# symforce.opt.barrier_functions module¶

max_power_barrier(x, x_nominal, error_nominal, dist_zero_to_nominal, power)[source]

A one-sided, non-symmetric scalar barrier function. The barrier passes through the points (x_nominal, error_nominal) and (x_nominal - dist_zero_to_nominal, 0) with a curve of the form x^power. The parameterization of the barrier by these variables is convenient because it allows setting a constant penalty for a nominal point, then adjusting the width and steepness of the curve independently. The barrier with power = 1 will look like:

** - (x_nominal, error_nominal) is a fixed point
** <- x^power is the shape of the curve
———-*****************———
|<-->| dist_zero_to_nominal is the distance from x_nominal to the point at which the error is zero

Note that when applying the barrier function to a residual used in a least-squares problem, a power = 1 will lead to a quadratic cost in the optimization problem because the cost equals 1/2 * residual^2. For example:

Cost (1/2 * residual^2) when the residual is a max_power_barrier with power = 1 (shown above):

** - (x_nominal, error_nominal^2)
** <- x^(2*power) is the shape of the cost curve
———*****************———
|<-->| dist_zero_to_nominal
Parameters:
• x (`float`) – The point at which we want to evaluate the barrier function.

• x_nominal (`float`) – x-value of the point at which the error is equal to error_nominal.

• error_nominal (`float`) – Error returned when x equals x_nominal.

• dist_zero_to_nominal (`float`) – The distance from x_nominal to the region of zero error. Must be a positive number.

• power (`float`) – The power used to describe the curve of the error tails.

Return type:

`float`

max_linear_barrier(x, x_nominal, error_nominal, dist_zero_to_nominal)[source]

Applies “max_power_barrier” with power = 1. When applied to a residual of a least-squares problem, this produces a quadratic cost in the optimization problem because cost = 1/2 * residual^2. See “max_power_barrier” for more details.

Parameters:
• x (`float`) –

• x_nominal (`float`) –

• error_nominal (`float`) –

• dist_zero_to_nominal (`float`) –

Return type:

`float`

min_power_barrier(x, x_nominal, error_nominal, dist_zero_to_nominal, power)[source]

A one-sided, non-symmetric scalar barrier function. The barrier passes through the points (x_nominal, error_nominal) and (x_nominal + dist_zero_to_nominal, 0) with a curve of the form x^power. The barrier with power = 1 will look like:

** |

(x_nominal, error_nominal) - ** |

** |

x^power is the shape of the curve -> ** |
** |

** |

———******************———

dist_zero_to_nominal |<->| |

Parameters:
• x (`float`) – The point at which we want to evaluate the barrier function.

• x_nominal (`float`) – x-value of the point at which the error is equal to error_nominal.

• error_nominal (`float`) – Error returned when x equals x_nominal.

• dist_zero_to_nominal (`float`) – The distance from x_nominal to the region of zero error. Must be a positive number.

• power (`float`) – The power used to describe the curve of the error tails. Note that when applying the barrier function to a residual used in a least-squares problem, a power = 1 will lead to a quadratic cost in the optimization problem.

Return type:

`float`

min_linear_barrier(x, x_nominal, error_nominal, dist_zero_to_nominal)[source]

Applies “min_power_barrier” with power = 1. When applied to a residual of a least-squares problem, this produces a quadratic cost in the optimization problem because cost = 1/2 * residual^2. See “min_power_barrier” for more details.

Parameters:
• x (`float`) –

• x_nominal (`float`) –

• error_nominal (`float`) –

• dist_zero_to_nominal (`float`) –

Return type:

`float`

symmetric_power_barrier(x, x_nominal, error_nominal, dist_zero_to_nominal, power)[source]

A symmetric barrier cenetered around x = 0, meaning the error at -x is equal to the error at x. The barrier passes through the points (x_nominal, error_nominal) and (x_nominal - dist_zero_to_nominal, 0) with a curve of the form x^power. For example, the barrier with power = 1 will look like:

** | **
** | ** - (x_nominal, error_nominal) is a fixed point
** | **
** | ** <- x^power is the shape of the curve
** | **

** | **

———-*****************———
|<-->| dist_zero_to_nominal is the distance from x_nominal to the point at which the error is zero

Note that when applying the barrier function to a residual used in a least-squares problem, a power = 1 will lead to a quadratic cost in the optimization problem because the cost equals 1/2 * residual^2. For example:

Cost (1/2 * residual^2) when the residual is a symmetric barrier with power = 1 (shown above):

** | ** - (x_nominal, 1/2 * error_nominal^2)
** | ** <- x^(2*power) is the shape of the cost curve
* | *

* | *

———*****************———
|<-->| dist_zero_to_nominal
Parameters:
• x (`float`) – The point at which we want to evaluate the barrier function.

• x_nominal (`float`) – x-value of the point at which the error is equal to error_nominal.

• error_nominal (`float`) – Error returned when x equals x_nominal.

