three.js/src/renderers/shaders/ShaderLib/distance.glsl.js

export const vertex = /* glsl */`
#define DISTANCE

varying vec3 vWorldPosition;

#include <common>
#include <batching_pars_vertex>
#include <uv_pars_vertex>
#include <displacementmap_pars_vertex>
#include <morphtarget_pars_vertex>
#include <skinning_pars_vertex>
#include <clipping_planes_pars_vertex>

void main() {

	#include <uv_vertex>

	#include <batching_vertex>
	#include <skinbase_vertex>

	#include <morphinstance_vertex>

	#ifdef USE_DISPLACEMENTMAP

		#include <beginnormal_vertex>
		#include <morphnormal_vertex>
		#include <skinnormal_vertex>

	#endif

	#include <begin_vertex>
	#include <morphtarget_vertex>
	#include <skinning_vertex>
	#include <displacementmap_vertex>
	#include <project_vertex>
	#include <worldpos_vertex>
	#include <clipping_planes_vertex>

	vWorldPosition = worldPosition.xyz;

}
`;

export const fragment = /* glsl */`
#define DISTANCE

uniform vec3 referencePosition;
uniform float nearDistance;
uniform float farDistance;
varying vec3 vWorldPosition;

#include <common>
#include <uv_pars_fragment>
#include <map_pars_fragment>
#include <alphamap_pars_fragment>
#include <alphatest_pars_fragment>
#include <alphahash_pars_fragment>
#include <clipping_planes_pars_fragment>

void main () {

	vec4 diffuseColor = vec4( 1.0 );
	#include <clipping_planes_fragment>

	#include <map_fragment>
	#include <alphamap_fragment>
	#include <alphatest_fragment>
	#include <alphahash_fragment>

	float dist = length( vWorldPosition - referencePosition );
	dist = ( dist - nearDistance ) / ( farDistance - nearDistance );
	dist = saturate( dist ); // clamp to [ 0, 1 ]

	gl_FragColor = vec4( dist, 0.0, 0.0, 1.0 );

}
`;