HRVDA: High-Resolution Visual Document Assistant

Introduction

Large Language Models (LLMs), such as ChatGPT , LLaMA , have taken a significant stride towards general artificial intelligence. By leveraging massive amounts of data, they have developed powerful reasoning and instruction understanding capabilities. The proliferation of LLMs has also faciliated the development of Multimodal Large Language Models (MLLMs), which can perceive and analyze information from images and other sources . Some existing works have demonstrated that MLLMs exhibit preliminary visual document understanding capabilities, as they can extract and comprehend information from complex documents containing textual and visual elements, such as tables, charts, and graphics . Given their ability to capture the relationships between textual and visual information, employing MLLMs for visual document understanding tasks shows great potential.

However, the document image processing capabilities of MLLMs are restricted in real-world scenarios, primarily due to two reasons: the limitations posed by low-resolution image inputs and the lack of document-oriented visual instruction tuning .

The restriction of low-resolution image inputs is a prevalent challenge in the multimodal community. Current models usually handle images with relatively low resolutions, typically $224\times 224$ pixels . While this resolution is sufficient for the majority of natural images, it can result in extensive text distortion when it comes to processing document images. As illustrated in Figure 1, clear text in high-resolution images becomes blurred when resized to a lower resolution.

Directly increasing the image resolution generates a large number of visual tokens, which will occupy the limited input capacity of LLMs, and induce considerable training costs and inference latency . Taking CLIP’s image encoder as an example, a $1536\times 1536$ image partitioned into $16\times 16$ patches results in 9216 visual tokens, which exceeds the context length of many existing open-source LLMs, such as LLaMA-2 with a context length of 4096. In addition, they exhibit quadratic computational complexity with respect to the length of the patch sequence.

On the other hand, general-purpose MLLMs suffer from a lack of document-oriented visual instruction tuning , leading to an incomplete understanding of document images. Unlike ordinary images, document images possess distinct layout and structural information, where the font, style, and color hold significant importance for comprehending the content .

To tackle these challenges, we propose a novel multimodal large language model called HRVDA (High-Resolution Visual Document Assistant), which employs a content filtering mechanism and an instruction filtering module designed to filter out content-agnostic visual tokens and instruction-agnostic visual tokens, respectively.

Specifically, content-agnostic visual tokens contribute a significant amount of redundant information, while the regions in document images that contain text, tables, charts, and other document content frequently provide the most valuable information. As shown in Figure 1, the pixels within these regions constitute only a small proportion of the entire image . To reduce the number of blank background tokens, our proposed content filtering mechanism, based on a content detector, can extract key features from document images. Conservatively estimated, this approach filters out approximately 50% of content-agnostic tokens in practice, resulting in a substantial reduction of 30% in training and inference latency without compromising performance.

Meanwhile, instruction-agnostic visual tokens refer to the parts that are not within the instruction attention region. In conventional document understanding tasks, such as information extraction, document-oriented instructions often rely on localized areas to generate answers . Therefore, we design the instruction filtering module to further filter instruction-agnostic visual tokens and significantly reduce the workload of the LLM.

To improve the document understanding capabilities of HDVDA, we construct a document-oriented visual instruction tuning dataset. This dataset covers an extensive array of tasks within the document domain, including information extraction, text recognition, and visual question answering. It also incorporates a variety of scenarios, such as tables, charts, natural images, and webpage screenshots. Furthermore, we employ ChatGPT to generate a diverse collection of instruction templates, thereby strengthening the generalization capabilities of the model.

Our experimental results on multiple document-oriented datasets demonstrate that HRVDA’s OCR-free document comprehension capabilities surpass current state-of-the-art MLLMs such as mPLUG-DocOwl , UReader .

In summary, our main contributionsare as follows:

We present HRVDA (High-Resolution Visual Document Assistant), which, to the best of our knowledge, is the first large multimodal model designed to directly accept high-resolution image inputs.

We propose a content filtering mechanism and an instruction filtering module to prune visual tokens, which significantly accelerate the model’s training and inference, making the processing of high-resolution image inputs computationally feasible.

We construct an extensive document-oriented visual instruction tuning dataset to enhance the model’s document analysis capabilities.

Experimental results on a series of document-oriented datasets demonstrate that HRVDA achieves state-of-the-art performance.

Related Work

Visual Document Understanding (VDU) refers to the automated process of analyzing, comprehending, and processing document images . Existing methods can be broadly categorized into two groups, OCR-dependent methods and OCR-free methods.

