I think in the left panel of the figure https://visionbook.mit.edu/gradient_descent.html#fig-gradient_descent-momentum_out1, the loss $J$ should be 1 when $\theta=-1$ or +1, not 0.5. It's also not clear why the colormap ranges between +1 and -1 if $J=abs(\theta)$ (also check the subsequent figures of the chapter, actually).
PS Amazing book!
I think in the left panel of the figure https://visionbook.mit.edu/gradient_descent.html#fig-gradient_descent-momentum_out1, the loss$J$ should be 1 when $\theta=-1$ or +1, not 0.5. It's also not clear why the colormap ranges between +1 and -1 if $J=abs(\theta)$ (also check the subsequent figures of the chapter, actually).
PS Amazing book!