add param computeLength and append v_distanceCamera when needed

[mickae1]

I'm working on custom shader, and I've some change needed to be done, on the core of cesium that I think will help others also.

By example, the dash material for polyline will be improved later ( the dash pattern depend of the position of the camera, which make the dash pattern move when the camera move which is not nice )

To help that, I need to know the distance of the fragment shader from the first point of the polyline.
That why, I added an attribute in the class PolylineGeometry. By default it's desactived, and it can be activated with the option computeLength.

In my custom shader, I need also to know the distance of the camera, which is already done here :

cesium/packages/engine/Source/Scene/Primitive.js

Line 1004 in c403b50

" distanceSq = max(distanceSq, 0.0); \n" +

so I just prepend in the vertexshader :
"out float v_distanceCamera; \n" +

when the function _appendDistanceDisplayConditionToShader is called.

which help me to show/hide, change object property in the custom shader depending of the distance of the camera from the object.

I think this will help also other.

Michael,

[github-actions]

Thank you for the pull request, @mickae1!

✅ We can confirm we have a CLA on file for you.