OCR-dependent methods typically rely on an external OCR interface to extract text content and coordinate information from document images . For instance, the LayoutLM family leverages multimodal pre-training to combine image layout features with textual features. DocFormer undergoes unsupervised pre-training through carefully designed tasks to encourage multimodal interactions. UDOP harmonizes image, text, and layout modalities into a unified and cohesive representation by leveraging the spatial relationships within the document. These methods typically face issues such as increased computational costs and error accumulation .

OCR-free methods aim to extract structured text directly from images in an end-to-end manner. This approach simplifies the information processing process, speeds up the reasoning and has gained significant attention in the VDU community recently . For example, both Donut and Dessurt utilize Swin Transformer to extract image features, followed by cross-attention operations between decoder models like BART and image features to generate text in an auto-regressive manner. SeRum goes a step further by employing selective region concentration to enhance the precision and speed of generation.

2 Multimodal Large Language Models

MLLMs have become a new research focus recently . According to the modality alignment approach, they can be roughly divided into two categories: query-based methods and projection-based methods.

Query-based methods involve utilizing a set of learnable query tokens to extract information through cross-attention mechanisms. Flamingo and BLIP-2 are the first to adopt this approach, which is later inherited by a series of works . However, this method essentially introduces a textual supervisory signal to extract image features and is not suitable for fine-grained prediction tasks. The experimental results are provided in the Appendix A.

Projection-based methods involve directly mapping visual tokens with the LLM’s input space . For instance, LLaVA employs a simple linear layer to project image features . LLaMA-Adapter applies a lightweight adapter module to align visual tokens and text tokens . This approach allows the LLM to perceive the entire image, offering a more promising perspective for effective multimodal learning.

3 Token Pruning

Token pruning is a technique aimed at reducing model parameters and computational complexity . It achieves model simplification and compression by removing certain weights or feature representations. Numerous methods for pruning vision transformers have been proposed . DynamicViT accelerates model inference by sparsifying less important tokens using lightweight prediction modules. SparseViT efficiently processes high-resolution images through sparse activations, enabling efficient dense prediction tasks. STVit achieves efficient global and local processing in ViTs by removing redundant image tokens and can serve as a backbone for downstream tasks. These pruning techniques are designed for natural images and are not suitable for document images.

HRVDA

In this section, we start with the model architecture (in Section 3.1), followed with a detailed explanation of the Content Filtering Mechanism (in Section 3.2) and the Instruction Filtering Module (in Section 3.3). Finally, we introduce the instruction tuning dataset constructed for document understanding (in Section 3.4) and the training strategy (in Section 3.5).

HRVDA is a large multimodal model designed to address the challenges posed by high-resolution requirements in visual document understanding tasks. As shown in Figure 2, it mainly consists of four modules: a content detector, an image encoder, an instruction filtering module (IFM), and an LLM.

The initial step involves partitioning the original image into a series of patches, which are subsequently converted into a sequence of visual tokens. These tokens are then processed by a content detector to assess the probability of each token containing significant information. Leveraging these probabilities, a content filtering mechanism enables the image encoder to selectively compute visual features and eliminate content-agnostic visual tokens. These encoded visual features are subsequently integrated with the instruction features using a self-attention mechanism within the instruction filtering module. A straightforward 2-layer MLP network is employed to classify these fused features and further exclude instruction-agnostic visual tokens. Ultimately, the highly refined visual tokens are concatenated with the instruction tokens and fed into the LLM for generating the anticipated response. This approach ensures a more efficient and effective representation of the image content, tailored specifically for the task at hand.

2 Content Filtering

In conventional Transformer architectures , high-resolution images are converted into long token sequences, which poses a substantial demand on computational resources. Moreover, elongated sequences introduce challenges in capturing long-range dependencies.

A potential solution to these challenges lies in the unique properties of document images: they typically consist of extensive areas of blank background, while content-rich regions provide the majority of valuable information . To leverage the sparse content information effectively and efficiently, we propose a content filtering mechanism, primarily involving two modules: the content detector and the image encoder.

Content Detector. A pluggable network is employed to identify whether each token contains important content. For document images, such content includes elements such as text, tables, and charts . The choice of network can be quite diverse. It could be a simple MLP network for token classification, a detection network like DETR , or a segmentation network like U-Net applied to reshaped feature maps. In this work, we employ a shallow PSENet , which is designed as a segmentation-based detector capable of localizing text instances of any shape. The content detector adopts a high recall rate strategy, ensuring that all visual tokens containing content are preserved.

