Title: Blockwise Flow Matching: Improving Flow Matching Models For Efficient High-Quality Generation
Venue: NeurIPS 2025
Accepted: yes
Keywords: Diffusion; Flow matching; efficient generation
Abstract: Recently, Flow Matching models have pushed the boundaries of high-fidelity data generation across a wide range of domains. It typically employs a single large network to learn the entire generative trajectory from noise to data. Despite their effectiveness, this design struggles to capture distinct signal characteristics across timesteps simultaneously and incurs substantial inference costs due to the iterative evaluation of the entire model. To address these limitations, we propose Blockwise Flow Matching (BFM), a novel framework that partitions the generative trajectory into multiple temporal segments, each modeled by smaller but specialized velocity blocks. This blockwise design enables each block to specialize effectively in its designated interval, improving inference efficiency and sample quality. To further enhance generation fidelity, we introduce a Semantic Feature Guidance module that explicitly conditions velocity blocks on semantically rich features aligned with pretrained representations.
Additionally, we propose a lightweight Feature Residual Approximation strategy that preserves semantic quality while significantly reducing inference cost.
Extensive experiments on ImageNet 256x256 demonstrate that BFM establishes a substantially improved Pareto frontier over existing Flow Matching methods, achieving 2.1x to 4.9x accelerations in inference complexity at comparable generation performance.

Blockwise Flow Matching: Improving Flow Matching Models For Efficient High-Quality Generation

Abstract

Recently, Flow Matching models have pushed the boundaries of high-fidelity data generation across a wide range of domains. It typically employs a single large network to learn the entire generative trajectory from noise to data. Despite their effectiveness, this design struggles to capture distinct signal characteristics across timesteps simultaneously and incurs substantial inference costs due to the iterative evaluation of the entire model. To address these limitations, we propose Blockwise Flow Matching (BFM), a novel framework that partitions the generative trajectory into multiple temporal segments, each modeled by smaller but specialized velocity blocks. This blockwise design enables each block to specialize effectively in its designated interval, improving inference efficiency and sample quality. To further enhance generation fidelity, we introduce a Semantic Feature Guidance module that explicitly conditions velocity blocks on semantically rich features aligned with pretrained representations. Additionally, we propose a lightweight Feature Residual Approximation strategy that preserves semantic quality while significantly reducing inference cost. Extensive experiments on ImageNet 256 × 256 demonstrate that BFM establishes a substantially improved Pareto frontier over existing Flow Matching methods, achieving 2.1 × to 4.9 × accelerations in inference complexity at comparable generation performance. Code is available at https://github.com/mlvlab/BFM.

1 Introduction

Recent advances in generative modeling have been driven by the success of diffusion and flowmatching frameworks [1-4], which learn to transform noise into high-quality data through a sequence of denoising steps. Powered by advanced transformer architectures [5, 6], these approaches have expanded the frontiers of high-fidelity data generation across many fields, including images [7-10], 3D data [11-14] and videos [15-19]. In particular, the Flow Matching (FM) framework [3, 20-22] has emerged as a simple yet effective training paradigm, adopted by several state-of-the-art generative models [23, 24].

A common design choice in FM models is to use a single large neural network to learn the entire velocity field, from noise to data. While parameter-efficient, this monolithic design faces two key limitations. First, the generative trajectory from noise to data inherently involves distinct signal characteristics across time [4]: our frequency-domain analysis (Figure 3) shows that early timesteps

Blockwise Flow Matching (Ours)

𝑥

!

𝑥

"

𝑡

𝑡

𝑥

𝑥

Velocity

Block 2

Velocity

Block 3

Velocity

Block 4

Velocity

Block 5

𝑥

!

𝑥

"

Velocity

Block 1

Smaller &

Specialized

4.9x more efficient

2.1x more efficient

FID (lower is better)

FLOPs (G) (lower is better)

MDTv2-XL

20

SiT-XL

REPA-XL

BFM-XLsF (Ours)

BFM-XLsF-RA (Ours)

8

6

4

2

0

2500

5000

750010000 12500 15000 17500 20000

are dominated by irregular, low-frequency patterns, while later timesteps require modeling refined, high-frequency details. This temporal heterogeneity imposes conflicting demands on a single shared model, limiting its ability to effectively capture both coarse global structure and subtle local variations simultaneously. Second, using one large model across timesteps incurs high inference costs, as the full model must be evaluated at every solver step, resulting in a total computational complexity of O ( SK ) , where S is the number of steps and K is the model's size in FLOPs. Together, these limitations make it challenging to balance the trade-off between sample quality and inference efficiency.

To address these challenges, we propose Blockwise Flow Matching (BFM), a novel framework that rethinks the monolithic architecture of standard FM. Instead of training a large, shared model, BFM partitions the flow trajectory into distinct temporal segments, each handled by a smaller, specialized velocity block (Figure 1). This segmentation strategy allows each block to focus on a narrower temporal window and better adapt to the specific signal characteristics, as supported by our spectral power analysis (Figure 6). As a result, BFM not only improves generation quality but also reduces inference cost by activating only a subset of the model at each timestep.

However, temporal segmentation limits each block's exposure to clean signals, potentially restricting their capacity to learn high-level semantic features. To mitigate this, we propose a Semantic Feature Guidance (SemFeat), a module that explicitly conditions each velocity block on semantic features aligned with powerful pretrained encoders, such as DINOv2 [25] (Figure 2a). SemFeat significantly enhances the generation fidelity, at a cost of increased computation. To reduce this, we propose an additional lightweight module, Feature Residual Approximation , which efficiently approximates semantic features during inference, preserving sample quality while significantly reducing computational cost (Figure 2b and 2c).

Through extensive experiments on ImageNet at 256 × 256 resolution, we demonstrate that BFM establishes a more favorable Pareto frontier, achieving substantial improvements in both generation quality and inference efficiency over existing state-of-the-art flow matching methods (Figure 1, Table 2). Notably, we achieve comparable or better generation quality with a 2.1 × to 4.9 × reduction in inference complexity compared to current best-performing models [26], highlighting the effectiveness of our proposed components.

Our main contributions are summarized as follows:

We propose Blockwise Flow Matching (BFM), a novel framework that divides the generative trajectory into temporal segments, each modeled by smaller but specialized velocity blocks. This blockwise modeling improves both generation quality and inference efficiency.

We introduce Semantic Feature Guidance and Feature Residual Approximation modules that significantly enhance visual fidelity while preserving efficiency gains enabled by BFM.

Extensive experiments demonstrate that BFM achieves a substantially improved Pareto frontier on ImageNet 256 × 256 , yielding 2.1 × to 4.9 × faster inference than state-of-the-art generative models with comparable performance.

2 Related Work

2.1 Diffusion and Flow Matching models

Diffusion models [1, 2, 15, 27, 28] have emerged as powerful tools for generative modeling, achieving state-of-the-art performance across diverse domains [4, 13, 15, 29-31]. More recently, Flow Matching (FM) [3, 20, 21, 32] has been proposed as a general framework that unifies and extends diffusion-based approaches. By directly modeling continuous-time velocity fields between noise and data, FM enables simulation-free training with improved stability and sample quality. Building on this foundation, transformer-based FM architectures like SiT [22] and FiT [6] have achieved strong performance across various generative tasks. However, these models remain computationally expensive at inference time, due to their large model size and the need for repeated evaluations across solver steps. In parallel, there has been growing interest in integrating semantic representation learning into generative models [26, 33, 34]. For instance, RCG [33], REPA [26], and DDT [35] show that semantic features from pretrained visual encoders (e.g., CLIP [36] or DINO [25]) can significantly improve generation fidelity. While these approaches enrich representations, they does not consider distinct characteristics at different diffusion timesteps and are computationally inefficient. In contrast, our method introduces Blockwise Flow Matching with our SemFeat module, enabling efficient and high-quality generation.

2.2 Efficient generation

Recent research has explored various strategies to improve the inference efficiency of diffusion and flow-based generative models. One direction involves distillation-based methods, such as consistency models [37-42], distribution matching distillation [43-46], which aim to reduce the number of solver steps by training a student model to imitate the behavior of a pretrained teacher. While effective in accelerating generation, these methods require strong teacher models, multi-stage optimization, and in some cases, adversarial training, which can increase both training complexity and instability. Our method is orthogonal to these approaches and could benefit from distillation techniques to further accelerate generation. Another line of work focuses on model-based compression, which reduces per-step computation by streamlining the network architecture. Prior works such as token merging [47], token pruning [48], and layer pruning [49] propose removing redundant computation during inference. Recently, dynamic inference approaches [50, 51] have introduced input- or timestepaware mechanisms to adjust the computational graph during generation. However, these methods rely on a single shared model to handle the entire generative trajectory. In contrast, our method partitions the generative trajectory into multiple temporal segments, each modeled by a smaller, specialized velocity block. BFM is complementary to pruning and dynamic routing strategies, which could be applied within individual blocks to further improve efficiency.

3 Preliminary

Diffusion and flow-based models [1-3, 20] aim to learn a continuous transformation between a simple reference distribution π 0 (e.g., Gaussian noise) and a target data distribution π 1 . Given samples x 0 ∼ π 0 and x 1 ∼ π 1 , the transformation is defined over a continuous time interval t ∈ [0 , 1] by the following ordinary differential equation:

where x t denotes a time-dependent interpolation between x 0 and x 1 , and v : R d × [0 , 1] → R d is the velocity field defined over the data-time joint domain. The interpolation is formulated as:

Different choices of interpolation coefficients α t and σ t yield different instantiations of diffusion or flow models [20, 52]. Following recent practices [22], we adopt a linear interpolation schedule: α t = t and σ t = 1 -t . To learn the velocity field, a neural network v θ is trained to approximate the ground-truth conditional velocity field v by minimizing the mean squared error:

This formulation is commonly referred to as the standard flow matching framework for training continuous-time generative models.

ℎ

!

(𝑓

"

)

ℎ

∗

𝑥

"

𝑡

𝑓

"

𝑥

"

𝑡

𝑐

Feature

Alignment

Network

𝑓

!

🔥

ℒ

!"#$%

𝑣

&

(()

ℒ

*+,

Velocity

Block

𝑣

"

($)

🔥

𝑡

$%&

𝑐

𝑡

𝑐

𝑓

"

!"#

𝑓

&

𝑡

𝑓 $

&

ℒ

!"#

Feature

Alignment

Network

𝑥

"

!"#

𝑥

"

𝑥

"

𝑡

𝑥

(a) Blockwise Flow Matching with Semantic Feature Guidance ( Section 4.1 & 4.2 )

(b) Feature Residual Approximation ( Section 4.3 )

(c) Inference ( Section 4.4 )

Feature

Alignment

Network

𝑡

$&'

𝑡

$

lightweight FRN

✅

lightweight FRN

❌

.

𝑥

!

!"#

𝑥

!

!

𝑥

!

