Maplibre COG Protocol

Custom protocol to load Cloud Optimized GeoTIFFs (COG) in Maplibre GL JS

Demo page

https://labs.geomatico.es/maplibre-cog-protocol

Usage

For better quality, use always tileSize: 256 to match the size of tiles delivered by the custom protocol.

$3

``html














`

$3

npm install @geomatico/maplibre-cog-protocol

`tsx
import maplibregl from 'maplibre-gl';
import {cogProtocol} from '@geomatico/maplibre-cog-protocol';
import Map from 'react-map-gl/maplibre';

maplibregl.addProtocol('cog', cogProtocol);

const App = () =>
style={{width: 600, height: 400}}
mapStyle="https://geoserveis.icgc.cat/contextmaps/icgc_mapa_base_gris_simplificat.json"
initialViewState={{longitude: 1.83369, latitude: 41.5937, zoom: 14}}
>



;
`




API

$3

COGs with three or four 8-bit bands can be displayed as RGB or RGBA images.

* Use a raster source with the url prepended with cog://
* Use a raster layer.

`javascript
map.addSource('sourceId', {
type: 'raster',
url: 'cog://https://labs.geomatico.es/maplibre-cog-protocol/data/image.tif',
});

map.addLayer({
id: 'imageId',
source: 'sourceId',
type: 'raster'
});
`

$3

COGs with a single band can be interpreted DEMs.

#### As Hillshading

* Use a raster-dem source with the url prepended with cog:// and appended with #dem
* Use a hillshade layer.

`javascript
map.addSource('sourceId', {
type: 'raster-dem',
url: 'cog://https://cdn.geomatico.es/pirineo_dem_cog_256.tif#dem',
});

map.addLayer({
id: 'hillshadeId',
source: 'sourceId',
type: 'hillshade'
});
`

#### As 3D Terrain

* Use a raster-dem source with the url prepended with cog:// and appended with #dem, same as above.
* Set it as the terrain.

`javascript
map.addSource('sourceId', {
type: 'raster-dem',
url: 'cog://https://cdn.geomatico.es/pirineo_dem_cog_256.tif#dem',
});

map.setTerrain({
source: 'sourceId'
});
`




$3

COGs with a single band can be also converted to images applying a color ramp.

* Use a raster source with the url prepended with cog:// and appended with #color: and the color ramp specification.
* Use a raster layer.

`javascript
map.addSource('sourceId', {
type: 'raster',
url: 'cog://https://labs.geomatico.es/maplibre-cog-protocol/data/kriging.tif#color:BrewerSpectral9,1.7,1.8,c',
});

map.addLayer({
id: 'imageId',
source: 'sourceId',
type: 'raster'
});
`

The syntax for the #color parameter is #color:,,,, where:

* : Mandatory parameter. One of the built-in color ramps, see the list of possible values in Color Ramp cheatsheet.
* , : Define the data range for color mapping, should map your data's actual range. These are required if we want predictable results, as we can't rely on COG "stats" metadata (not always provided or correctly informed) and cannot read the whole file to get them (that's the point of the library, not having to).
* : Some characters representing additional configuration. We support:
* c continuous color interpolation (vs discrete).
* - reverse scale.

Some examples:

* Apply discrete CartoEarth ramp between 1 and 100: #color:CartoEarth,1,100
* Apply continuous BrewerYlOrRd7 ramp between -1 and 1: #color:BrewerYlOrRd7,-1,1,c
* Same as above, reversed (so colors go red-orange-yellow instead of yellow-orange-red): #color:CartoEarth,-1,1,c-.

See other usages in examples. If you need more flexibility, use a Custom Color Function.




$3

In case you want to apply any other coloring logic, you can provide a function that
converts pixel values to RGBA color values, and assign it to the COG URL where it needs
to be applied.

Use the setColorFunction method, which needs two arguments:
* cogUrl: the COG to which the custom color function will be applied. Don't prepend the cog:// protocol here.
* colorFunction: A function that maps pixel values to color values, whose arguments are:
* pixel: A TypedArray with the raw pixel data as read from the geotiff, one value per band.
* color: An Uint8ClampedArray of exactly 4 elements. Set the pixel color by setting the first, second, third and fourth element to red, green, blue and alpha values respectively.
* metadata: CogMetadata structure with information about the COG, such as noData, offset or scale values.

