Training a latent diffusion model differs from standard diffusion models primarily in how they handle data representation and computational efficiency. Standard diffusion models operate directly in the pixel space of images, gradually adding and removing noise during training and inference. This approach requires high computational resources, especially for large images, since every pixel is processed. Latent diffusion models (LDMs), on the other hand, compress input data into a lower-dimensional latent space using an autoencoder. The diffusion process occurs in this compressed space, reducing computational demands while maintaining quality. For example, Stable Diffusion uses a variational autoencoder (VAE) to encode images into latent representations, allowing the diffusion model to work with smaller tensors (e.g., 64x64 instead of 512x512 pixels), which speeds up training and inference.
The training process for LDMs involves two main components: the autoencoder and the diffusion model. First, the autoencoder is trained separately to encode data into a meaningful latent space and decode it back accurately. This step ensures the latent space retains essential features of the original data. Once the autoencoder is stable, the diffusion model is trained in the latent space. Similar to standard diffusion, noise is iteratively added to the latent vectors, and the model learns to reverse this process by predicting the noise at each step. However, LDMs often incorporate conditioning mechanisms, such as text prompts, using cross-attention layers. For instance, in text-to-image generation, the model learns to associate textual embeddings with corresponding latent features during training. This conditioning allows controlled generation while keeping computations manageable due to the reduced dimensionality.
A practical advantage of LDMs is their scalability. By avoiding pixel-level processing, they require less memory and compute power, making them accessible for developers with limited resources. For example, training a standard diffusion model on high-resolution images might need multiple high-end GPUs, whereas an LDM like Stable Diffusion can be fine-tuned on a single consumer GPU. However, LDMs introduce dependencies on the autoencoder’s quality—poor compression can lead to artifacts or loss of detail. Developers must balance the autoencoder’s compression ratio with reconstruction fidelity. Additionally, LDMs often use techniques like classifier-free guidance to improve sample quality, which adjusts the conditioning strength during inference. These trade-offs make LDMs a flexible choice for tasks like image generation, inpainting, or style transfer, where efficiency and control are priorities.