SDF(Signed-distance-field: 有向距离场)WebGL GLSL(1)

Demo:http://www.artvily.com/sample?sample=sdfBase01

感谢:https://www.shadertoy.com/view/Xds3zN

请参见:

http://jamie-wong.com/2016/07/15/ray-marching-signed-distance-functions/

http://www.iquilezles.org/www/articles/normalsSDF/normalsSDF.htm

http://iquilezles.org/www/articles/distfunctions/distfunctions.htm