essentially this: 
```glsl
    highp vec2 point_start = (point - start);
    highp vec2 start_end = (end - start);

    highp float dot1 = dot(point_start, start_end);
    highp float dot2 = dot(start_end, start_end);

    highp float h = clamp(dot1 / dot2, 0.0, 1.0);

    highp vec2 p = point_start - start_end * h;

    highp float l = sqrt(p.x * p.x + p.y * p.y) ;

    highp float c = clamp(l * 100.0, 0.0, 1.0);

    gl_FragColor = vec4(c, c, c, 1.0);
```