How to determine Boundary Box corner in CRS units?

Hi everyone.

For my project, I am using cesium to display on a map WMS layers (On 2D map). To increase performances, I use the provider SingleTileImageryProvider to request an only tile.

The point is : the request need a parameter called “BBOX” which corresponds to boundary box corner in CRS units.

I am wondering how can I get these values to request the entire shown part of the map.

For instance, I was using WebMapServiceImageryProvider before and Cesium was attributing this BBOX automaticaly to each tile. But it does not work with SingleTileImageryProvider.

I was able to get a rectangle of the map I guess, by using Camera.computeViewRectangle
But coordinates are probably in radians, not in CRS units. I do not know if it is the right way or not.

An other solution could be requesting the entire WMS Layer, in spite of hidden parts.

I saw a lot of things on internet, but nothing to answer my question.

I someone could help me, please.

Thank you all.