Complex shape character outline

Question

Say I have this character and I want allow user to select it, so when it s selected I want to show an outline around it.

Answer 1

An generic solution that applies to geometries of any complexity might be to apply a fragment shader via the ShaderMaterial class in three.js. Not sure what your experience level is at, but if you need it an introduction to shaders can be found here.

A good example where shaders are used to highlight geometries can be found here. In their vertex shader, they calculate the normal for a vertex and a parameter used to express intensity of a glow effect:

uniform vec3 viewVector;
uniform float c;
uniform float p;
varying float intensity;
void main() 
{
    vec3 vNormal = normalize( normalMatrix * normal );
    vec3 vNormel = normalize( normalMatrix * viewVector );
    intensity = pow( c - dot(vNormal, vNormel), p );

    gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );
}

These parameters are passed to the fragment shader where they are used to modify the color values of pixels surrounding the geometry:

uniform vec3 glowColor;
varying float intensity;
void main() 
{
    vec3 glow = glowColor * intensity;
    gl_FragColor = vec4( glow, 1.0 );
}