# View Source Bumblebee.Diffusion.DdimScheduler (Bumblebee v0.4.2)

Denoising diffusion implicit models (DDIMs).

This sampling method was proposed as a follow up to the original denoising diffusion probabilistic models (DDPMs) in order to heavily reduce the number of steps during inference. DDPMs model the diffusion process as a Markov chain; DDIMs generalize this considering non-Markovian diffusion processes that lead to the same objective. This enables a reverse process with many less samples, as compared to DDPMs, while using the same denoising model.

DDIMs were shown to be a simple variant of pseudo numerical methods
for diffusion models (PNDMs), see `Bumblebee.Diffusion.PndmScheduler`

and the corresponding paper for more details.

## Configuration

`:num_train_steps`

- the number of diffusion steps used to train the model. Defaults to`1000`

`:beta_schedule`

- the beta schedule type, a mapping from a beta range to a sequence of betas for stepping the model. Either of`:linear`

,`:quadratic`

, or`:squared_cosine`

. Defaults to`:linear`

`:beta_start`

- the start value for the beta schedule. Defaults to`0.0001`

`:beta_end`

- the end value for the beta schedule. Defaults to`0.02`

`:prediction_type`

- prediction type of the denoising model. Either of:`:noise`

(default) - the model predicts the noise of the diffusion process`:angular_velocity`

- the model predicts velocity in angular parameterization. See Section 2.4 in Imagen Video: High Definition Video Generation with Diffusion Models, then Section 4 in Progressive Distillation for Fast Sampling of Diffusion Models and Appendix D

. Defaults to

`:noise`

`:alpha_clip_strategy`

- each step $t$ uses the values of $\bar{\alpha}_t$ and $\bar{\alpha}_{t-1}$, however for $t = 0$ there is no previous alpha. The strategy can be either`:one`

($\bar{\alpha}_{t-1} = 1$) or`:alpha_zero`

($\bar{\alpha}_{t-1} = \bar{\alpha}_0$) . Defaults to`:one`

`:timesteps_offset`

- an offset added to the inference steps. You can use a combination of`timesteps_offset: 1`

and`alpha_clip_strategy: :alpha_zero`

, so that the last step $t = 1$ uses $\bar{\alpha}_1$ and $\bar{\alpha}_0$, as done in stable diffusion . Defaults to`0`

`:clip_denoised_sample`

- whether to clip the predicted denoised sample ($x_0$ in Equation (12)) into $[-1, 1]$ for numerical stability. . Defaults to`true`

`:rederive_noise`

- whether the noise (output of the denoising model) should be re-derived at each step based on the predicted denoised sample ($x_0$) and the current sample. This technique is used in OpenAI GLIDE . Defaults to`false`

`:eta`

- a weight for the noise added in a denoising diffusion step. This scales the value of $\sigma_t$ in Equation (12) in the original paper, as per Equation (16) . Defaults to`0.0`