𝑥

"

𝑥

#

𝑥

#

𝑥

"

ℎ

$

🔥

𝑡

$&'

𝑡

$

FRN

𝑓

!

🔥

m

"#

block

Pretrained

Encoder

ℰ

𝑥

#

𝑣 %

&

(()

1

装

4 Method

4.1 Blockwise Flow Matching (BFM)

Standard Flow Matching (FM) models [3, 20, 22] train a single large neural network to model the entire flow trajectory from noise to data. However, this design imposes two key limitations. First, it forces a network to simultaneously handle the distinct spectral characteristics at different timesteps [4, 53]. As illustrated in Figure 3, early timesteps are dominated by irregular, low-frequency signals, while later timesteps contain more structured, high-frequency content. This gives conflicting demands on a single model, which may struggle to represent them effectively. Second, it results in high inference cost, as the full model must be evaluated at every solver step, yielding a total complexity of O ( SK ) , where S is the number of solver steps and K is the model's size in FLOPs.

To address these limitations, we propose Blockwise Flow Matching (BFM) , which partitions the flow trajectory into a sequence of specialized velocity blocks of model size K ′ ,

Irregular

and

low frequency

Regular

and

high frequency

each focusing on a distinct temporal segment. By dedicating each block to a smaller temporal region, these blocks can efficiently capture interval-specific dynamics, allowing the use of smaller networks with size K ′ < K . Consequently, at each timestep, only the corresponding block is evaluated, reducing the inference complexity. Formally, consider flow trajectories defined from pure noise at timestep t = 0 to clean data at timestep t = 1 . We divide this interval into M non-overlapping segments defined by intervals { [ t m -1 , t m ) } M m =1 , where 0 = t 0 < t 1 < · · · < t M = 1 , and each segment is modeled by its velocity block v ( m ) θ . To train the m -th block on segment [ t m -1 , t m ) , we

define the corresponding start and end points of the segment by interpolating between a clean data sample and a Gaussian noise sample:

We use sample noise for Equation (4) and (5) to enhance the straightness of the flow trajectory across segments. For a randomly sampled t ∈ [ t m -1 , t m ) , we sample intermediate data x t as:

where a m ( t ) = ( t -t m -1 ) / ( t m -t m -1 ) . Then, the ground-truth conditional velocity at time t within the m -th segment is given by:

We train each block v ( m ) θ to predict the target velocity using the blockwise flow-matching loss:

where c denotes optional conditioning information, e.g., class labels. We omit t for notation simplicity. During inference, given a current state x t and timestep t , we evaluate only the corresponding velocity block v ( m ) θ . As empirically demonstrated in Table 1, our proposed BFM-S with six segments (second row) reduces overall inference complexity (FLOPs) while improving generation quality (FID) compared to the standard FM baseline (SiT-S [22]) using the identical transformer blocks.

4.2 Semantic Feature Guidance

While our blockwise modeling enables efficient inference and segment-wise specialization, the temporal scope of each velocity block restricts its exposure to the clean data distribution. In particular, blocks operating in early timesteps primarily observe noisy samples, making it challenging to capture high-level semantic features. Motivated by recent studies [26, 33, 34] demonstrating the effectiveness of semantic representations in generative performance, we propose a semantic conditioning technique, called Semantic Feature Guidance (SemFeat), which enhances each velocity block by conditioning rich semantic context f t (Figure 2a).

Specifically, we introduce a shared feature alignment network f ϕ which aims to extract robust semantic features from noisy intermediate states x t . To ensure these extracted features carry strong global semantic information, we train this alignment network by aligning its output features with embeddings from pretrained visual encoder E , such as DINOv2 [25]. Given an intermediate noisy input x t at timestep t , the alignment network produces a semantic feature: f t = f ϕ ( x t , c ) , where f ϕ : R d x × [0 , 1] × R d c → R d x and c ∈ R d c denotes conditioning information. This feature is trained to match the pretrained embedding h ∗ = E ( x 1 ) by minimizing the following feature alignment loss:

where h ψ is the learnable projection MLP layer to match the dimensionalities, and d ( · , · ) is a distance metric such as cosine similarity. Then, the velocity block v ( m ) θ takes the current state

x t , timestep t , condiction c , and semantic feature f t as inputs to estimate the conditional velocity field: ˆ v ( m ) t = v ( m ) θ ( x t , c, f t ) . The blockwise flow matching loss with the semantic feature guidance becomes:

Finally, the overall training objective is a weighted sum of the blockwise flow matching and feature alignment losses:

where λ controls the contribution of the feature alignment objective. Refer to Algorithm 1 for training pseudo code. This conditioning strategy allows each block to access semantically rich guidance, significantly improving generation quality. As demonstrated in Table 1, integrating SemFeat into BFM (third row in Table 1) achieves substantial FID gains compared to the vanilla BFM.

Comparision with REPA [26]. Our SemFeat is inspired by recent works in representation-aligned diffusion, particularly REPA. However, there is a key difference in design: REPA aligns the internal hidden states of the velocity model directly with semantic features. In contrast, SemFeat introduces a dedicated alignment network f ϕ whose output is explicitly conditioned on the velocity blocks, thereby separating feature alignment from velocity prediction. We observe that this modular design leads to a more coherent and rich representation (Figure 8) and improved generation quality (Table 6).

4.3 Feature Residual Approximation

The feature alignment network f ϕ significantly improves generation quality by providing rich semantic guidance. However, evaluating the alignment network at every timestep during inference can undermine the efficiency gains by our BFM. To address this, we propose training a lightweight network that enables an efficient approximation of semantic features during inference (Figure 2b).

Specifically, after the initial training phase where we jointly optimize the feature alignment network f ϕ and the velocity blocks v θ , we freeze the parameters of both sets of networks. Then, in a second training stage, we introduce a smaller network f η , denoted as Feature Residual Network (FRN), to approximate semantic features within the m -th temporal segment [ t m -1 , t m ) . This is motivated by the observation in Figure 5 that the discrepancy between start point feature f t m -1 and intermediate feature f t increases as t increases within [ t m -1 , t m ) .

Given the start point x t m -1 , we first compute its corresponding semantic feature using the alignment network: f t m -1 = f ϕ ( x t m -1 , c ) . For the subsequent timestep t where t ∈ [ t m -1 , t m ) , we approximate its feature incrementally via a residual connection defined by: ˆ f t = f t m -1 + b m ( t ) · f η ( x t , c ) , where b m ( t ) = t -t m -1 t m -t m -1 is normalized offset from t m -1 . This allows the network f η to scale the residual contribution based on how far the timestep t is from the segment start point t m -1 . FRN f η is trained to match the output of the frozen alignment network f ϕ at intermediate timestep t , i.e., f t = f ϕ ( x t , c ) , using the following feature regression loss:

By leveraging this lightweight residual approximation, we drastically reduce inference cost, since the feature alignment network f ϕ is only computed once per segment, and subsequent semantic features within each segment are computed efficiently by the FRN (see Figure 2c). Empirically, our analysis demonstrates that this residual approximation (third row in Table 1) retains over 98% of the BFM-SSF fidelity while reducing inference complexity by 41% (5.01 to 2.96). Refer to Algorithm 2 for pseudo code.

4.4 Inference

At the beginning of the m -th segment, we compute the semantic feature f t m -1 using the feature alignment network f ϕ . For subsequent timesteps t ∈ [ t m -1 , t m ) , we approximate the semantic feature either using FRN f η or the full alignment network f ϕ , depending on the desired trade-off between speed and fidelity. Then, the velocity block v ( m ) θ predicts the velocity: ˆ v ( m ) t = v ( m ) θ ( x t , c, f t ) . The intermediate state is updated iteratively via a numerical solver using the predicted velocity. This process repeats until reaching t = 1 , yielding the final sample x 1 ∼ p data . A full pseudocode implementation of the inference process is provided in the Appendix (Algorithm 3 and 4).

5 Experiment

We primarily compare our method with recent flow matching models built on diffusion transformer architectures [5, 6, 22, 26], demonstrating state-of-the-art performance in generative tasks. We follow the experimental setup established by SiT [22] and REPA [26]. Most of the experiments are conducted on the ImageNet dataset at a resolution of 256 × 256, unless stated otherwise. We utilize an off-the-shelf pretrained V AE from Stable Diffusion [4], which applies an 8× downsampling factor. We evaluate generation quality using several established metrics, including FID [54], sFID [55], Inception Score (IS) [56], Precision, and Recall [57]. Following recent architectural best practices [5, 6, 22], we adopt a DiT-style transformer backbone for both v θ , f ϕ , and f η . To integrate conditions to v θ , we sum the semantic feature f t with the timestep embeddings element-wise and inject it at the attention layers via AdaLN-Zero modulation [5]. Our model configurations are based on -S and -XL architectures with a patch size of 2 and M = 6 . More implementation details are available in the Appendix A.

5.1 Comparison with state-of-the-art models

In Table 2, we present a comprehensive comparison of our Blockwise Flow Matching (BFM) framework against recent state-of-the-art generative models. Our BFM with Semantic Feature Guidance (BFM-XLSF) achieves an FID score of 1.75, surpassing closely related diffusion transformer models such as FiTv2 [6], DIT [5], SiT [22], and REPA [26]. Notably, compared to the previous best-performing model, REPA, our model demonstrates consistent improvements across FID and IS with a notable improvement of IS by 5.4 points, while also reducing inference complexity in terms of FLOPs by approximately 5% (from 114.5 to 107.8). Furthermore, by incorporating our efficient Feature Residual Approximation module (BFM-XLSF-RA), we achieve an even more substantial inference efficiency gain, a 67% reduction in FLOPs over REPA, corresponding to roughly a 3 × speed-up. When benchmarked against the recent efficiency-focused model, DyDiT-XL λ =0 . 5 [10], our BFM-XLSF-RA not only reduces inference complexity from 57.9 to 37.8 GFLOPs but also significantly improves generation quality, reflected by a 30.1 points increase in IS. These results highlight our model's superior capability in balancing both generation performance and inference efficiency compared to current state-of-the-art approaches.

Few-step sampling. We further evaluate our model's performance under a small number of solver steps in Table 3, an essential setting for achieving efficient and practical image generation. In real-world scenarios, reducing the number of function evaluations (NFEs) directly translates to faster sampling and lower inference cost, often at the expense of generation quality. Remarkably, BFM-XLSF achieves an Inception Score of 248.2 and an FID of 8.01 using only six solver steps , demonstrating strong robustness with minimal degradation as the number of steps decreases. In contrast, SiT-XL and REPA-XL exhibit

substantial performance drops of 91.1 and 122.8 in Inception Score, respectively, whereas BFMXLSF shows a much smaller reduction of 62.8. Furthermore, BFM-XLSF attains an FID of 8.01, outperforming SiT-XL and REPA-XL by large margins of 4.9 and 5.01 FID points, respectively. These results indicate that BFM maintains high-quality generation even under extremely low NFE settings, validating its effectiveness for fast and resource-efficient inference. We attribute this superior few-step performance to the coherent and semantically rich representations provided by our Semantic Feature Guidance , which stabilize the velocity field and enable more consistent dynamics even at early timesteps, as visualized in Figure 8.

