Paper: LaVie: High-Quality Video Generation with Cascaded Latent Diffusion Models (ICLR-2024)

Reviewer comment (Novelty):
I think the article lacks technical innovation. Its core contribution involves extending the pre-trained LDM to a video generation model, introducing temporal attention, and emphasizing joint image-video training. However, similar techniques have been mentioned in previous works, such as VDM and Align your latent. The authors should clarify the distinguishing aspects of these innovation.

Author response:
we consider our contributions are system-level rather than model-level. We leverage a "base-interpolation-video super resolution" system design.

For the entire system, we made the following contributions:
- Model design: we propose a simple yet effective method to involve RoPE-equipped temporal self-attention in our base, interpolation and video super resolution models. Experiments show that such design is able to produce videos with temporal coherency.
- Training: we studied different training schemes in base, interpolation and super-resolution models, and proved that when given a pre-trained text-to-image generation model, joint image-video fine-tuning is an effective way to overcome catatrophical forgeting, as well as to obtain high quality videos. 
- Evaluation: T2V is a novel video generation task, where we think novel evaluation is required. Different from other approaches by simply reporting human preference and FVD, we conduct a systematic human evaluation on various dimensions to compare with state-of-the-art.
- Dataset: We propose a novel text-video dataset Vimeo25M to improve the performance of text-to-video generation tasks.

Reviewer comment (Experiments):
The article presents several assertions without strong empirical support or ablation study. For instance, the introduction of RoPE as a positional encoding lacks corresponding experimental evidence to demonstrate its effectiveness and advantages over other encoding methods. The statement, "we validate that the process of joint image-video fine-tuning plays a pivotal role," lacks experimental substantiation in the article.

Author response:
We have conducted small-scale experiments on UCF101 for comparison. We explore three different settings. 1)  Our proposed temporal module, 2) replacing RoPE in our module with absolute PE, 3) spatial-temporal self-attention without PE used in [1], We train three models using same iterations and report FVD in Table


|Method|Ours|absolute PE|spatio-temporal self-attention|
|----|----|----|----|
|FVD|611|625|635|

Small-scale experimental results demonstrate the effectiveness of our proposed temporal module. 
We show results with and without image-video joint fine-tuning in our updated supplementary material website. Results show that, without image-video joint tuning, catatrophical forgeting appear in the generated results. Such as the teddy bear and Iron Man could not be well generated, the entire system overfit to training video data.

Reviewer comment (Writing):
The writing should be improved, and there exist some unclear explanations. For instance, the modules in Figure 2 are not adequately introduced in the text, causing confusion. For example, I want to know whether 'E' denotes the encoder or denoiser? Additionally, the article mentions "By applying rigorous filtering criteria" to construct Vimeo25M, but the specific criteria are not outlined. Will this dataset be made publicly available?

Author response:
Thanks to the reviewer for pointing it out. E represents Encoder in VAE. We have updated the figure, as well as the caption in the revised manuscript.

The original videos are sourced from Vimeo. To obtain the dataset, we utilized PySceneDetect for scene detection and segmentation of the primary videos. We implemented camera motion detection and static video analysis primarily to filter out videos with camera shake and static footage. We retained only those segments where the camera was either stationary or moving gently, while segments with entirely static footage were eliminated. To ensure the quality of captions, we filtered out captions with fewer than three words and excluded video segments with fewer than 16 frames.

Yes, Vimeo25M will be publicly available.

[1] Wu et al, Tune-a-video: One-shot tuning of image diffusion models for text-to-video generation, CVPR 2023

Reviewer comment (Novelty):
The technical novelty is limited. The authors present two findings which have both been proposed in previous works. 1) Temporal attention over a pretrained text-to-image model could address the temporal consistency model well. This idea is first stated in “AnimateDiff: Animate Your Personalized Text-to-Image Diffusion Models without Specific Tuning” and then pointed out in "MagicEdit: High-Fidelity and Temporally Coherent Video Editing". 2) Joint image-video training is a well-known technique to improve the effectiveness of training text-to-video models. It was first proposed by Jonathan Ho et al. in "Video Diffusion Models".