Image Encoder. A visual backbone network is used to extract image features. In contrast to most MLLMs that utilize ViT , we adopt the Swin Transformer as our image encoder, which utilizes a window-based mechanism for self-attention computation, mitigating computational burdens. Moreover, it incorporates a token merge mechanism to prevent the direct loss of information. The Swin Transformer’s downsampling of feature maps also contributes to a further reduction of the number of visual tokens.

As shown on the left side of Figure 2, a skip connection is introduced in each Swin Transformer block to accelerate computation:

where $F^{j}$ represents the operation in the $j$-th Swin Transformer block, and $h^{j}$ is the hidden state of the visual tokens.

For a well-trained content detector, we employ a threshold $\epsilon_{c}$ to adjust the probability values in $P$ for tokens containing content:

Utilizing these probabilities, if none of the tokens within a window is considered to contain content, the window bypasses the attention computation and is directly passed to the next block, thereby achieving computational acceleration.

It is worth highlighting once again that content-agnostic tokens are not directly removed, making the four merging adjacent patches spatially close. The shifted window partitioning approach in the Swin Transformer enables interactions between different tokens, thereby preserving potentially useful layout information and enhancing modeling capabilities.

The patch merging operation in Swin Transformer consolidates adjacent $2\times 2$ regions into a single new patch, and the probability of the merged patch containing content is set to the maximum value among the probabilities of the 4 original regions:

To further preserve global information, the threshold value $\epsilon$ is progressively increased from shallow to deeper layers. Preserving more tokens in the shallow layers can reduce the loss of visual information.

3 Instruction Filtering

Document-oriented instructions are highly specific, typically referring only to particular regions within the image, which necessitates further filtering of visual tokens.

Several existing methods, for instance, the Q-Former module in BLIP-2 and the Visual Abstractor in mPLUG-owl , employ learnable queries to extract valuable information. Nevertheless, this approach inadvertently leads to a diminished representation of visual information, making it less suitable for fine-grained prediction tasks. Moreover, the inclusion of query vectors essentially relies on text as a supervisory signal, yet the textual descriptions of images are often insufficient to provide accurate representations. On the other hand, we experimentally discover that for high-resolution images, approximately 500 query vectors are required to maintain performance without significant degradation. This indicates that this approach does not offer a computational advantage in terms of processing speed.

In this study, we utilize a more direct instruction filtering module (IFM) that avoids excessive compression of visual information, thus preserving its integrity.

Formally, the visual vectors obtained from the image encoder and the instruction vectors are concatenated and then fed into the instruction filtering module for further processing. Then, a Transformer layer is employed to facilitate the fusion of these feature vectors:

where $SA$ stands for the self-attention layer, $FFN$ represents the feedforward layer, and $V$, $I$ denote the visual vectors and instruction vectors, respectively. The fused visual features $V^{\prime}$ are then sent to a 2-layer MLP for binary classification to filter out visual tokens that are irrelevant to the instructions . Similar to the content detector, the instruction filtering module also adopts a filtering threshold $\epsilon_{i}$, as in Equation 3, to increase the classification recall rate, ensuring that visual tokens related to instructions are not discarded.

Ultimately, following content-agnostic and instruction-agnostic filtering, the visual token sequences are fed into the LLM.

4 Visual Instruction Tuning

In this section, we primarily introduce the task of visual instruction tuning and the data sources.

Tuning Tasks. To enhance HRVDA’s generalization in visual document understanding, we organize a wide range of document tasks into an instruction format. In this work, we primarily focus on tasks such as document classification (DC), information extraction (IE), visual question answering (VQA), optical character recognition (OCR), visual grounding (VG), image captioning (IC), and table reconstruction (TR). Table 1 presents some fundamental examples.

To diversify the range of prompts, we first manually craft 10 prompt templates for each task. Subsequently, we employ ChatGPT to generate 50 similar prompts, which are then reviewed by human experts to ensure their alignment with the intended meaning. Additional templates can be found in the Appendix B.1.

Instruction Data Resources. A large number of real-world and synthetic datasets are collected. The real-world datasets used in this study include IIT-CDIP , CORD , SROIE , DocVQA , InfographicsVQA , DeepForm , Kleister Charity , WikiTableQuestions , TabFact , ChartQA , TextVQA , TextCaps , VisualMRC , PubTabNet , etc. Given the limited availability of open source data, in this work a significant amount of data synthesis methods are applied, such as SynthText , Synth90K and SynthDoG . Due to space constraints, more details can be seen in the Appendix B.2.

