpec	The spec used for this job.
sourceLoadErrors	SourceLoadError[]	Array of SourceLoadError objects.

SourceLoadError type

A SourceLoadError object contains the URL for tile sources which produced errors while loading data.

field	type	description
url	string	URL of the ol.source that encountered at least one 'tileloaderror' or 'imageloaderror'.

PrintableImage class

A PrintableImage is essentially a wrapper around a native image or canvas, with added information about its real world size.

method	return type	description
getImage()	HTMLImageElement or HTMLCanvasElement	Returns the native image to be drawn or printed.
getRealWorldDimensions(units: string)	[number, number]	Returns the real world dimensions of the image for a given unit (e.g. mm).
getDpi()	number	Returns the image DPI.

Architecture

Under the hood, inkmap will attempt to install a service worker on the page it is called. The service worker will then be in charge of loading all the map images and data, composing them together and giving them back to the application code.

Contributing

See CONTRIBUTING.

License

CeCILL-C