5.2 Generalization and Scalability

Compatibility to MeanFlow To further demonstrate the effectiveness of our method, we integrate BFM into the recently proposed MeanFlow [66], which enables high-quality generation using only a few solver steps. Specifically, we adapt our approach by constructing four specialized velocity blocks corresponding to the four-step diffusion trajectory in MeanFlow and applying the MeanFlow objective independently to each temporal segment. This design allows each block to learn distinct dynamics across the generative process, enabling efficient sampling with only four function evaluations (one per segment). As summarized in Table 10, our integrated model (BFMSF+MeanFlow) achieves a substantially lower FID of 9.7 compared to the MeanFlow baseline of 13.2, despite using the same number of sampling steps. This result highlights the strong compatibility of BFM with existing few-step generative frameworks and demonstrates its effectiveness in improving both efficiency and sample quality with lower inference cost.

Training at higher resolution. To examine the scalability of our approach to higher image resolutions, we compare our model against the SiT baseline on ImageNet at 512 2 resolution. For this setting, we adopt the S-architecture configuration with a patch size of 4. The results, summarized in Table 11, clearly demonstrate that our BFM with Semantic Feature Guidance substantially outperforms the SiT baseline, achieving a remarkable 14.69 improvement in FID. This consistent improvement at a higher resolution highlights the robustness and scalability of our framework in capturing fine-grained visual details and maintaining semantic coherence across larger spatial resolution.

Seg. Lay.

FLOPs (G)

4

8

6

8

12

8

8

3.64

3.64

3.64

FID

85.1

81.5

79.1

8

3.64

76.7

Block number

Average MSE

MSE

Block3

Block4

Block 5

Block 6

0.5

0.7

0.8

1.0

0.026

0.024

0.014

0.010

Block3Block 4Block5Block6

5.3 Analysis

To show the efficacy of our components, we conduct a comprehensive component-wise analysis.

Effect of temporal segments. First, we investigate how varying the number of temporal segments M in BFM influences generation quality and computational efficiency under two complementary settings. This analysis provides insight into how temporal decomposition affects specialization and scalability within the flow-matching framework.

1) Fixed per-segment capacity. In Table 4, we fix the number of layers within each segment while varying the total number of segments M , thereby keeping the per-segment capacity constant. We observe that increasing M consistently improves FID scores, suggesting that finer-grained temporal partitioning enables each velocity network to specialize more effectively in modeling localized dynamics along the generative trajectory. Importantly, these improvements come without any increase in inference complexity, highlighting BFM's ability to scale up without added inference cost.

2) Fixed total network capacity : In Table 5, we fix the overall model capacity by increasing the number of segments while proportionally decreasing the number of layers per segment. This setup demonstrates a trade-off: while more segments provide greater temporal specialization, shallower blocks may lack sufficient capacity to model complex signals. Empirically, we find that a moderate segmentation level (e.g., M = 6 ) achieves the optimal balance, allowing for effective specialization while preserving sufficient expressiveness within each block.

Spectral analysis. To further investigate whether blockwise modeling promotes specialized learning across different stages of the generative trajectory, we conduct a frequency-domain analysis of generated images. Following the methodology of [67], we compute the 2D Fourier power spectrum for both real and generated images and apply azimuthal integration to derive their mean spectral power distributions. This analysis allows us to assess how well each model captures structural details across spatial frequencies, from coarse global layouts to fine-grained textures.

As illustrated in Figure 6, the Fréchet distance between the spectral distributions of real images and those generated by BFM-XLSF (0.049) is smaller than that of SiT-XL (0.054), indicating that our model more faithfully reproduces the frequency characteristics of natural images across the spectrum. This result supports our core design hypothesis: segment-wise specialization enables different blocks

Seg. Lay.

FLOPs (G)

4

12

6

8

12

1.0

0.8

0.4

0.2

0.0

8

6

4

Low frequency

Real

BFM (Ours)

SiT (baseline)

25

50

75

FID

81.7

81.5

88.1

95.2

Fréchet distance

(

×𝟏𝟎

!𝟑

54.9

5.46

3.64

2.72

1.82

SiT

BFM

49.5

High frequency

150

)

w.o Residual w Residual

12000

4000

8000

Training steps

𝐅𝐑𝐍

𝓛

0.96

BFM

REPA

t = 0

BFM

REPA

source image

t = 1

BFM-XL

SF-RA

BFM-XL

SF

to more effectively capture diverse signal characteristics across the diffusion trajectory, leading to improved spectral fidelity.

SemFeat vs. REPA. We analyze the effectiveness of our proposed SemFeat by comparing it against the closely related approach, REPA [26]. As discussed in Section 4.2, SemFeat differs from REPA in a key design: SemFeat leverages an external feature alignment network that explicitly conditions features on the velocity model, whereas REPA aligns semantic features implicitly within the velocity model itself. This modular design separates representation learning from generative modeling, enabling SemFeat to learn more coherent and stable semantic representations.

To empirically validate this, we conducted a feature-level analysis by applying Principal Component Analysis (PCA) to the semantic features f t extracted from each model. We visualize the top three components in Figure 8: SemFeat exhibits high semantic consistency across varying noise levels, capturing temporally stable and semantically meaningful information. In contrast, REPA generates noticeably noisier and less consistent semantic features, even within the same object region, highlighting the advantage of SemFeat's modular conditioning. Quantitative comparisons in Table 6 confirm these observations. Starting from standard FM with REPA (a), replacing REPA with SemFeat (b) yields a significant FID improvement of 4.4 points, underscoring the effectiveness of explicit modular conditioning. Moreover, integrating SemFeat within our BFM framework (c) further improves the FID score by an additional 1.6 points, while reducing inference complexity.

Effectiveness of feature residual approximation. To demonstrate the effectiveness of our feature residual approximation approach, we first compare it against a direct approximation method that independently predicts semantic features, i.e., ˆ f t = f η ( x t , c ) without residual connections. Figure 7 shows that our residual approximation method converges faster and achieves lower feature approximation loss ( L FRN), highlighting the efficacy of modeling semantic features as residual increments from the segment's start point. Additionally, as demonstrated in Table 1 and 2, incorporating residual approximation significantly reduces inference complexity: BFM-SSF-RA reduces 41% FLOPs compared to BFM-SSF and BFM-XLSF-RA reduces 65% FLOPs compared to BFM-XLSF, all while preserving generation quality. Qualitative visual comparisons in Figure 9 also demonstrate marginal perceptual differences, validating efficient feature approximation.

6 Conclusion

In this paper, we introduced Blockwise Flow Matching (BFM), a novel generative framework that partitions the generative trajectory into distinct temporal segments, each handled by compact and specialized velocity blocks. This design allowed each block to effectively capture the unique signal characteristics of its interval, leading to improved generation quality and efficiency. By further incorporating our proposed Semantic Feature Guidance and Feature Residual Approximation modules, we demonstrated that BFM establishes a significantly improved Pareto-frontier between generation performance and inference complexity on ImageNet 256 × 256: 2.1 × and 4.9 × acceleration in inference complexity compared to the existing state-of-the-art generative methods.

References

Jonathan Ho, Ajay Jain, and Pieter Abbeel. Denoising diffusion probabilistic models. In Advances in Neural Information Processing Systems, NeurIPS , 2020.

Jiaming Song, Chenlin Meng, and Stefano Ermon. Denoising diffusion implicit models. In arXiv preprint arXiv:2010.02502 , 2020.

Yaron Lipman, Ricky TQ Chen, Heli Ben-Hamu, Maximilian Nickel, and Matthew Le. Flow matching for generative modeling. In International Conference on Learning Representations, ICLR , 2023.

Robin Rombach, Andreas Blattmann, Dominik Lorenz, Patrick Esser, and Björn Ommer. Highresolution image synthesis with latent diffusion models. In Conference on Computer Vision and Pattern Recognition, CVPR , 2022.

William Peebles and Saining Xie. Scalable diffusion models with transformers. In Conference on Computer Vision and Pattern Recognition, CVPR , 2023.

ZiDong Wang, Zeyu Lu, Di Huang, Cai Zhou, Wanli Ouyang, et al. Fitv2: Scalable and improved flexible vision transformer for diffusion model. In arXiv preprint arXiv:2410.13925 , 2024.

Wan-Cyuan Fan, Yen-Chun Chen, DongDong Chen, Yu Cheng, Lu Yuan, and Yu-Chiang Frank Wang. Frido: Feature pyramid diffusion for complex scene image synthesis. In Proceedings of the AAAI conference on artificial intelligence, AAAI , 2023.

Chitwan Saharia, William Chan, Huiwen Chang, Chris Lee, Jonathan Ho, Tim Salimans, David Fleet, and Mohammad Norouzi. Palette: Image-to-image diffusion models. In ACMSIGGRAPH 2022 conference proceedings, SIGGRAPH , 2022.

Chitwan Saharia, William Chan, Saurabh Saxena, Lala Li, Jay Whang, Emily L Denton, Kamyar Ghasemipour, Raphael Gontijo Lopes, Burcu Karagol Ayan, Tim Salimans, et al. Photorealistic text-to-image diffusion models with deep language understanding. In Advances in neural information processing systems, NeurIPS , 2022.

Yuzhe Zhang, Jiawei Zhang, Hao Li, Zhouxia Wang, Luwei Hou, Dongqing Zou, and Liheng Bian. Diffusion-based blind text image super-resolution. In Conference on Computer Vision and Pattern Recognition, CVPR , 2024.

Gabriela Ben Melech Stan, Diana Wofk, Scottie Fox, Alex Redden, Will Saxton, Jean Yu, Estelle Aflalo, Shao-Yen Tseng, Fabio Nonato, Matthias Muller, et al. Ldm3d: Latent diffusion model for 3d. In arXiv preprint arXiv:2305.10853 , 2023.

Alex Nichol, Heewoo Jun, Prafulla Dhariwal, Pamela Mishkin, and Mark Chen. Point-e: A system for generating 3d point clouds from complex prompts. In arXiv preprint arXiv:2212.08751 , 2022.

Gal Metzer, Elad Richardson, Or Patashnik, Raja Giryes, and Daniel Cohen-Or. Latent-nerf for shape-guided generation of 3d shapes and textures. In Conference on Computer Vision and Pattern Recognition, CVPR , 2023.

Jaehyeok Shim, Changwoo Kang, and Kyungdon Joo. Diffusion-based signed distance fields for 3d shape generation. In Conference on Computer Vision and Pattern Recognition, CVPR , 2023.

Jonathan Ho, Tim Salimans, Alexey Gritsenko, William Chan, Mohammad Norouzi, and David J Fleet. Video diffusion models. In Advances in Neural Information Processing Systems, NeurIPS , 2022.

