lesson 3 / 6·Reparameterisation trick15 min · Bloom's apply

Reparameterisation trick

objective Trace gradients through sampling

md[1]

3 · Reparameterisation trick

Instead of sampling `z ~ N(μ, σ²)` directly (which blocks gradients), we reparameterise as `z = μ + σ · ε` where `ε ~ N(0, I)`. Kingma & Welling 2014 showed this makes the sampling differentiable w.r.t. `μ` and `σ`.

In [7][2]
python
import torch

def reparameterise(mu, log_var):
    sigma = torch.exp(0.5 * log_var)
    eps = torch.randn_like(sigma)
    return mu + sigma * eps

mu = torch.zeros(1, 32)
logvar = torch.zeros(1, 32)
z = reparameterise(mu, logvar)
print(z.shape, z.mean().item())
out [7]
torch.Size([1, 32]) -0.018
viz[3]
Sampled z points in latent space (2-d projection, 200 samples).
ex[4]
exercise
Task — change `mu` to a non-zero tensor and `logvar` to `-2.0`. What happens to the variance of `z`? Run it, then answer the quiz below.
?[5]
check your understanding

What is the role of `ε` in the reparameterisation trick?

sh[6]
$ python train.py --epochs 3 --lr 1e-3
Epoch 1/3 · loss 184.2
Epoch 2/3 · loss 168.7
Epoch 3/3 · loss 156.1
Checkpoint → ./runs/vae_mnist.pt
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Last updated: June 2026