// xs_begin
// author : 'ephtracy'
// arg : { var = 'm_side'  name = 'Side'  value = '3'   range = '3 64'    step = '1'    precision = '0' }
// arg : { var = 'm_angle' name = 'Angle' value = '0'   range = '0 360'   step = '5'    precision = '0' }
// xs_end

//===== built-in args =====
// uniform vec3		i_volume_size;		// volume size [1-256]
// uniform float	i_color_index;		// current color index [0-255]
// uniform int		i_num_color_sels;	// number of color selections [1-255]

//===== built-in functions ===== 
// float voxel( vec3 v );				// get voxel color index
// float color_sel( float k );			// get kth selected color index
// vec4 palette( float index );			// get palette color

// generate a new voxel color index [0, 255] at position v ( v is at the center of voxel, such as vec3( 1.5, 2.5, 4.5 ) )
float map( vec3 v ) {
	const float PI = 3.1415926;

	// model center
	vec3 center = i_volume_size * 0.5;
	
	// 2D pos
	vec2 u = ( v - center ).xy;

	// poly
	float angle = PI * 2.0 / max( floor( m_side ), 3.0 );
	float t = min( center.x, center.y );

	// polar coord
	float r = length( u );
	float theta = atan( u.y, u.x ) + PI + m_angle / 180.0 * PI;

	r *= cos( angle * abs( fract( theta / angle ) - 0.5 ) );
	t *= cos( angle * 0.5 );

	r = max( r, 0.0 );
	t = max( t, 0.0 );

	return i_color_index * step( r, t );
}