Preventing use of higher level tiles when lower level ones are unavailable (WMTS)

[Thomas LEROY]

Hello,

 

I have a WMTS server having only data from zoom levels 6 to 16 and want to display its tiles in Cesium. According to the API reference of the WebMapTileServiceImageryProvider (https://cesiumjs.org/Cesium/Build/Documentation/WebMapTileServiceImageryProvider.html#WebMapTileServiceImageryProvider), I should be able to use the 'minimumLevel' and 'maximumLevel' properties to indicate the available tile range.

 

I used the following code (the URL is not the one I used, as my server is not accessible from Internet):

 

var viewer = new Cesium.Viewer('cesiumContainer');

 

var wmts = new Cesium.WebMapTileServiceImageryProvider({

    url : "https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryTopo/MapServer/WMTS/tile/1.0.0/USGSImageryTopo/{Style}/{TileMatrixSet}/{TileMatrix}/{TileRow}/{TileCol}",

    layer : 'USGSImageryTopo',

    style : 'default',

    format : 'image/png',

    tileMatrixSetID : 'default028mm',

    minimumLevel: 6,

    maximumLevel: 16

});

 

viewer.imageryLayers.addImageryProvider(wmts);

 

 

I get the following error: "Uncaught DeveloperError: The imagery provider's rectangle and minimumLevel indicate that there are 4096 tiles at the minimum level. Imagery providers with more than four tiles at the minimum level are not supported."

 

I guess the reason why Cesium tries to download data from zoom level 6 when the view is currently at zoom level 0 is to "compute" the missing tiles from higher level ones.

 

I tried creating an ImageryLayer and specifying the minimumTerrainLevel, but that's not valid workaround, because the check is done in the WebMapTileServiceImageryProvider constructor (https://github.com/AnalyticalGraphicsInc/cesium/blob/3ef9dad02f4be88894e58501a602118008c5b5c6/Source/Scene/WebMapTileServiceImageryProvider.js#L214).

The only workaround I could find was that, if you work on a relatively small area, you can specify your extent of interest with the 'rectangle' property of the options.

 

I see there was an issue created to raise the allowed number of tiles at minimum level (https://github.com/AnalyticalGraphicsInc/cesium/issues/4372).

But would not it be better to totally disable this "extrapolation" mechanism" when it is not wanted?

 

It could be done like that:

1. Remove the check of the number of tiles at minimum level (https://github.com/AnalyticalGraphicsInc/cesium/blob/3ef9dad02f4be88894e58501a602118008c5b5c6/Source/Scene/WebMapTileServiceImageryProvider.js#L214)
2. Add a new optional property to the ImageryLayer options named for instance 'extrapolateLowerLevelTiles'. True would mean the current behavior (use higher level tiles to compute missing lower level ones), and false would mean that the layer should not try to display anything. Defaut value could be true to keep current behavior.
3. Change the following block (https://github.com/AnalyticalGraphicsInc/cesium/blob/3ef9dad02f4be88894e58501a602118008c5b5c6/Source/Scene/ImageryLayer.js#L545)

 

if (defined(imageryProvider.minimumLevel)) {

    var minimumLevel = imageryProvider.minimumLevel;

    if (imageryLevel < minimumLevel) {

        imageryLevel = minimumLevel;

    }

}

 

by something like:

 

if (defined(imageryProvider.minimumLevel)) {

    var minimumLevel = imageryProvider.minimumLevel;

    if (imageryLevel < minimumLevel) {

        if (this.extrapolateLowerLevelTiles) {

            imageryLevel = minimumLevel;

        } else {

            // Do not create missing tiles from higher level ones.

            return false;

        }

    }

}

 

Would it be of any interest for the product?

 

Best regards

[Gabby Getz]

Hi Thomas,

As mentioned in the other thread, step #1 may not be the best approach in all cases, as it's trying to avoid loading too many tiles in at once when viewing the entire globe. I think you can circumvent that check if that optional "extrapolate" is set to false.

I think your approach makes sense, we'd be happy to review a pull request with this proposed change! See CONTRIBUTING.md to get started.

Thanks!

Gabby

[Thomas LEROY]

Hi Gabby,

Thanks for your answer. I will gladly propose a pull request as soon as possible.

Thomas

[Matthew Amato]

Thomas,

This is a really poorly documented feature in Cesium and something we can do a lot better with.   Just to jump right into it, I think this example code will provide exactly what you need, no changes to Cesium necessary:

var viewer = new Cesium.Viewer('cesiumContainer', {

    imageryProvider : false,

    baseLayerPicker: false

});

var wmts = new Cesium.WebMapTileServiceImageryProvider({

    url : "https://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryTopo/MapServer/WMTS/tile/1.0.0/USGSImageryTopo/{Style}/{TileMatrixSet}/{TileMatrix}/{TileRow}/{TileCol}",

    layer : 'USGSImageryTopo',

    style : 'default',

    format : 'image/png',

    tileMatrixSetID : 'default028mm',

    maximumLevel: 16

});

var layers = viewer.imageryLayers;

layers.add(new Cesium.ImageryLayer(wmts, { minimumTerrainLevel: 6 }));

Here's a Runnable demo version: https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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

Basically, I defined a single layer and have it disabled completely until you hit zoom level 6, so the globe is the default blue when the demo starts but as you zoom in it will appear when you hit level 6.  You could also use a different base imagery layer as the base and turn it off at layer  6 at the same time yours turns on.

The poorly named minimumTerrainLevel/maximumTerrainLevel options to the ImageryLayer constructor turn on/off visualization of that layer at the specified zoom levels.

I'm going to write up an issue to better improve the API here and make it much more obvious how to implement this (often requested) functionality.

Let me know if you still have questions.  Thanks!

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[Gabby Getz]

Thanks Matt! Can you link to the issue you open here?

[Matthew Amato]

Thanks for the reminder.  Submitted https://github.com/AnalyticalGraphicsInc/cesium/issues/6564