Author response:
1) We think LaVie is a concurrent work compared to AnimateDiff and MagicEdit since both works have not been peer-reviewed and are only on arXiv. 

2) Joint image-video training is a well-known technique for both video understanding [1] and video generation for a long time. In this work, we studied how to leverage this technique to fine-tune a pretrained Stable Diffusion initialized T2V system. We demonstrate that joint image-video fine-tuning is the key to obtaining good results for base, interpolation, as well as video super resolution models. We have also shown that such technique helps to prevent catastrophic forgeting. However, there are still open questions about using this technique, such as 1) is there any balance between image and video, and 2) how to also leverage images in temporal modules to make the best usage of the entire training data. Such questions are also related to design effective learning approaches in video diffusion models. Therefore, we consider that our contributions are not in this technique itself, but the analysis of this technique, as well as opening new doors for future usage and improvement.

Reviewer comment (Novelty):
The idea of the cascaded diffusion model, which first trains a base model then temporally interpolates it, and finally performs super-resolution spatially, is a standard procedure to learn a text-to-video model. Similar ideas are also proposed in "Make-A-Video: Text-to-video Generation Without Text-Video Data", "Imagen Video: High Definition Video Generation with Diffusion Models" and "Align your Latents: High-Resolution Video Synthesis with Latent Diffusion Models".

Author response:
We have already discussed the differences with listed works in the original manuscript in Related Works.  

Since the "base-interpolation-SR" framework has become a standard design for high-quality video generation, we consider our contributions are system-level rather than model-level. 

For the entire system, we made the following contributions:
- Model design: we propose a simple yet effective method to involve RoPE-equipped temporal self-attention in our base, interpolation and video super resolution models. Experiments show that such design is able to produce videos with temporal coherency.
- Training: we studied different training schemes in base, interpolation and super-resolution models, and proved that when given a pre-trained text-to-image generation model, joint image-video fine-tuning is an effective way to overcome catatrophical forgeting, as well as to obtain high quality videos. 
- Evaluation: T2V is a novel video generation task, where we think novel evaluation is required. Different from other approaches by simply reporting human preference and FVD, we conduct a systematic human evaluation on various dimensions to compare with state-of-the-art.
- Dataset: We propose a novel text-video dataset Vimeo25M to improve the performance of text-to-video generation tasks.

Reviewer comment (Reproducibility):
Will the authors release the newly proposed video dataset Vimeo25M? Since it seems that there is no promise from the authors that they will release the dataset. I would say that the major contribution of the paper is the dataset.

Author response:
Yes, Vimeo25m will be released.

Reviewer comment (Reproducibility):
Availability of Contributed Dataset: A significant contribution highlighted in this paper is the introduction of the Vimeo25M dataset. However, it's crucial to clarify that, as of now, this dataset has not been made publicly available for utilization and further exploration.

Author response:
**A1.** Yes, the Vimeo25M dataset will be publicly available soon.

Reviewer comment (Experiments):
The claim:  

> simple temporal self-attention coupled with RoPE (Su et al., 2021) adequately captures temporal correlations inherent in video data. More complex architectural design only results in marginal visual improvements to the generated outcomes  

isn't supported by experimental results.

Author response:
**A2.**  For the temporal module, we have conducted small-scale experiments on UCF101 for comparison. We explore three different settings. 1)  Our proposed temporal module, 2) replacing RoPE in our module with absolute PE, 3) spatial-temporal self-attention used in [1], We train three models using same iterations and report FVD in Table 

|Method|Ours|absolute PE|spatio-temporal self-attention|
|----|----|----|----|
FVD|611|625|635|

While 3) is much complex than our proposed temporal module,  we didn't find improvement in visual quality and training is slower. Small-scale quantitative evaluation demonstrates the effectiveness of our proposed temporal module.

Reviewer comment (Experiments):
Positional Embedding: Is the utilization of Rotary Positional Encoding (RoPE) distinctly advantageous compared to other available options, such as learnable embeddings? The clarity of the benefits associated with using RoPE, as opposed to its counterparts, would be appreciated.