5 Training Strategies

In order to achieve visual token filtering and enhance the model’s document-oriented instruction understanding capabilities, a multi-stage training strategy is adopted in this work as shown in Figure 3.

Stage 1 focuses on training the content detector. We employ external OCR tools and detection networks to obtain the coordinates of various elements, including text, charts, tables, etc. These coordinates can be used to provide supervised signals for the PSENet, determining whether each visual token contains content or not. Stage 2 concentrates on the pretraining of the image encoder. Our encoder is integrated with m-BART via cross-attention to perform the task of recognizing all text within the images . Stage 3 involves the training of the instruction filtering module. For data with fixed layouts, a high filtering threshold is used. Conversely, we utilize a low filtering threshold for data characterized by variable layouts. Stage 4 entails implementing low-rank adaptation techniques to preserve the general conversational capabilities of the LLM . Additional training details can be found in the Appendix C.

Experiments

In this section, we conduct experiments on numerous publicly available document-oriented datasets to validate the effectiveness of our proposed HRVDA model.

In visual document understanding, information extraction and text-oriented visual question answering are challenging tasks, which also have widespread applications in practice.

Information Extraction involves extracting structured key-value pair data from documents. In this study, we use the two most commonly used datasets for evaluation, CORD and SROIE . They are all scanned receipt images and have good image quality. The F1 score is reported, which is the weighted harmonic mean of Precision and Recall.

Text-oriented Visual Question Answering is a highly generalizable task, capable of addressing various problems through appropriate prompts. We evaluate HRVDA on a wide range of publicly available datasets, including DocVQA , InfoVQA , TextVQA , ChartQA , DeepForm , KLC , WTQ , TableFact , VisualMRC , and TextCaps . Different metrics, including ANLS, CIDEr, Accuracy, and F1 Score are reported in accordance with the methodologies employed in previous works. A detailed description can be found in the Appendix B.2.

2 Implementation Details

Model Architecture. Our HRVDA model employs Swin-L as the image encoder. Its layer and window sizes are set to 2, 2, 18, 2, and 10, respectively, with a patch size of $4\times 4$. Additionally, the image resolution is set to $1536\times 1536$. In this study, we conduct experiments based on LLaMA-2-7B , which has a context length of 4096.

Training Details. We employ the Adam optimizer for each stage of training, with an initial learning rate of 1e-4. The learning rate schedule uses a linear warmup during the first 20% of steps. For LoRA, we set the rank to 8. Unless otherwise specified, the detection thresholds for content filtering in the Swin Transformer are set to $\epsilon_{c}=[0.25,0.25,0.5,0.5]$ in 4 stages, while the threshold for instruction filtering is set to $\epsilon_{i}=0.5$. The batch size is set at 128. All training is conducted on 128 Tesla V100 GPUs for 10 epochs.

3 Comparisons with Previous Approaches

We conduct a comparative analysis of HRVDA against OCR-free models, including Donut , SeRum , Pix2Struct , Qwen-VL , mPLUG-Doc , and UReader , utilizing 12 publicly available datasets for evaluation.

These models can be broadly categorized into two classes: encoder-decoder models and MLLMs. The first class utilizes a cross-attention mechanism to fuse image and text, resulting in computational efficiency for high-resolution image inputs while simultaneously requiring task-specific fine-tuning. The second class leverages LLMs, offering exceptional understanding capabilities, but often unable to directly process high-resolution inputs.

As demonstrated in Table 2, HRVDA achieves the best results across the 9 datasets. In information extraction tasks, our model significantly surpasses current state-of-the-art performance, owing to our robust visual pretraining (Stage 2). In visual question answering tasks, understanding the question becomes crucial, particularly in datasets with a high prevalence of elements from natural scene . The semantic analysis capabilities of the decoder in the first category are limited, which prevents them from achieving optimal performance. Previous MLLMs are constrained by the visual information distortion caused by low-resolution image input, which also prevents them from achieving desirable results. Consequently, our HRVDA model directly processes high-resolution image inputs, minimizing the loss of visual information and thereby delivering substantial performance enhancements.

In terms of efficiency evaluation, we use Qwen-VL as our baseline and evaluate the forward-inference latency on a Tesla V100 GPU. The results reveal that HRVDA’s speed is significantly faster than Qwen-VL’s across various filtering thresholds, as illustrated in Table 3. Remarkably, when both thresholds are set to 0.5, HRVDA reduces the runtime by 61%. However, due to the constraints of GPU memory usage, we do not further increase the resolution.