Tsai-Shien Chen, Chieh Hubert Lin, Hung-Yu Tseng, Tsung-Yi Lin, and Ming-Hsuan Yang. Motion-conditioned diffusion model for controllable video synthesis. In arXiv preprint arXiv:2304.14404 , 2023.

Andreas Blattmann, Robin Rombach, Huan Ling, Tim Dockhorn, Seung Wook Kim, Sanja Fidler, and Karsten Kreis. Align your latents: High-resolution video synthesis with latent diffusion models. In Conference on Computer Vision and Pattern Recognition, CVPR , 2023.

Aram Davtyan, Sepehr Sameni, and Paolo Favaro. Efficient video prediction via sparsely conditioned flow matching. In Conference on Computer Vision and Pattern Recognition, CVPR , 2023.

Aram Davtyan and Paolo Favaro. Learn the force we can: Enabling sparse motion control in multi-object video generation. In Proceedings of the AAAI conference on artificial intelligence, AAAI , 2024.

Xingchao Liu, Chengyue Gong, and Qiang Liu. Flow straight and fast: Learning to generate and transfer data with rectified flow. In International Conference on Learning Representations, ICLR , 2023.

Michael S Albergo, Nicholas M Boffi, and Eric Vanden-Eijnden. Stochastic interpolants: A unifying framework for flows and diffusions. In arXiv preprint arXiv:2303.08797 , 2023.

Nanye Ma, Mark Goldstein, Michael S Albergo, Nicholas M Boffi, Eric Vanden-Eijnden, and Saining Xie. Sit: Exploring flow and diffusion-based generative models with scalable interpolant transformers. In European Conference on Computer Vision, ECCV , 2024.

Patrick Esser, Sumith Kulal, Andreas Blattmann, Rahim Entezari, Jonas Müller, Harry Saini, Yam Levi, Dominik Lorenz, Axel Sauer, Frederic Boesel, et al. Scaling rectified flow transformers for high-resolution image synthesis. In International Conference on Machine Learning, ICML , 2024.

Black Forest Labs. Flux: A generative model by black forest labs. https://github.com/ black-forest-labs/flux , 2024. Accessed: 2025-05-14.

Maxime Oquab, Timothée Darcet, Théo Moutakanni, Huy Vo, Marc Szafraniec, Vasil Khalidov, Pierre Fernandez, Daniel Haziza, Francisco Massa, Alaaeldin El-Nouby, et al. Dinov2: Learning robust visual features without supervision. In arXiv preprint arXiv:2304.07193 , 2023.

Sihyun Yu, Sangkyung Kwak, Huiwon Jang, Jongheon Jeong, Jonathan Huang, Jinwoo Shin, and Saining Xie. Representation alignment for generation: Training diffusion transformers is easier than you think. In arXiv preprint arXiv:2410.06940 , 2024.

Jonathan Ho, Chitwan Saharia, William Chan, David J Fleet, Mohammad Norouzi, and Tim Salimans. Cascaded diffusion models for high fidelity image generation. Journal of Machine Learning Research , 2022.

Jonathan Ho and Tim Salimans. Classifier-free diffusion guidance. In arXiv preprint arXiv:2207.12598 , 2022.

Dogyun Park, Sihyeon Kim, Sojin Lee, and Hyunwoo J Kim. Ddmi: Domain-agnostic latent diffusion models for synthesizing high-quality implicit neural representations. In International Conference on Learning Representations, ICLR , 2024.

Juyeon Ko, Inho Kong, Dogyun Park, and Hyunwoo J Kim. Stochastic conditional diffusion models for robust semantic image synthesis. In International Conference on Machine Learning, ICML , 2024.

Sojin Lee, Dogyun Park, Inho Kong, and Hyunwoo J Kim. Diffusion prior-based amortized variational inference for noisy inverse problems. In European Conference on Computer Vision, ECCV , 2024.

Yaron Lipman, Marton Havasi, Peter Holderrieth, Neta Shaul, Matt Le, Brian Karrer, Ricky TQ Chen, David Lopez-Paz, Heli Ben-Hamu, and Itai Gat. Flow matching guide and code. In arXiv preprint arXiv:2412.06264 , 2024.

Tianhong Li, Dina Katabi, and Kaiming He. Return of unconditional generation: A selfsupervised representation generation method. In Advances in Neural Information Processing Systems, NeurIPS , 2024.

Rui Zhu, Yingwei Pan, Yehao Li, Ting Yao, Zhenglong Sun, Tao Mei, and Chang Wen Chen. Sd-dit: Unleashing the power of self-supervised discrimination in diffusion transformer. In Conference on Computer Vision and Pattern Recognition, CVPR , 2024.

Shuai Wang, Zhi Tian, Weilin Huang, and Limin Wang. Ddt: Decoupled diffusion transformer. In arXiv preprint arXiv:2504.05741 , 2025.

Alec Radford, Jong Wook Kim, Chris Hallacy, Aditya Ramesh, Gabriel Goh, Sandhini Agarwal, Girish Sastry, Amanda Askell, Pamela Mishkin, Jack Clark, et al. Learning transferable visual models from natural language supervision. In International Conference on Machine Learning, ICML , 2021.

Yang Song, Prafulla Dhariwal, Mark Chen, and Ilya Sutskever. Consistency models. In International Conference on Machine Learning, ICML , 2023.

Yang Song and Prafulla Dhariwal. Improved techniques for training consistency models. In arXiv preprint arXiv:2310.14189 , 2023.

Dongjun Kim, Chieh-Hsin Lai, Wei-Hsiang Liao, Naoki Murata, Yuhta Takida, Toshimitsu Uesaka, Yutong He, Yuki Mitsufuji, and Stefano Ermon. Consistency trajectory models: Learning probability flow ode trajectory of diffusion. In arXiv preprint arXiv:2310.02279 , 2023.

Jianbin Zheng, Minghui Hu, Zhongyi Fan, Chaoyue Wang, Changxing Ding, Dacheng Tao, and Tat-Jen Cham. Trajectory consistency distillation: Improved latent consistency distillation by semi-linear consistency function with trajectory mapping. In arXiv preprint arXiv:2402.19159 , 2024.

Dogyun Park, Sojin Lee, Sihyeon Kim, Taehoon Lee, Youngjoon Hong, and Hyunwoo J Kim. Constant acceleration flow. In Advances in Neural Information Processing Systems, NeurIPS , 2024.

Simian Luo, Yiqin Tan, Longbo Huang, Jian Li, and Hang Zhao. Latent consistency models: Synthesizing high-resolution images with few-step inference. arXiv preprint arXiv:2310.04378 , 2023.

Tianwei Yin, Michaël Gharbi, Richard Zhang, Eli Shechtman, Fredo Durand, William T Freeman, and Taesung Park. One-step diffusion with distribution matching distillation. In Conference on Computer Vision and Pattern Recognition, CVPR , 2024.

Tianwei Yin, Michaël Gharbi, Taesung Park, Richard Zhang, Eli Shechtman, Fredo Durand, and Bill Freeman. Improved distribution matching distillation for fast image synthesis. In Advances in Neural Information Processing Systems, NeurIPS , 2024.

Yujin Kim, Dogyun Park, Dohee Kim, and Suhyun Kim. Naturalinversion: Data-free image synthesis improving real-world consistency. In Proceedings of the AAAI conference on artificial intelligence, AAAI , 2022.

Weijian Luo. Diff-instruct++: Training one-step text-to-image generator model to align with human preferences. In arXiv preprint arXiv:2410.18881 , 2024.

Daniel Bolya and Judy Hoffman. Token merging for fast stable diffusion. In Conference on Computer Vision and Pattern Recognition, CVPR , 2023.

Hongjie Wang, Difan Liu, Yan Kang, Yijun Li, Zhe Lin, Niraj K Jha, and Yuchen Liu. Attentiondriven training-free efficiency enhancement of diffusion models. In Conference on Computer Vision and Pattern Recognition, CVPR , 2024.

Bo-Kyeong Kim, Hyoung-Kyu Song, Thibault Castells, and Shinkook Choi. Bk-sdm: A lightweight, fast, and cheap version of stable diffusion. In European Conference on Computer Vision, ECCV , 2024.

Wangbo Zhao, Yizeng Han, Jiasheng Tang, Kai Wang, Yibing Song, Gao Huang, Fan Wang, and Yang You. Dynamic diffusion transformer. arXiv preprint arXiv:2410.03456 , 2024.

Minglei Shi, Ziyang Yuan, Haotian Yang, Xintao Wang, Mingwu Zheng, Xin Tao, Wenliang Zhao, Wenzhao Zheng, Jie Zhou, Jiwen Lu, et al. Diffmoe: Dynamic token selection for scalable diffusion transformers. arXiv preprint arXiv:2503.14487 , 2025.

Tero Karras, Miika Aittala, Timo Aila, and Samuli Laine. Elucidating the design space of diffusion-based generative models. In Advances in Neural Information Processing Systems, NeurIPS , 2022.

Yogesh Balaji, Seungjun Nah, Xun Huang, Arash Vahdat, Jiaming Song, Qinsheng Zhang, Karsten Kreis, Miika Aittala, Timo Aila, Samuli Laine, et al. ediff-i: Text-to-image diffusion models with an ensemble of expert denoisers. In arXiv preprint arXiv:2211.01324 , 2022.

Martin Heusel, Hubert Ramsauer, Thomas Unterthiner, Bernhard Nessler, and Sepp Hochreiter. Gans trained by a two time-scale update rule converge to a local nash equilibrium. In Advances in Neural Information Processing Systems, NeurIPS , 2017.

Charlie Nash, Jacob Menick, Sander Dieleman, and Peter W Battaglia. Generating images with sparse representations. In arXiv preprint arXiv:2103.03841 , 2021.

Tim Salimans, Ian Goodfellow, Wojciech Zaremba, Vicki Cheung, Alec Radford, and Xi Chen. Improved techniques for training gans. In Advances in Neural Information Processing Systems, NeurIPS , 2016.

Tuomas Kynkäänniemi, Tero Karras, Samuli Laine, Jaakko Lehtinen, and Timo Aila. Improved precision and recall metric for assessing generative models. In Advances in Neural Information Processing Systems, NeurIPS , 2019.

Tuomas Kynkäänniemi, Miika Aittala, Tero Karras, Samuli Laine, Timo Aila, and Jaakko Lehtinen. Applying guidance in a limited interval improves sample and distribution quality in diffusion models. In arXiv preprint arXiv:2404.07724 , 2024.

Prafulla Dhariwal and Alexander Nichol. Diffusion models beat gans on image synthesis. In Advances in Neural Information Processing Systems, NeurIPS , 2021.

Diederik Kingma and Ruiqi Gao. Understanding diffusion objectives as the elbo with simple data augmentation. In Advances in Neural Information Processing Systems, NeurIPS , 2023.

