//    回転体作成シェーダ "spin"
//        ver 1.2 2020/09/06
//        by yarunashi@dooon

float map(vec3 v){

	float center = iVolumeSize.x * 0.5;
	float x = v.x - center;
	float y = v.y - center;
	float r = sqrt(x*x + y*y);
	float far = iVolumeSize.y - 1.0; //最背面
	return voxel(vec3(center + r, far, v.z));

}