# Visualizing high-dimensional functions with cross-sections

Last September, I gave a talk which included a bunch of two-dimensional plots of a high-dimensional objective I was developing specialized algorithms for optimizing. A month later, at least three of my colleagues told me that my plots had inspired them to make similar plots. The plotting trick is really simple and not original, but nonetheless I'll still write it up for all to enjoy.

Example plot: This image shows cross-sections of two related functions: a non-smooth (black) and a smooth approximating function (blue). The plot shows that the approximation is faithful to the overall shape, but sometimes over-smooths. In this case, we miss the maximum, which happens near the middle of the figure.

Details: Let $f: \mathbb{R}^d \rightarrow \mathbb{R}$ be a high-dimensional function ($d \gg 2$), which you'd like to visualize. Unfortunately, you are like me and can't see in high-dimensions what do you do?

One simple thing to do is take a nonzero vector $\boldsymbol{d} \in \mathbb{R}^d$, take a point of interest $\boldsymbol{x}$, and build a local picture of $f$ by evaluating it at various intervals along the chosen direction as follows,

$$f_i = f(\boldsymbol{x} + \alpha_i \ \boldsymbol{d}) \ \ \text{for } \alpha_i \in [\alpha_\min, \alpha_\max]$$

Of course, you'll have to pick a reasonable range and discretize it. Note, $\boldsymbol{x}$ and $\boldsymbol{d}$ are fixed for all $\alpha_i$. Now, you can plot $(\alpha_i,f_i)$.

Picking directions: There are many alternatives for picking $\boldsymbol{d}$, my favorites are:

1. Gradient (if it exists), this direction is guaranteed to show a local increase/decrease in the objective, unless it's zero.

2. Coordinate vectors. Varying one dimension per plot.

3. Random. I recommend directions drawn from a spherical Gaussian.1 The reason being that such a vector is uniformly distributed across all unit-length directions (i.e., the angle of the vector, not it's length). We will vary the length ourselves via $\alpha$. It's probably best that our plots don't randomly vary in scale.

Extension to 3d: It's pretty easy to extend this generating 3d plots by using 2 vectors, $\boldsymbol{d_1}$ and $\boldsymbol{d_2}$, and varying two parameters $\alpha$ and $\beta$,

$$f(\boldsymbol{x} + \alpha \ \boldsymbol{d_1} + \beta \ \boldsymbol{d_2})$$

Closing remarks: These types of plots are probably best used to: empirically verify/explore properties of an objective function, compare approximations, test sensitivity to certain parameters/hyperparameters, visually debug optimization algorithms.

## Notes

1. More formally, vectors drawn from a spherical Gaussian are points uniformly distributed on the surface of a $d$-dimensional unit sphere, $\mathbb{S}^d$. Sampling a vector from a spherical Gaussian is straightforward: sample $\boldsymbol{d'} \sim \mathcal{N}(\boldsymbol{0},\boldsymbol{I})$, $\boldsymbol{d} = \boldsymbol{d'} / \| \boldsymbol{d'} \|_2$