Fan Bao, Shen Nie, Kaiwen Xue, Yue Cao, Chongxuan Li, Hang Su, and Jun Zhu. All are worth words: A vit backbone for diffusion models. In Conference on Computer Vision and Pattern Recognition, CVPR , 2023.

Shanghua Gao, Pan Zhou, Ming-Ming Cheng, and Shuicheng Yan. Mdtv2: Masked diffusion transformer is a strong image synthesizer. arXiv preprint arXiv:2303.14389 , 2023.

Hongkai Zheng, Weili Nie, Arash Vahdat, and Anima Anandkumar. Fast training of diffusion models with masked transformers. In Transactions on Machine Learning Research, TMLR , 2024.

Shuai Wang, Zexian Li, Tianhui Song, Xubin Li, Tiezheng Ge, Bo Zheng, and Limin Wang. Exploring DCN-like architecture for fast image generation with arbitrary resolution. In Advances in Neural Information Processing Systems, NeurIPS , 2024.

Emiel Hoogeboom, Jonathan Heek, and Tim Salimans. simple diffusion: End-to-end diffusion for high resolution images. In International Conference on Machine Learning, ICML , 2023.

Zhengyang Geng, Mingyang Deng, Xingjian Bai, J Zico Kolter, and Kaiming He. Mean flows for one-step generative modeling. In arXiv preprint arXiv:2505.13447 , 2025.

Ricard Durall, Margret Keuper, and Janis Keuper. Watch your up-convolution: CNN based generative deep neural networks are failing to reproduce spectral distributions. In Conference on Computer Vision and Pattern Recognition, CVPR , 2020.

Hugo Touvron, Thibaut Lavril, Gautier Izacard, Xavier Martinet, Marie-Anne Lachaux, Timothée Lacroix, Baptiste Rozière, Naman Goyal, Eric Hambro, Faisal Azhar, et al. Llama: Open and efficient foundation language models. In arXiv preprint arXiv:2302.13971 , 2023.

Jianlin Su, Murtadha Ahmed, Yu Lu, Shengfeng Pan, Wen Bo, and Yunfeng Liu. Roformer: Enhanced transformer with rotary position embedding. In Neurocomputing , 2024.

Agrim Gupta, Lijun Yu, Kihyuk Sohn, Xiuye Gu, Meera Hahn, Fei-Fei Li, Irfan Essa, Lu Jiang, and José Lezama. Photorealistic video generation with diffusion models. In European Conference on Computer Vision, ECCV , 2024.

Dogyun Park and Suhyun Kim. Probabilistic precision and recall towards reliable evaluation of generative models. In International Conference on Computer Vision, ICCV , 2023.

Efrat, Guibas, Sariel Har-Peled, and Murali. New similarity measures between polylines with applications to morphing and polygon sweeping. In Discrete & Computational Geometry , 2002.

A Implementation details

We provide the overall hyperparameter setup in Table 7.

Training. We implement our model based on the original SiT implementation [22] with recent improvements such as SwiGLU [68] activations, RMS normalization [68], and Rotary Positional Embeddings (RoPE) [69]. Following FiTv2 [6], we reassign model parameters and adopt AdaLNLoRA [70] within transformer blocks to mitigate the parameter overhead of AdaLN modules. This adjustment substantially reduces the total parameter count while maintaining comparable performance.

We use pre-computed latent vectors derived from raw pixels via the Stable Diffusion VAE, without applying data augmentation, following [26]. For feature projection, we employ a three-layer MLP with SiLU activations. We adopt uniform temporal partitioning with t m = m M for simplicity, though preliminary experiments indicate that non-uniform, data-adaptive partitioning could further improve performance. To train multiple velocity networks across temporal segments, one simple approach is to randomly sample a velocity network per iteration; however, this may lead to uneven updates among networks. Instead, we uniformly divide each training batch according to the number of temporal segments and assign corresponding timesteps to each subset. For example, with a batch size of 256 and four temporal segments, each segment receives 64 samples. This training strategy ensures balanced updates across all velocity networks and reduces training variance by evenly sampling timesteps across the entire time horizon. All experiments are conducted with a global batch size of 864 using eight NVIDIA A100 GPUs. Full training pseudocode is provided in Algorithms 1 and 2.

Training complexity. Our Blockwise Flow Matching (BFM) can be viewed as a form of sparse activation, analogous to a Mixture-of-Experts (MoE) model with a fixed router, where each velocity network serves as an expert. Similar to MoEs, BFM increases the total parameter count while maintaining comparable computational cost and latency. We evaluate the training resource usage on ImageNet 256 × 256 for XL configuration using NVIDIA A100 GPU with a batch size of 32. We use deepspeed profiler to measure the FLOPs and results are summarized in Table 8. We use multiply-accumulates (MACs) to represent FLOPs. For both simple and balanced training strategies, BFM achieves per-iteration FLOPs and training throughput comparable to SiT and REPA. Although BFM introduces more parameters, its memory footprint increases only moderately (from 30-31 GB to 47 GB), remaining well within the capacity of modern GPUs. These results demonstrate that BFM is both practical and scalable for large-scale training. Furthermore, it is possible to train each block parallelly (and separately) across multiple GPUs, which can further improve the training scalability.

Inference. For inference, we use the Euler solver with the ODE (Tables 1,3,4,5,6 in the main paper) or the Euler-Maruyama solver [22] with the SDE (Table 2 in the main paper). We keep the 246 solver steps (41 solver steps for each segment) except for Table 3 in the main paper.

Specifically, given an initial noise input x t 0 = x 0 ∼ p noise, the generation proceeds sequentially through each temporal segment [ t m -1 , t m ) . At the beginning of the m -th segment, we compute the semantic feature f t m -1 using the feature alignment network f ϕ . For subsequent timesteps t ∈ [ t m -1 , t m ) , we approximate the semantic feature either using FRN f η or the full alignment network f ϕ , depending on the desired trade-off between speed and fidelity. Then, the velocity block v ( m ) θ predicts the velocity: ˆ v ( m ) t = v ( m ) θ ( x t , c, f t ) . The intermediate state is updated iteratively via a numerical solver using the predicted velocity. This process repeats until reaching t = 1 , yielding the final sample x 1 ∼ p data. A full pseudocode implementation of the inference process is provided in Algorithm 3 and 4.

Inference complexity. We benchmark the wall-clock inference time of our models against baseline methods on a single NVIDIA A100 GPU with a batch size of 32. The results are summarized in Table 9. Empirically, our primary model, BFMSF, achieves a 9.4% reduction in inference time than the baselines while attaining better FID scores. Furthermore, the most efficient variant, BFMSF-RA, achieves a remarkable 63% reduction in runtime while maintaining performance comparable to SiT. These results demonstrate that the reduction in FLOPs achieved by BFM translates directly into real-world speedups, validating its inference efficiency.

Metrics. We evaluate generation performance using standard metrics including FID [54] (Fréchet Inception Distance), IS [56] (Inception Score), and Precision/Recall [57, 71]. Unless otherwise noted, we follow the evaluation protocol of [59] and report results using 50K generated samples.

FID is the most commonly used metric, measuring the feature distance between real and generated image distributions. It is computed using the Inception-V3 network under the assumption that both feature distributions follow multivariate Gaussian statistics.

IS, also based on Inception-V3, evaluates both image quality and diversity by measuring the KLdivergence between the marginal and conditional label distributions derived from the network logits.

Finally, Precision captures the fraction of generated samples that lie close to the real data manifold, while Recall measures how well the generated distribution covers real data samples. Together, these complementary metrics provide a more holistic assessment of generative performance in terms of both fidelity and diversity.

B Analysis details

Component analysis (Table 1). We provide additional implementation details for the experimental configurations reported in Table 1 of main paper. For the vanilla Flow Matching model, we train

Algorithm 1 Training Blockwise Flow Matching (BFM)

Require: Feature alignment network f ϕ , velocity network v θ , and projection layer h ψ 1: repeat

2: B { ( x ( i ) , x ( i ) , c ( i ) ) } B

Sample a mini-batch of size : 0 1 i =1 ▷ x 0 : data, x 1 : noise

3: Initialize losses: L BFM ← 0 , L align ← 0

4: Partition sample indices i uniformly into M disjoint groups: I 1 , . . . , I M

5: for each segment m = 1 , . . . , M do

6: for each sample index i ∈ I do

7:

m ▷ All samples can be computed parallel

(

i

)

t

(

t

1

-

m

t

-

)

i

m

1

m

-

1

(

i

)

t

m

-

t

-

(

t

m

m

m

10:

,

a

(

t

)

x

(1

a

(

t

)

)

x

+

a

(

-

11: Semantic features: h ∗ ( i ) ←E ( x ( i ) 1 ) , f ( i ) t ← f ϕ ( x ( i ) t , c ( i ) )

13:

Per-sample losses:

16: end for

17: end for

18: Normalize losses: L ← 1 B ( L BFM + λ L align )

19: Update θ, ϕ, ψ with a gradient step on L

20: until convergence

Algorithm 2 Training Feature Residual Network (FRN)

1: Freeze parameters v θ , f ϕ h ψ , and introduce FRN parameters f η

2: repeat

3: Sample m ∼ U [1 , ..., M ]

4: Sample t ∼ U [ t m -1 , t m )

5: Compute semantic feature at segment start: f t m -1 = f ϕ ( x t m -1 , c )

6: Compute semantic target at intermediate step t : f t = f ϕ ( x t , c )

7: Compute normalized time offset b m ( t ) = t -t m -1 t m -t m -1

8: Approximate semantic feature at intermediate step t : ˆ f t = f t m -1 + b · f η ( x t , c )

9: Compute residual approximation loss L FRN = ∥ ˆ f t -f t ∥ 2

10: Update FRN parameters η via gradient step on L FRN

11: until convergence

the original SiT-S model (12 transformer blocks) with architectural improvements. For our vanilla BFM-S model (second row of Table 1), we train the same transformer architecture (except that we reassign model parameters with AdaLN-LoRA) with six temporal segments. We assign each velocity block with 8 transformer blocks. When adding a feature alignment network (BFM-SSF, third row of Table 1), we reduce the number of layers of each velocity block, as shown in Table 7.

Spectral Entropy and High-Frequency Ratio (Figure 3). To analyze the frequency characteristics of generated images across timesteps, we compute two key metrics: Spectral Entropy (SE) and the High-Frequency Ratio (HFR). Spectral Entropy quantifies the distributional complexity of a signal in the frequency domain. Specifically, we treat the normalized 2D power spectrum of an image as a probability distribution and compute its Shannon entropy. A higher SE indicates a more uniform, less structured distribution of spectral energy (i.e., more randomness), while a lower SE reflects a concentration of energy in fewer frequencies, indicating more structured signals. High-Frequency

i

)

(

i

)

t

)

(

i

)

x

(

i

)

t

m

Ratio measures the proportion of total spectral energy in the high-frequency range. It captures the relative contribution of fine-grained details in the image.