4 Ablation Study

In this section, we separately explore the impact of filtering thresholds in the visual filtering mechanism and instruction filtering module.

Figure 4 showcases several examples of token pruning. It can be observed that for text-dense images, the proportion of filtered pixels is considerably high. In contrast, for images containing charts and natural elements, the filtering ratio is lower, as more visual semantic information is required for these types of images. On the other hand, we quantitatively evaluate the impact of filtering thresholds in the content filtering mechanism and instruction filtering module on prediction accuracy and inference latency, as shown in Figure 5. As the threshold increases, the accuracy of the prediction gradually improves, reaching its peak at 50% and then experiencing a decline. The inference latency decreases almost linearly with the filtering threshold. These results indicate that appropriate token pruning not only accelerates computation but also improves performance, as removing redundant information can reduce the difficulty for the model to extract key information.

5 Qualitative Analyzes

As shown in Figure 6, HRVDA can recognize text in specific areas based on location hints. This is extremely useful in practical applications, as people often describe vague locations to obtain information. HRVDA also successfully identifies the highly blurred text Menu, which may be due to the influence of visual semantic cues. Utilizing comprehensive document-oriented visual instruction tuning, HRVDA exhibits outstanding capabilities in following document instructions. More cases can be found in the Appendix D.

Conclusion

In this work, we propose a new OCR-free multimodal large language model, HRVDA, which can directly accept high-resolution image inputs and is suitable for fine-grained prediction tasks. To the best of our knowledge, HRVDA is the first MLLM to utilize the Swin Transformer as an encoder. Additionally, we employ a content filtering mechanism and an instruction filtering module to alleviate the computational challenges brought about by high-resolution inputs. Experimental results demonstrate that our HRVDA model achieves state-of-the-art results on a series of public datasets, while also exhibiting significantly faster speeds compared to previous MLLMs. In the future, we will continue to investigate high-resolution challenges.

Acknowlegement

This research was supported by the National Key Research and Development Program of China (Grant No. 2022YFB3103100), the National Natural Science Foundation of China (Grant No. 62276245).

References

Appendix

Appendix A Analysis of Query-based Feature Extraction

In this section, we offer an experimental analysis to clarify our reasoning behind not choosing a query-based feature fusion approach.

Building upon the Donut framework , we employ Q-Former to extract image features and conduct cross-attention operations with Bart using the extracted features. We fine-tune the model on the DocVQA dataset, and the experimental results are illustrated in Figure 7. When the image resolution is set to 1280, we observe that an insufficient number of query vectors can significantly degrade the model’s performance. To mitigate this decline while maintaining the model’s performance, 500 query vectors are required. However, this approach to information extraction is not highly efficient in practice. Consequently, we choose a direct fusion approach in the instruction filtering module to retain visual information to the greatest extent possible.

Appendix B Visual Instruction Tuning

As shown in Table 4, we present additional instruction templates. A greater number of instruction templates can significantly enhance the model’s generalization capabilities and improve its performance in real-world applications. It is worth noting that users’ perspectives in posing questions are diverse; therefore, having an adequate number of templates allows the model to better understand and respond to real-world instructions.

B.2 Details of Datasets

In this section, we provide a detailed introduction to the various datasets used in our experiments.

CORD The CORD dataset comprises 800 training receipts, 100 validation receipts, and 100 test receipts. Each receipt is accompanied by a photo and a set of OCR annotations. The dataset identifies 30 fields across four categories, and the task’s objective is to correctly assign each word to the appropriate field. The evaluation metric used is the entity-level F1 score, and official OCR annotations are utilized.

SROIE The SROIE dataset is designed for extracting data from digitized receipts. It consists of 626 training samples and 347 testing samples. The objective is to retrieve information for up to four specific keys from each receipt: company, date, address, and total. The assessment metric used is the entity-level F1 score. Official OCR annotations are utilized, and the test set outcomes are supplied by the authorized evaluation platform.

DocVQA The DocVQA dataset comprises 50,000 questions based on more than 12,000 pages from a diverse range of documents. The pages are divided into training, validation, and test sets at a ratio of approximately 8:1:1. The task’s evaluation employs an edit distance-based metric called ANLS (average normalized Levenshtein similarity).

InfoVQA The InfographicVQA dataset consists of 30,035 questions and 5,485 images, originating from 2,594 distinct web domains. This dataset employs the ANLS metric for evaluation, where higher scores are assigned if the predicted answer has a smaller difference from at least one of the ground-truth answers.

