That error is from using it with SingleTileImageryProvider.url.
It kind of works when I use Rectangle.material, but there are some issues:
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I’m back to using the WGS84 view instead of Mercator (that’s what I set on line 12 of CesiumHeatmap._getImageryProvider) which distorts the heatmap to not fit (see my third message in this thread).
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The Entity also seems to only use the first heatmap canvas it can find, in stead of using the one I supply to it. Maybe it’s getting the image by doing a getElementsByClassName() and using the first one it finds?