To compute both metrics, we randomly sample 10,000 real images from the ImageNet set and transform them to noisy versions at specific timesteps by interpolation formulation. Each image is converted to the frequency domain via a 2D Fourier Transform. We then calculate the power spectrum and apply azimuthal integration to obtain the spectral distribution. SE is computed as the Shannon entropy of the normalized spectrum, while HFR is obtained by summing the energy above a predefined frequency threshold (=0.5) and dividing by the total energy.

Fourier power spectrum (Figure 6). We perform a frequency-domain analysis to evaluate how well the spectral characteristics of generated images match those of real images. This experiment tests whether our blockwise modeling strategy improves spectral fidelity by allowing different blocks to specialize in capturing distinct frequency components along the generative trajectory. Specifically, we randomly sample 100 images from each model (BFM-XLSF and SiT-XL [22]) and 100 real images from ImageNet. We compute the 2D Fourier power spectrum for each image and apply azimuthal integration to obtain a 1D mean spectral power distribution, following the procedure in [67]. We then calculate the Fréchet Distance [72] between the spectral distributions of generated and real images to quantify their similarity.

Discrepancy between f t m -1 and f t (Figure 5). For each segment m , we randomly sample 50 images from the ImageNet dataset and convert them into their corresponding noisy versions x t m -1 and x t at timesteps t m -1 and t ∈ [ t m -1 , t m ) , respectively We then evaluate the temporal consistency of the semantic features extracted by the alignment network by computing the mean squared error (MSE) between f t m -1 = f ϕ ( x t m -1 , c ) and f t = f ϕ ( x t , c ) . A lower MSE indicates that the feature alignment network learns temporally stable, time-invariant representations across the segment.

Quantitative comparison between REPA [26] and SemFeat (Table 6). We provide additional implementation details for the experimental configurations reported in Table 6. All models are trained for 100K iterations using DINOv2-B [25] as the target representation, with a loss weight of λ = 0 . 5 and negative cosine similarity as the alignment objective. Feature projection layers are implemented with identical configurations across all methods for a fair comparison. (a): We implement REPA using the SiT-S architecture (12 transformer layers), aligning the hidden representations at the 8th transformer layer to the DINOv2-B targets. (b): We incorporate our proposed SemFeat into the standard FM framework by decomposing the SiT-S model into two modules: a feature alignment network composed of 8 transformer layers, and a velocity model composed of 4 transformer layers. (c): This setting corresponds to our BFM-SSF in Table 7.

PCA results (Figure 8, 10) For the PCA analysis, we extract semantic features from noisy inputs x t using both REPA-XL and BFM-XLSF. In the case of REPA-XL, we use the hidden representations from the 8th transformer layer, as this layer is aligned with DINO features during training. For BFM-XLSF, we directly use the outputs of the feature alignment network, f t = f ϕ ( x t , c ) . We perform Principal Component Analysis across features collected from multiple timesteps to capture their dominant variations. The top three principal components are then mapped to the RGB channels for visualization, allowing us to assess the semantic consistency and structure of features across time.

C Additional PCA results

In Figure 10, we present additional PCA visualizations of semantic features across varying noise levels. SemFeat consistently produces semantically coherent and temporally stable representations, maintaining structural consistency even under significant noise. In contrast, REPA exhibits greater variability and noise in its feature representations, with noticeable inconsistencies across the same object regions. These observations further support the effectiveness of SemFeat's modular conditioning in capturing robust and meaningful semantic information throughout the generative trajectory.

source image

t = 0

t = 1

REPA

BFM

source image

t = 0

t = 1

REPA

BFM

source image

t = 0

t = 1

REPA

BFM

source image

t = 0

t = 1

REPA

BFM

source image

t = 0

t = 1

REPA

BFM

source image

t = 0

t = 1

REPA

BFM

source image

REPA

BFM

t = 0

t = 1

source image

t = 0

t = 1

REPA

BFM

D More qualitative results

2171

ealiiastaceno

E Limitations and Future work

Reliance on external features. Our method relies on semantic features extracted from a pretrained visual encoder (e.g., DINOv2) to guide the velocity models through SemFeat. While this enables stronger semantic conditioning and improves generation quality, it introduces an external dependency and limits the model's generality to domains where such pretrained encoders are available. In future work, we plan to explore self-supervised alternatives that jointly learn semantic representations during training, potentially eliminating the need for external networks.

Multi-modal large-scale datasets. Our current evaluation focuses on single-modality image generation using ImageNet. While BFM scales well in this setting, applying the same framework to multi-modal, large-scale datasets (e.g., text-to-image or video datasets) remains an open challenge. Future research could investigate how to adapt blockwise modeling and semantic conditioning to settings that require more complex cross-modal reasoning and longer temporal consistency, such as text-driven video generation.

Non-uniform partitioning schedule. In this work, we adopt a uniform temporal segmentation strategy for simplicity. However, the generative trajectory exhibits varying complexity across timesteps, suggesting that certain regions (e.g., early noisy stages or late refinement phases) may benefit from finer-grained modeling. Future work could explore non-uniform or learnable partitioning schedules that allocate computational resources adaptively based on local signal complexity, potentially improving both performance and efficiency.

F Broader impact

The goal of this work is to improve the efficiency and scalability of generative models. By introducing Blockwise Flow Matching (BFM), we reduce the computational burden of high-quality generation, making diffusion-based models more accessible for researchers and practitioners with limited resources. This has potential societal benefits in democratizing access to generative AI, enabling smaller labs, startups, and educational institutions to experiment with cutting-edge models without requiring extensive computational infrastructure. However, improved efficiency also lowers the barrier for misuse. Generative models can be exploited to produce content such as deepfakes or misinformation at scale. We acknowledge that its capabilities could be repurposed in harmful ways. Mitigating such misuse requires responsible downstream deployment, as well as efforts from the broader community to develop and enforce ethical standards around generative AI.

NeurIPS Paper Checklist

1. Claims

Question: Do the main claims made in the abstract and introduction accurately reflect the paper's contributions and scope?

Answer: [Yes]

Justification: Section 5

Guidelines:

The answer NA means that the abstract and introduction do not include the claims made in the paper.

The abstract and/or introduction should clearly state the claims made, including the contributions made in the paper and important assumptions and limitations. A No or NA answer to this question will not be perceived well by the reviewers.

The claims made should match theoretical and experimental results, and reflect how much the results can be expected to generalize to other settings.

It is fine to include aspirational goals as motivation as long as it is clear that these goals are not attained by the paper.

2. Limitations

Question: Does the paper discuss the limitations of the work performed by the authors?

Answer: [Yes]

Justification: Appendix

Guidelines:

The answer NA means that the paper has no limitation while the answer No means that the paper has limitations, but those are not discussed in the paper.

The authors are encouraged to create a separate "Limitations" section in their paper.

The paper should point out any strong assumptions and how robust the results are to violations of these assumptions (e.g., independence assumptions, noiseless settings, model well-specification, asymptotic approximations only holding locally). The authors should reflect on how these assumptions might be violated in practice and what the implications would be.

The authors should reflect on the scope of the claims made, e.g., if the approach was only tested on a few datasets or with a few runs. In general, empirical results often depend on implicit assumptions, which should be articulated.

The authors should reflect on the factors that influence the performance of the approach. For example, a facial recognition algorithm may perform poorly when image resolution is low or images are taken in low lighting. Or a speech-to-text system might not be used reliably to provide closed captions for online lectures because it fails to handle technical jargon.

The authors should discuss the computational efficiency of the proposed algorithms and how they scale with dataset size.

If applicable, the authors should discuss possible limitations of their approach to address problems of privacy and fairness.

While the authors might fear that complete honesty about limitations might be used by reviewers as grounds for rejection, a worse outcome might be that reviewers discover limitations that aren't acknowledged in the paper. The authors should use their best judgment and recognize that individual actions in favor of transparency play an important role in developing norms that preserve the integrity of the community. Reviewers will be specifically instructed to not penalize honesty concerning limitations.

3. Theory assumptions and proofs

Question: For each theoretical result, does the paper provide the full set of assumptions and a complete (and correct) proof?

Answer: [NA]

Justification: No theoretical proof

Guidelines:

The answer NA means that the paper does not include theoretical results.

All the theorems, formulas, and proofs in the paper should be numbered and crossreferenced.

All assumptions should be clearly stated or referenced in the statement of any theorems.

The proofs can either appear in the main paper or the supplemental material, but if they appear in the supplemental material, the authors are encouraged to provide a short proof sketch to provide intuition.

Inversely, any informal proof provided in the core of the paper should be complemented by formal proofs provided in appendix or supplemental material.

Theorems and Lemmas that the proof relies upon should be properly referenced.

4. Experimental result reproducibility

Question: Does the paper fully disclose all the information needed to reproduce the main experimental results of the paper to the extent that it affects the main claims and/or conclusions of the paper (regardless of whether the code and data are provided or not)?

Answer: [Yes]

Justification: Supplementary material

Guidelines:

The answer NA means that the paper does not include experiments.

If the paper includes experiments, a No answer to this question will not be perceived well by the reviewers: Making the paper reproducible is important, regardless of whether the code and data are provided or not.

If the contribution is a dataset and/or model, the authors should describe the steps taken to make their results reproducible or verifiable.

Depending on the contribution, reproducibility can be accomplished in various ways. For example, if the contribution is a novel architecture, describing the architecture fully might suffice, or if the contribution is a specific model and empirical evaluation, it may be necessary to either make it possible for others to replicate the model with the same dataset, or provide access to the model. In general. releasing code and data is often one good way to accomplish this, but reproducibility can also be provided via detailed instructions for how to replicate the results, access to a hosted model (e.g., in the case of a large language model), releasing of a model checkpoint, or other means that are appropriate to the research performed.

While NeurIPS does not require releasing code, the conference does require all submissions to provide some reasonable avenue for reproducibility, which may depend on the nature of the contribution. For example

(a) If the contribution is primarily a new algorithm, the paper should make it clear how to reproduce that algorithm.

(b) If the contribution is primarily a new model architecture, the paper should describe the architecture clearly and fully.

(c) If the contribution is a new model (e.g., a large language model), then there should either be a way to access this model for reproducing the results or a way to reproduce the model (e.g., with an open-source dataset or instructions for how to construct the dataset).

(d) We recognize that reproducibility may be tricky in some cases, in which case authors are welcome to describe the particular way they provide for reproducibility. In the case of closed-source models, it may be that access to the model is limited in some way (e.g., to registered users), but it should be possible for other researchers to have some path to reproducing or verifying the results.

5. Open access to data and code

Question: Does the paper provide open access to the data and code, with sufficient instructions to faithfully reproduce the main experimental results, as described in supplemental material?

Answer: [Yes]

Justification: We will release code.

Guidelines:

The answer NA means that paper does not include experiments requiring code.

Please see the NeurIPS code and data submission guidelines ( https://nips.cc/ public/guides/CodeSubmissionPolicy ) for more details.

