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After a few changes to other parts of the scene, I find that the fog no longer takes any notice of the surface or of the light colour, and I just get a white cone the shape of the spotlight beam.
I've tried deleting the light and making a new one, but that doesn't work. I've checked that the shadow casting and receiving properties of the light and the surfaces involved are switched on. The light does actually cast a shadow, it's just the fog that won't co-operate.
The only way I've found to get out of this problem is to go back to a version of the scene saved before I put any lights in and start again from there. But I've done too much work now to go back that far, and I promised the client a rough render today...
Please, please, can someone tell me how to fix this.