cds.healpix

Interface AuxiliaryAxis

public interface AuxiliaryAxis

Define an auxiliary axis, i.e. a 3rd dimension to be taken into account when computing an HEALPix index.
To compute a hash using the auxiliary axis, the values must be projected along the z-axis ranging from 0 (inclusive) to 1 (exclusive).
The maximum depth when using an auxiliary axis is 19 (4 + 19 * 3 = 61 bits). So the maximum number of intervals is 2^19 = 524288.
At a given depth, the [0.0, 1.0[ interval is divided into NSIDE (=2^depth) segments.
The resolution at a given depth is thus abs(unproject(1) - unproject(0)) / NSIDE.
If we want to represent temporal data, we can represent:

• about 1 years with intervals being time ranges of 1 min;
• 60.68 years with intervals being time ranges of 1 hour.

We recall that when a double is divided by a power of two, there is no loss in precision since the mantissa remains unchanged (only the exponent part changes).
Example: depth 12 => NSIDE = 4096 => angular resolution = 51.5 arcsec => 4096 / 365 = 11.222 years with a resolution = 1 day

Author:

F.-X. Pineau

Method Summary

Modifier and Type	Method and Description
java.lang.String	name() Returns the name of the auxiliary axis.
double	project(double x) Project (or normalize) the given auxiliary axis value into the [0.0, 1.0[ range.
double	unproject(double y) Unproject (denormalize) the value from the [0.0, 1.0[ range.

Method Detail

name

java.lang.String name()

Returns the name of the auxiliary axis.

Returns:

the name of the auxiliary axis.

project

double project(double x)

Project (or normalize) the given auxiliary axis value into the [0.0, 1.0[ range.

Parameters:

x - the auxiliary axis value to be projected (normalized).

Returns:

the projected (normalized) value on the auxiliary axis, in [0.0, 1.0[.

unproject

double unproject(double y)

Unproject (denormalize) the value from the [0.0, 1.0[ range.

Parameters:

y - the projected (normalized) value on the auxiliary axis, in [0.0, 1.0[

Returns:

the unprojected (denormalize) value on the auxiliary axis.