While we encourage the release of code and data, we understand that this might not be possible, so 'No' is an acceptable answer. Papers cannot be rejected simply for not including code, unless this is central to the contribution (e.g., for a new open-source benchmark).

The instructions should contain the exact command and environment needed to run to reproduce the results. See the NeurIPS code and data submission guidelines ( https: //nips.cc/public/guides/CodeSubmissionPolicy ) for more details.

The authors should provide instructions on data access and preparation, including how to access the raw data, preprocessed data, intermediate data, and generated data, etc.

The authors should provide scripts to reproduce all experimental results for the new proposed method and baselines. If only a subset of experiments are reproducible, they should state which ones are omitted from the script and why.

At submission time, to preserve anonymity, the authors should release anonymized versions (if applicable).

Providing as much information as possible in supplemental material (appended to the paper) is recommended, but including URLs to data and code is permitted.

6. Experimental setting/details

Question: Does the paper specify all the training and test details (e.g., data splits, hyperparameters, how they were chosen, type of optimizer, etc.) necessary to understand the results?

Answer: [Yes]

Justification: Appendix

Guidelines:

The answer NA means that the paper does not include experiments.

The experimental setting should be presented in the core of the paper to a level of detail that is necessary to appreciate the results and make sense of them.

The full details can be provided either with the code, in appendix, or as supplemental material.

7. Experiment statistical significance

Question: Does the paper report error bars suitably and correctly defined or other appropriate information about the statistical significance of the experiments?

Answer: [No]

Justification: Used metrics are average statistics.

Guidelines:

The answer NA means that the paper does not include experiments.

The authors should answer "Yes" if the results are accompanied by error bars, confidence intervals, or statistical significance tests, at least for the experiments that support the main claims of the paper.

The factors of variability that the error bars are capturing should be clearly stated (for example, train/test split, initialization, random drawing of some parameter, or overall run with given experimental conditions).

The method for calculating the error bars should be explained (closed form formula, call to a library function, bootstrap, etc.)

The assumptions made should be given (e.g., Normally distributed errors).

It should be clear whether the error bar is the standard deviation or the standard error of the mean.

It is OK to report 1-sigma error bars, but one should state it. The authors should preferably report a 2-sigma error bar than state that they have a 96% CI, if the hypothesis of Normality of errors is not verified.

For asymmetric distributions, the authors should be careful not to show in tables or figures symmetric error bars that would yield results that are out of range (e.g. negative error rates).

If error bars are reported in tables or plots, The authors should explain in the text how they were calculated and reference the corresponding figures or tables in the text.

8. Experiments compute resources

Question: For each experiment, does the paper provide sufficient information on the computer resources (type of compute workers, memory, time of execution) needed to reproduce the experiments?

Answer: [Yes]

Justification: Appendix

Guidelines:

The answer NA means that the paper does not include experiments.

The paper should indicate the type of compute workers CPU or GPU, internal cluster, or cloud provider, including relevant memory and storage.

The paper should provide the amount of compute required for each of the individual experimental runs as well as estimate the total compute.

The paper should disclose whether the full research project required more compute than the experiments reported in the paper (e.g., preliminary or failed experiments that didn't make it into the paper).

9. Code of ethics

Question: Does the research conducted in the paper conform, in every respect, with the NeurIPS Code of Ethics https://neurips.cc/public/EthicsGuidelines ?

Answer: [Yes]

Justification:

Guidelines:

The answer NA means that the authors have not reviewed the NeurIPS Code of Ethics.

If the authors answer No, they should explain the special circumstances that require a deviation from the Code of Ethics.

The authors should make sure to preserve anonymity (e.g., if there is a special consideration due to laws or regulations in their jurisdiction).

10. Broader impacts

Question: Does the paper discuss both potential positive societal impacts and negative societal impacts of the work performed?

Answer: [Yes]

Justification: Appendix

Guidelines:

The answer NA means that there is no societal impact of the work performed.

If the authors answer NA or No, they should explain why their work has no societal impact or why the paper does not address societal impact.

Examples of negative societal impacts include potential malicious or unintended uses (e.g., disinformation, generating fake profiles, surveillance), fairness considerations (e.g., deployment of technologies that could make decisions that unfairly impact specific groups), privacy considerations, and security considerations.

The conference expects that many papers will be foundational research and not tied to particular applications, let alone deployments. However, if there is a direct path to any negative applications, the authors should point it out. For example, it is legitimate to point out that an improvement in the quality of generative models could be used to

generate deepfakes for disinformation. On the other hand, it is not needed to point out that a generic algorithm for optimizing neural networks could enable people to train models that generate Deepfakes faster.

The authors should consider possible harms that could arise when the technology is being used as intended and functioning correctly, harms that could arise when the technology is being used as intended but gives incorrect results, and harms following from (intentional or unintentional) misuse of the technology.

If there are negative societal impacts, the authors could also discuss possible mitigation strategies (e.g., gated release of models, providing defenses in addition to attacks, mechanisms for monitoring misuse, mechanisms to monitor how a system learns from feedback over time, improving the efficiency and accessibility of ML).

11. Safeguards

Question: Does the paper describe safeguards that have been put in place for responsible release of data or models that have a high risk for misuse (e.g., pretrained language models, image generators, or scraped datasets)?

Answer: [NA]

Justification:

Guidelines:

The answer NA means that the paper poses no such risks.

Released models that have a high risk for misuse or dual-use should be released with necessary safeguards to allow for controlled use of the model, for example by requiring that users adhere to usage guidelines or restrictions to access the model or implementing safety filters.

Datasets that have been scraped from the Internet could pose safety risks. The authors should describe how they avoided releasing unsafe images.

We recognize that providing effective safeguards is challenging, and many papers do not require this, but we encourage authors to take this into account and make a best faith effort.

12. Licenses for existing assets

Question: Are the creators or original owners of assets (e.g., code, data, models), used in the paper, properly credited and are the license and terms of use explicitly mentioned and properly respected?

Answer: [Yes]

Justification: Cited

Guidelines:

The answer NA means that the paper does not use existing assets.

The authors should cite the original paper that produced the code package or dataset.

The authors should state which version of the asset is used and, if possible, include a URL.

The name of the license (e.g., CC-BY 4.0) should be included for each asset.

For scraped data from a particular source (e.g., website), the copyright and terms of service of that source should be provided.

If assets are released, the license, copyright information, and terms of use in the package should be provided. For popular datasets, paperswithcode.com/datasets has curated licenses for some datasets. Their licensing guide can help determine the license of a dataset.

For existing datasets that are re-packaged, both the original license and the license of the derived asset (if it has changed) should be provided.

If this information is not available online, the authors are encouraged to reach out to the asset's creators.

13. New assets

Question: Are new assets introduced in the paper well documented and is the documentation provided alongside the assets?

Answer: [NA]

Justification:

Guidelines:

The answer NA means that the paper does not release new assets.

Researchers should communicate the details of the dataset/code/model as part of their submissions via structured templates. This includes details about training, license, limitations, etc.

The paper should discuss whether and how consent was obtained from people whose asset is used.

At submission time, remember to anonymize your assets (if applicable). You can either create an anonymized URL or include an anonymized zip file.

14. Crowdsourcing and research with human subjects

Question: For crowdsourcing experiments and research with human subjects, does the paper include the full text of instructions given to participants and screenshots, if applicable, as well as details about compensation (if any)?

Answer: [NA]

Justification:

Guidelines:

The answer NA means that the paper does not involve crowdsourcing nor research with human subjects.

Including this information in the supplemental material is fine, but if the main contribution of the paper involves human subjects, then as much detail as possible should be included in the main paper.

According to the NeurIPS Code of Ethics, workers involved in data collection, curation, or other labor should be paid at least the minimum wage in the country of the data collector.

15. Institutional review board (IRB) approvals or equivalent for research with human subjects

Question: Does the paper describe potential risks incurred by study participants, whether such risks were disclosed to the subjects, and whether Institutional Review Board (IRB) approvals (or an equivalent approval/review based on the requirements of your country or institution) were obtained?

Answer: [NA]

Justification:

Guidelines:

The answer NA means that the paper does not involve crowdsourcing nor research with human subjects.

Depending on the country in which research is conducted, IRB approval (or equivalent) may be required for any human subjects research. If you obtained IRB approval, you should clearly state this in the paper.

We recognize that the procedures for this may vary significantly between institutions and locations, and we expect authors to adhere to the NeurIPS Code of Ethics and the guidelines for their institution.

For initial submissions, do not include any information that would break anonymity (if applicable), such as the institution conducting the review.

16. Declaration of LLM usage

Question: Does the paper describe the usage of LLMs if it is an important, original, or non-standard component of the core methods in this research? Note that if the LLM is used only for writing, editing, or formatting purposes and does not impact the core methodology, scientific rigorousness, or originality of the research, declaration is not required.

Answer: [NA]

Justification:

Guidelines:

The answer NA means that the core method development in this research does not involve LLMs as any important, original, or non-standard components.

Please refer to our LLM policy ( https://neurips.cc/Conferences/2025/LLM ) for what should or should not be described.

Peer reviews:
{"IS_META_REVIEW": false, "comments": "SUMMARY:\nThis paper introduces Blockwise Flow Matching (BFM), a method that splits the generative trajectory in flow matching models into multiple temporal “blocks,” each with their own smaller, specialized network for a subset of timesteps. Combined with a Semantic Feature Guidance module (using pretrained DINOv2 features) and a lightweight Feature Residual Approximation for fast inference, BFM aims to improve both sample quality and efficiency. Experiments on ImageNet 256x256 show state-of-the-art FID at significantly reduced FLOPs, suggesting BFM dramatically pushes the quality/speed Pareto frontier.\n\nSTRENGTHS AND WEAKNESSES:\n**Strengths**\n\n- The paper is clear and easy to follow.\n- Integrating pretrained semantic features makes sense and performs better than previous representation alignment techniques.\n- Demonstrates robust improvements across quality and FLOPs, backed by systematic ablations.\n\n**Weaknesses**\n\n- I questioned the novelty of the BFM. Previous works have already divided the entire time interval into multiple segments and then optimized the inference quality for each of them. Some mentioned works can be listed as [1], [2], [3]. For example, the work [2] uses a similar loss function to yours; the difference is that they use only one network, which is specialized for all sub-segments. Similar principles can be found in [2] and [3]. I believed that deploying one neural network for inference is much resource-friendly than the paper’s approach. Could a distilled single model recover BFM’s benefits?\n\n- The comparison with the works mentioned in the first drawback is highly recommended.\n- Results are limited to ImageNet and strong pretrained image encoders; adaptation to other data types or domains isn’t addressed.\n- Increased total parameters from block specialization may strain memory and training scalability, not fully discussed.\n- The choice and allocation of temporal blocks lacks theory or automation—performance appears sensitive to segmentation hyperparameters.\n\nQUESTIONS:\n**Questions** \n\n- In line 138, are the intervals uniform? Could the optimal non-uniform intervals in [2] provide any motivation or benefits to your design?\n\n**References**\n\n[1] Yan, Hanshu, et al. \"Perflow: Piecewise rectified flow as universal plug-and-play accelerator.\" arXiv preprint arXiv:2405.07510 (2024).\n\n[2] Nguyen, Bao, Binh Nguyen, and Viet Anh Nguyen. \"Bellman optimal stepsize straightening of flow-matching models.\" arXiv preprint arXiv:2312.16414 (2023).\n\n[3] Yoon, Jongmin, and Juho Lee. \"Sequential Flow Straightening for Generative Modeling.\" arXiv preprint arXiv:2402.06461 (2024).\n\nLIMITATIONS:\nyes", "TITLE": "Official Review", "RECOMMENDATION": 4, "REVIEWER_CONFIDENCE": 4, "CLARITY": 3, "IMPACT": 2, "ORIGINALITY": 2, "DATE": "2025-06-17"}

