How to do that like Sketchfab setting border fade in shadow ground plane

Questions
5

For regular shadows (not contact shadows) a trick is to patch ShadowNode.prototype.setupShadowFilter with TSL (when using WebGPURenderer) to implement custom logic for the shadow (return a node that calculates values from 0 (shadow) to 1 (no shadow):

			const originalSetupShadowFilter = THREE.ShadowNode.prototype.setupShadowFilter;

			THREE.ShadowNode.prototype.setupShadowFilter = function ( builder, inputs ) {

				// base is the current value 0 to 1 for the current shadow fragment
				const base = originalSetupShadowFilter.call( this, builder, inputs );
				
				return yourCalculation( base, 123 )
			};

Here’s an example.