• dist_zero_to_nominal (`float`) – Distance from x_nominal to the closest point at which the error is zero. Note that dist_zero_to_nominal must be less than x_nominal and greater than zero.

• power (`float`) – The power used to describe the curve of the error tails.

Return type:

`float`

min_max_power_barrier(x, x_nominal_lower, x_nominal_upper, error_nominal, dist_zero_to_nominal, power)[source]

A symmetric barrier centered between x_nominal_lower and x_nominal_upper. See symmetric_power_barrier for a detailed description of the barrier function. As an example, the barrier with power = 1 will look like:

** | **

** | **

(x_nominal_lower, error_nominal) - ** | ** - (x_nominal_upper, error_nominal)
** | **
** | ** <- x^power is the shape of the curve

** | **

———-*************———

dist_zero_to_nominal |<->| | |<->| dist_zero_to_nominal

Parameters:
• x (`float`) – The point at which we want to evaluate the barrier function.

• x_nominal_lower (`float`) – x-value of the point at which the error is equal to error_nominal on the left-hand side of the barrier function.

• x_nominal_upper (`float`) – x-value of the point at which the error is equal to error_nominal on the right-hand side of the barrier function.

• error_nominal (`float`) – Error returned when x equals x_nominal_lower or x_nominal_upper.

• dist_zero_to_nominal (`float`) – The distance from either of the x_nominal points to the region of zero error. Must be less than half the distance between x_nominal_lower and x_nominal_upper, and must be greater than zero.

• power (`float`) – The power used to describe the curve of the error tails. Note that when applying the barrier function to a residual used in a least-squares problem, a power = 1 will lead to a quadratic cost in the optimization problem.

Return type:

`float`

min_max_linear_barrier(x, x_nominal_lower, x_nominal_upper, error_nominal, dist_zero_to_nominal)[source]

Applies “min_max_power_barrier” with power = 1. When applied to a residual of a least-squares problem, this produces a quadratic cost in the optimization problem because cost = 1/2 * residual^2. See “min_max_power_barrier” for more details.

Parameters:
• x (`float`) –

• x_nominal_lower (`float`) –

• x_nominal_upper (`float`) –

• error_nominal (`float`) –

• dist_zero_to_nominal (`float`) –

Return type:

`float`

min_max_centering_power_barrier(x, x_nominal_lower, x_nominal_upper, error_nominal, dist_zero_to_nominal, power, centering_scale)[source]

This barrier is the maximum of two power barriers which we call the “bounding” barrier and the “centering” barrier. Both barriers are centered between x_nominal_lower and x_nominal_upper. As an example, the barrier with power = 1 may look like:

BARRIER (max of bounding and centering barriers):
** | **
** <-(x_nominal_lower, error_nominal) ** <-(x_nominal_upper, error_nominal)
** | **
** | **
** | **
** | ** <- x^power is the shape of upper/lower curve

** **

——————***——————-

It may be easier to vizualize the bounding and centering barriers independently:

BOUNDING BARRIER:
** | **
** <-(x_nominal_lower, error_nominal) ** <-(x_nominal_upper, error_nominal)
** | **
** | **
** | ** <- x^power is the shape of the curve
** | **

** | **

——***************************——-
|<-->| dist_zero_to_nominal
CENTERING BARRIER:

** | **

** | **

nominal_lower ^ ** | ** ^ nominal_upper
** | **

** ** <- x^power is the shape of the curve

——————***——————-
^-((x_nominal_lower + x_nominal_upper) / 2, 0)
where:

nominal_lower = (x_nominal_lower, centering_scale * error_nominal) nominal_upper = (x_nominal_upper, centering_scale * error_nominal)

and the only point with zero error is the midpoint of x_nominal_lower and x_nominal_upper.

Parameters:
• x (`float`) – The point at which we want to evaluate the barrier function.

• x_nominal_lower (`float`) – x-value of the point at which the error is equal to error_nominal on the left-hand side of the barrier function.

• x_nominal_upper (`float`) – x-value of the point at which the error is equal to error_nominal on the right-hand side of the barrier function.

• error_nominal (`float`) – Error returned when x equals x_nominal_lower or x_nominal_upper.

• dist_zero_to_nominal (`float`) – Used with the “bounding barrier” to define the distance from either of the x_nominal points to the region of zero error. Must be less than half the distance between x_nominal_lower and x_nominal_upper, and must be greater than zero.

• power (`float`) – The power used to describe the curve of the error tails. Note that when applying the barrier function to a residual used in a least-squares problem, a power = 1 will lead to a quadratic cost in the optimization problem.

• centering_scale (`float`) – Used to define the shape of the “centering barrier”. Must be between zero and one. The centering barrier passes through (x_nominal_lower, centering_scale * error_nominal), ((x_nominal_lower + x_nominal_upper) / 2, 0), and (x_nominal_upper, centering_scale * error_nominal).

Return type:

`float`