{"IS_META_REVIEW": false, "comments": "SUMMARY:\nThe authors of this work study a novel training scheme for flow matching models, which are state-of-the-art in image, text, etc, generation. In standard flow matching training, the practitioner has access to the start and end distributions and trains a neural network to learn vector fields at random times within the interval [0,1]. The hardest part in training is being able to capture the precise moments where the feature-learning takes place, and at inference, the need to discretize the flow into many steps is also expensive. To remedy these issues, this work proposes BlockwiseFlowMatching (BFM), where they train more (but smaller) specialized velocity blocks for particular time intervals. They compare their approach to several other works and they achieve SOTA results on large imagenet datasets.\n\nSTRENGTHS AND WEAKNESSES:\nA huge strength of this paper lies in its simplicity---the idea is very natural from an algorithmic/training perspective. My understanding is that many folks have tried to basically over- or under-tune training via stepsize schedulers, and I view this work as a complementary perspective on the matter (namely, to segment the time interval and train in block instead). I really appreciated the figures in the main text and appendix that highlight the \"semantic features over timesteps\"---this was very cool.\n\nA weakness is that the work is purely empirical, but that is less of a weakness and more of just where the paper fits in the ML space. The clarity could be improved in some cases; see below. I am also not an expert, but it is not clear to me whether or not their experimentation is exhaustive enough with respect to the existing literature. Looking closer at Table 2, the number of parameters required is sometimes larger than other models (see e.g., the reference [45]) with comparable FID and the number of FLOPs is lower. \n\nThe numerical results are not entirely compelling and seem a bit catered for the datasets in question. That being said, I do appreciate the novelty this idea brings to this space, and I thus recommend weak accept.\n\nQUESTIONS:\n- Is it possible to distill the training ideas into a few key steps in the main text? I personally found Figure 2 and the accompanying caption very difficult to understand compared to the pseudocode in the appendix\n- Do you attempt to see what happens when you take other paths?\n- Following the \"strengths\" comment above, if they are available, is it possible to see what the images look like for various times (apart from just the semantic features)?\n- From the semantic features: why does it look like the model very early on learns the image? This is a bit strange?\n- Is Table1 inconsistent with the \"lower is better\"? I feel like it would be better if the first column said something like 0.66 and 1/1.09, and so on\n\nLIMITATIONS:\nN/A", "TITLE": "Official Review", "RECOMMENDATION": 5, "REVIEWER_CONFIDENCE": 3, "CLARITY": 2, "IMPACT": 3, "ORIGINALITY": 3, "DATE": "2025-06-26"}

{"IS_META_REVIEW": false, "comments": "SUMMARY:\nThe paper proposes partitioning the original diffusion process into multiple time intervals, where each interval is managed by a separate, compact network. This design reduces the model feedforward cost incurred at every evaluation step.\n\nSTRENGTHS AND WEAKNESSES:\nStrenths:\n- Decomposing a cumbersome network into multiple smaller ones is a sensible idea where each handles an assigned temporal block.  \n- This gives a flexibility in eliminating computational cost while obtaining good generation quality.\n- The exposition is clear and well-motivated.\n\nWeaknesses:\n- The proposed components, including semantic guidance and the feature residual module, add considerable complexity to the method and could benefit from clearer motivation to justify their inclusion.\n- There is a compromise of model performance when using the Feature Residual Approximation which is understandable. I would be helpful to include a speed vs. FID chart for demonstration of the tradeup.\n\nQUESTIONS:\nQ1. Why FRN is needed? Why is not not trained jointly with feature alignment network?\n\nQ2. Using non-overlapping time interval is reasonable since it is easier to manage. However, it also has a limitation that there is a disconnected information at the starting and ending point between each interval. Have the authors considered this issue in the method? \n\nQ3. In semantic feature guidance, what is the model performance when only aligment loss is used and no conditioning semantic feature $f_t$ is used. This allows to compare with the naive REPA baseline.\n\nQ4. As concern in Q2, this seems that the authors addressed a different issue via Feature Residual Approximation which is the consistency of starting point and intermediate point within the same interval. However, there is an inevitable discrepancy between the ending point of the previous time interval and the starting point of the next time interval, even though they should theoretically be identical. The feature residual only addresses the discrepancy of within-interval features, not the temporal discontinuity between consecutive intervals.\n\nQ5. Have the authors tried a simple baseline that aligns the features of velocity block with pretrained features, without the presence of Semantic module? This is similar to REPA method.\n\nQ6. Feature residual approximation is just one approach. Have the authors tried a simple interpolation between the start and end points instead so there is no need of additional learnt module.\n\nLIMITATIONS:\nyes but please also address the concern of discontinuity of non-overalaping inverals above.", "TITLE": "Official Review", "RECOMMENDATION": 4, "REVIEWER_CONFIDENCE": 5, "CLARITY": 3, "IMPACT": 3, "ORIGINALITY": 3, "DATE": "2025-06-30"}

{"IS_META_REVIEW": false, "comments": "SUMMARY:\nThis paper introduces Blockwise Flow Matching (BFM), a novel framework to address the efficiency and performance limitations of Flow Matching models. It partitions the generative trajectory into segments, each handled by a smaller, specialized network block to improve both modeling accuracy and inference speed. Furthermore, by incorporating semantic guidance and an efficient feature approximation strategy, the method achieves both quality and speed improvement. Overall, this paper proposes a brand new paradigm for Flow Matching models.\n\nSTRENGTHS AND WEAKNESSES:\nStrengths\n1. The main idea of this paper, partitioning the generative trajectory into segments handled by specialized, smaller blocks, is a novel design and effectively deals with the temporal heterogeneity of the generation process. \n2. The experiments are rigorous, comprehensive, and conducted on a competitive benchmark (ImageNet 256x256). The detailed ablation studies and visualizations systematically validate the contribution of each component of the proposed method, demonstrating high-quality research practice.\n3. This is a well-organized paper. The authors do an excellent job of motivating the problem, explaining their method, and presenting their results in an accessible manner.\n\nWeaknesses\n1. Some key experiments validating the effectiveness of the proposed modules are missing, which I elaborate on in my Questions.\n\nQUESTIONS:\n1. In Equation 7, the authors supervise the denoising process by velocity within the $m$-th segment. I wonder whether supervising by velocity to $x_1$, namely, $\\frac{x_{t_m}-x_1}{t_m-1}$ will result in performance degradation.\n2. It is suggested to provide an ablation study where the feature alignment network is trained without the $\\mathcal{L}_{align}$, which demonstrates the benefits of explicitly aligning to pretrained representations.\n3. In Section 4.2 (Line 163), the authors mention that BFM can take various additional conditions as inputs (e.g., class labels or text prompts). However, I only see class-label-conditioned generation in the paper. Can the authors provide analysis regarding text-conditioned generation?\n4. The design of the Semantic Feature Guidance module is similar to recent work DDT [1]. The authors are advised to add a citation and discussion. \n5. It would be better if the authors provided a discussion or visualization of typical failure cases to demonstrate the limitations of BFM.\n\n[1] Wang, Shuai, et al. \"Ddt: Decoupled diffusion transformer.\" arXiv preprint arXiv:2504.05741 (2025).\n\nLIMITATIONS:\nyes", "TITLE": "Official Review", "RECOMMENDATION": 4, "REVIEWER_CONFIDENCE": 4, "CLARITY": 3, "IMPACT": 3, "ORIGINALITY": 3, "DATE": "2025-07-02"}

{"IS_META_REVIEW": false, "comments": "SUMMARY:\nThe paper presents an interesting approach to flow-matching based methods by partitioning the generative trajectory into multiple temporal segments. Each segment is handled by a specialized smaller model for prediction. The proposed algorithm is called blockwise flow matching. Experimental results on ImageNet shows reasonable accelerations, like up to 4.9x over the baseline, without compromising the accuracy.\n\nSTRENGTHS AND WEAKNESSES:\n### Strengths\n- The idea of using blockwise flow matching is well motivated. As the paper claims, there are different characteristics for different timesteps during the flow trajectory. For example, early timesteps are dominated by irregular, low frequency patterns while the later timesteps require high-frequency details. Thus, using specialized models for different timesteps is reasonable.\n\n- The design of the proposed algorithm in Fig. 2 can well reduce the computation cost of the vanilla flow-matching based algorithm while maintaining feasible performance during the generation process.\n\n- The presentation of the paper is clear. The paper provides sufficient implementation details which are easy to reproduce the results.\n\n- The paper presents extensive ablation studies to validate the effectiveness of the proposed algorithm. \n\n\n### Weaknesses\n\n- Although the proposed algorithm reports reasonable speed up (based on FLOPs) as well as superior performance gain over the baseline, as shown in Table 2, blockwise flow matching actually increases the model parameters. Thus it will also consume more training memory, which makes the training cost larger than the vanilla flow matching algorithms. Besides, for the inference time, the decrease of FLOPs does not mean the model will have similar speed-up gain. Is it possible to provide the inference time cost for these algorithms reported in Table 2？\n\n- The proposed algorithm is based on ImageNet 256x256. How about the performance on the challenging text-to-image benchmark with larger generation size, like 1024x1024. Also, currently, the algorithm is based on the small model like SiT and REPA. Is it possible to be used in state-of-art image generation models?\n\n- For the performance evaluation, FID may not be consistent with human preference. Is it possible to provide a user-study to further validate the effectiveness of the proposed algorithm. \n\n- Is the proposed algorithm applicable to the algorithms with smaller diffusion steps, like 4-step model?\n\nQUESTIONS:\nThe main questions are in the experimental section. Please well address the comments in the weakness section.\n\nLIMITATIONS:\nThe paper does not explicitly provide the limitations of the proposed algorithm. Please reference the weakness section above for more details listed.", "TITLE": "Official Review", "RECOMMENDATION": 4, "REVIEWER_CONFIDENCE": 4, "CLARITY": 3, "IMPACT": 3, "ORIGINALITY": 2, "DATE": "2025-07-06"}