Author response:
**A3.** We have conducted experiments to compare RoPE with absolute PE on UCF101. Results show that we are able to obtain lower FVD using RoPE. 

In LLM, compared to absolute PE, RoPE is able to extropolate longer text length beyond training length. Our motivation is to adapt this property also for video generation, e.g., training on 16 frames and inference for 32 or even more frames. We show results in updated supplementary material website. We train on 16 frames, and generate 32, 48, 64 frames for each prompt. Results show that we are able to generate longer videos with good spatial quality. While our method is able to generate video length beyond training data, we found flickering becomes larger when the generated videos become longer. Hence, we believe that it is still an open question about how to design better PE for video generation.

Reviewer comment (Writing):
Clarification on Image Concatenation: The process involving the concatenation of M images along the temporal axis to formulate a T-frame video raises a query. Specifically, is there an equivalence between the variables `M` and `T` in this context?

Author response:
**A4.** In our experiments, we try to make the best use of our GPU memory towards adding as many images as we could for each video. Currently, we add T=4 images to each M=16 frames videos. Benchmarking M and T is hard at current stage as it requires conducting abalation experiments on large-scale datasets, which may require months computing. We will leave it in our future work to explore.

Reviewer comment (Reproducibility):
Query on SA-T Removal in Image Training: Figure 3 (c) seems to imply that the SA-T is omitted during the image training phase. Could you provide a more comprehensive explanation or clarification regarding this aspect?

Author response:
**A5.** Yes, SA-T is omitted during the image training phase. The reason is, temporal self-attention is not able to be conducted on images since there is no temporal correlation between random concatenated images. Images are only used for optimizing spatial parts.

Reviewer comment (Theory):
Interaction of Text Conditioning with Spatial Attention: The paper suggests that text conditioning primarily interacts with spatial attention. Given this, how does text effectively influence or control motion within the model? More details regarding the interplay between text and motion would enhance understanding.

Author response:
**A6.** We have explored to add an extra temporal-level cross-attention for text input. However, we didn't observe huge differences in the generated results. The reason could be while text has been used in spatial-level cross-attention, it can still be considered as a type of condition for the entire video. When the entire spatio-temporal networks are optimized, such conditions will as well affect the temporal module, hence control motion.

Reviewer comment (Novelty):
The technical novelty of the proposed method is limited as it is a combination of existing techniques, including text-to-image generation, frame interpolation, and super-resolution.

Author response:
Since the "base-interpolation-SR" framework has become a standard design for high-quality video generation, we consider our contributions are system-level rather than model-level. 
For the entire system, we made the following contributions:
- Model design: we propose a simple yet effective method to involve RoPE-equipped temporal self-attention in our base, interpolation and video super resolution models. Experiments show that such design is able to produce videos with temporal coherency.
- Training: we studied different training schemes in base, interpolation and super-resolution models, and proved that when given a pre-trained text-to-image generation model, joint image-video fine-tuning is an effective way to overcome catatrophical forgetting, as well as to obtain high quality videos. 
- Evaluation: T2V is a novel video generation task, where we think novel evaluation is required. Different from other approaches by simply reporting human preference and FVD, we conduct a systematic human evaluation on various dimensions to compare with state-of-the-art.
- Dataset: We propose a novel text-video dataset Vimeo25M to improve the performance of text-to-video generation tasks.

Reviewer comment (Novelty):
Dandi et al.'s work on “Jointly Trained Image and Video Generation using Residual Vectors" is relevant to this paper's exploration of joint image-video training. However, this paper is neither cited nor discussed.

Author response:
We have included this paper in Related Works in the revised manuscript.

This paper proposed a GAN-based framework to generate videos. It combined an LSTM and an image GAN to learn disentangled temporal and spatial information. Joint image-video training is conducted to learn the entire framework. 

