// xs_begin
// author : 'ephtracy'
// 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;
	v = ( v - i_volume_size * 0.5 ) / ( i_volume_size * 0.5 );
	return i_color_index * step( length( v ), 1.0 );
}