The following example paints values below a given threshold as red, and green otherwise:

`javascript
const cogUrl = 'https://labs.geomatico.es/maplibre-cog-protocol/data/kriging.tif';
const threshold = 1.75;

// Function is called for every pixel, keep it fast!
MaplibreCOGProtocol.setColorFunction(cogUrl, (pixel, color, metadata) => {
if (pixel[0] === metadata.noData) {
color.set([0, 0, 0, 0]); // Transparent
} else if (pixel[0] < threshold) {
color.set([255, 0, 0, 255]); // Red
} else {
color.set([0, 255, 0, 255]); // Green
}
});

map.addSource('sourceId', {
type: 'raster',
url: cog://${cogUrl}, // Use the same URL as in setColorFunction, preppended with "cog://".
});

map.addLayer({
id: 'imageId',
source: 'sourceId',
type: 'raster'
});
`

This function will be called for each pixel, keep it as fast as possible!

Some other interesting usages:

* Apply other color scales not listed in the builtin standard ColorBrewer or CartoColors catalog.
* Use custom breakpoints or interpolations.
* Display other bands.
* Combine bands of a multispectral image to calculate indicators on the fly.




$3

Use the getCogMetadata(url) to obtain metadata about a COG file. Some interesting information it provides:
offset, scale, noData value (for the first band), bbox. It also exposes some other internals that may change
in future releases, so use with caution.

It returns a promise. Usage example:

`javascript
MaplibreCOGProtocol.getCogMetadata(url).then(metadata => console.log(metadata?.bbox));
`




$3

The locationValues(url, location, zoom?) method reads raw pixel values for a given location. It returns an array of numbers, one for each band in the COG. NaNs are returned when querying outside of the image. If zoom is indicated, it will query the nearest overview corresponding to that zoom level.

Example usage in conjunction with maplibre API to get COG values on mouse hover:

`javascript
import {locationValues} from '@geomatico/maplibre-cog-protocol';

map.on('mousemove', ({lngLat}) => {
locationValues(
'./data/kriging.tif',
{latitude: lngLat.lat, longitude: lnglat.lon},
map.getZoom()
).then(console.log);
});
`

locationValues doesn't depend on Maplibre API or the CogProtocol, so it can be used to query raster values in applications without a map:

`javascript
import {locationValues} from '@geomatico/maplibre-cog-protocol';

const url = 'https://labs.geomatico.es/maplibre-cog-protocol/data/kriging.tif';
locationValues(url, {latitude: 41.656278, longitude: 0.501394}).then(console.log);
`




COG generation tips

COG should be in EPSG:3857 (Google Mercator) projection, as this library doesn't reproject and won't understand any other projection.

For better performance, use the Google Maps tiling scheme with 256x256 blocksize.

For RGB images, JPEG yCbCr (lossy) compression is recommended.
For lossless compression, deflate gives good decoding performance on the browser.

Sample GDAL commands (using docker for convenience, but not needed):

#### RGB Image (lossy compression)

`bash
docker run --rm -v .:/srv ghcr.io/osgeo/gdal:alpine-small-3.9.1 gdalwarp /srv/.tif /srv/.tif -of COG -co BLOCKSIZE=256 -co TILING_SCHEME=GoogleMapsCompatible -co COMPRESS=JPEG -co OVERVIEWS=IGNORE_EXISTING -co ADD_ALPHA=NO -dstnodata NaN
`

#### Digital Elevation Model

`bash
docker run --rm -v .:/srv ghcr.io/osgeo/gdal:alpine-small-3.9.1 gdalwarp /srv/.tif /srv/.tiff -of COG -co BLOCKSIZE=256 -co TILING_SCHEME=GoogleMapsCompatible -co COMPRESS=DEFLATE -co RESAMPLING=BILINEAR -co OVERVIEW_RESAMPLING=NEAREST -co OVERVIEWS=IGNORE_EXISTING -co ADD_ALPHA=NO -dstnodata NaN
`

For developers

$3

`
npm version [patch | minor | major]

npm run build
npm publish --access public

git push origin tag vX.X.X
npm run gh-publish # publish examples to labs.geomatico.es
``