Our framework is a large-scale diffusion-based system. Since diffusion model does not have a compressed latent space, our temporal module is added after each spatial block. Our joint image-video fine-tuning uses different large-scale image and video datasets, and in each batch, we concatenate images with videos in the temporal axis. Dandi et al. proposed using the same video to optimize both spatial and temporal modules. We train spatial and temporal modules jointly while Dandi et al. proposed a two-step training scheme to train the framework separately.

Reviewer comment (Experiments):
There is no analysis of the contribution of individual components or techniques on the video generation performance. The paper mentions the temporal module, joint image-video training, and usage of Vimeo25M dataset for training. However, we don’t see a comprehensive analysis on the impact of each of them on the performance. It’s hard to understand which component has a higher impact on the final model. The only analysis we see is in Fig 10 which is very limited by showing only three images.

Author response:
**For the temporal module**, we have conducted small-scale experiments on UCF101 for comparison. We explore three different settings. 1)  Our proposed temporal module, 2) replacing RoPE in our module with absolute PE, 3) spatial-temporal self-attention used in [1], We train three models for same iterations and report FVD in Table
| Method | Ours | absolute PE | spatio-temporal self-attention |
| ---- | ----  | ---- | ---- |
| FVD |  611 | 625 | 635 |

Quantitative results demonstrate the effectiveness of our proposed temporal module.

**For joint image-video tune-tuning**, we qualitatively show several examples in the updated Supplementary Material website. From the results, we can observe that without joint image-video fine-tuning, the network catastrophically forgets previous learned concepts and overfits to training data very quickly. On the opposite, using images with videos together to fine-tune the entire framework helps combine different concepts in both image and video data, which we found can help improve the diversity of the results.

**For the usage of Vimeo25M data**, we have updated Tab.1 in the revised manuscript for comparison.  Without using Vimeo, we obtain higher FVD on zero-shot generation on UCF101. In addition, as shown in Tab. 4, since Vimeo25M has better aesthetics, and large-amount human data, the generated results have smoother motion and better face/body/hand quality compared with other method.

Reviewer comment (Reproducibility):
The role of the temporal self-attention module (SA-T) during image-only training phases is ambiguous. It's unclear from Figure 3 whether SA-T is frozen or entirely excluded from the process.

Author response:
Image and video are simultaneously used to train the model. Video data is used to optimize both spatial and temporal modules while images are only used to optimize the spatial part.  Since SA-T is not able to be applied to images, when optimizing the network using images, SA-T is entirely excluded from the process.

Reviewer comment (Novelty):
It’s not clear what is the benefit of the technique explained on page 4: "our approach differs from conventional video frame interpolation methods, as each frame generated through interpolation replaces the corresponding input frame. In other words, every frame in the output is newly synthesized"

Author response:
In our interpolation process, input frames are also reconstructed. In conventional video interpolation methods, input frames are directly combined with the generated frames. We found, directly reusing the input frames in generated videos leads to flickering in the final results. Generating the entire video, including input frames, makes the results have more temporally coherency.

Reviewer comment (Experiments):
How did authors make sure there is enough correlation between text and video segment? Details of text-video pair selection are missing.

Author response:
In the current version, we didn't conduct text-video selection process. We use the entire WebVid10M dataset, as well as filtered Vimeo25M dataset. In future work, we plan to use [2] to compute text-video correlation score for selecting videos which have higher correspodance with text.

Reviewer comment (Presentation):
In Fig. 9 (b) statistics on resolution are not clear. what “99.9%” means?

Author response:
It means that the height of 99.9% of videos in the WebVid10M dataset is in the range of 400~600. We will improve the figure in the final version.

Reviewer comment (Evaluation):
There is no comprehensive evaluation of diversity (since the paper claimed to improve it) besides Fig. 4. Did the authors consider evaluating diversity with human evaluation?

Author response:
At current stage, we use FVD to report both generated video quality and diversity, and we show improvement compared to other approaches. In future work, we plan to designed around 20 prompts and generate 10 videos per prompt using different methods. Human raters will be asked to compare which method is able to generate more diverse results per prompt. Final diversity will be reported based on the average of all the prompt results.