DeepForm DeepForm is a socially important documents related to election spending with the objective of extracting contract numbers, advertiser names, payment amounts, and advertisement broadcast dates from advertisement disclosure forms. The dataset comprises 700 training samples, 100 validation samples, and 300 testing samples. The evaluation metric used is the F1 score.

KCL Kleister Charity is a document understanding dataset designed for the extraction of information related to charitable organizations. It consists of 1,700 training samples, 400 validation samples, and 600 testing samples. The evaluation metric employed is the F1 Score.

WTQ WikiTableQuestions is a question-answering dataset that comprises semi-structured HTML tables sourced from Wikipedia. It includes 1,400 training samples, 300 validation samples, and 400 testing samples. The evaluation metric employed is accuracy.

TabFact TabFact is a dataset designed for investigating fact verification tasks in the context of semi-structured evidence. It consists of 13.2K training samples, 1.7K validation samples, and 1.7K testing samples. The evaluation metric employed is accuracy.

ChartQA ChartQA is a question-answering dataset targeting data visualizations in the form of charts, involving both visual and logical reasoning. It comprises 9.6K manually curated questions and 23.1K questions automatically generated from manually curated chart summaries. The evaluation metric employed is relaxed accuracy.

TextVQA The TextVQA dataset is constructed by extracting images and questions from the Open Images v3 dataset. It consists of 34,602 training samples, 5,000 validation samples, and 5,734 testing samples. The evaluation metric employed is accuracy.

VisualMRC The VisualMRC dataset aims to enable machines to read and comprehend text in real-world documents and respond to natural language questions. This dataset comprises over 30,000 question and abstractive answer pairs derived from more than 10,000 document images spanning multiple web domains. The evaluation metric employed is CIDEr. The computation of CIDEr is based on syntactic consistency, content consistency, consistency metrics, and diversity evaluation, synthesizing the similarity and consistency scores between the generated image descriptions and multiple reference descriptions.

TextCaps The TextCaps dataset consists of 28,408 images and 142,040 captions, requiring models to read and comprehend textual information within the images and generate coherent descriptions. The evaluation metric employed is CIDEr.

Appendix C Training

In this section, we primarily provide a detailed description of Stage 2 of our training strategy.

Stage 2 essentially involves the pretraining of the image encoder. Currently, open-source image encoders mainly focus on two aspects: one is performing image classification tasks using datasets like ImageNet, and the other is aligning image and text features based on contrastive learning. These two pretraining paradigms are not suitable for generative tasks such as text recognition, as there is a significant difference between the pretraining methods and downstream tasks.

To make the image encoder more suitable for text recognition and generation tasks, we employ a method similar to Donut for pretraining the image encoder, as illustrated in Figure 8. We primarily construct a temporary model to perform a pseudo-OCR task, which involves recognizing all text in the image in a top-to-bottom and left-to-right order. This pretraining task is more consistent with the downstream tasks, enabling our final HRVDA model to possess strong text recognition capabilities.

Appendix D Qualitative Experimental Analysis

In this section, we provide some supplementary qualitative analysis.

As depicted in the first two rows of Figure 9, HRVDA can recognize colors, positions, and artistic fonts, which is primarily attributed to its visual pretraining. Furthermore, leveraging the semantic understanding capabilities of the LLM, HRVDA can also recognize text in complex regions, such as identifying the field above a particular field. Even when dealing with images containing long text, HRVDA demonstrates strong full-text OCR capabilities.

Nonetheless, HRVDA struggles with certain highly challenging examples, as illustrated in the last row of Figure 9. For example, the HRVDA model faces comprehension difficulties when processing images that have an exceptionally high density of text and exhibit intricate structural relationships. Moreover, HRVDA is not well-suited for images with extreme proportions. As demonstrated in Figure 9-(h), the model can only manage such images by performing multiple cropping operations, which inevitably compromises its grasp of the overall image structure. Furthermore, HRVDA is incapable of generating an adequate understanding for exceedingly complex instructions. To address these extremely challenging examples, we plan to further increase the resolution and employ a more powerful LLM in future iterations.

We also evaluate the performance of HRVDA using open-domain data, as shown in Figure 10. HRVDA performs exceptionally well in information extraction tasks for common fields, such as dates, amounts, fax numbers, etc. Overall, if the answer relies more on simple text recognition, HRVDA can perform very well, significantly advancing the practical application of MLLMs.