Comparison of JPEG’s competitors for document images

Mostafa Darwiche1, The-Anh Pham1 and Mathieu Delalandre1 1 Laboratoire d’Informatique, 64 Avenue Jean Portalis, 37200 Tours, France e-mail: fi[email protected]

Abstract— In this work, we carry out a study on the per- in [6] concerns assessing quality of common image formats formance of potential JPEG’s competitors when applied to (e.g., JPEG, TIFF, and PNG) that relies on optical charac- document images. Many novel codecs, such as BPG, Mozjpeg, ter recognition (OCR) errors and peak signal to noise ratio WebP and JPEG-XR, have been recently introduced in order to substitute the JPEG. Nonetheless, there is a lack of (PSNR) metric. The authors in [3] compare the performance performance evaluation of these codecs, especially for a particular of different coding methods (JPEG, JPEG 2000, MRC) using category of document images. Therefore, this work makes an traditional PSNR metric applied to several document samples. attempt to provide a detailed and thorough analysis of the Since our target is to provide a study on document images, aforementioned JPEG’s competitors. To this aim, we first provide we then use a large dataset with different image resolutions, a review of the most famous codecs that have been considered as being JPEG replacements. Next, some experiments are performed them at very low bit-rate and after that evaluate the to study the behavior of these coding schemes. Finally, we extract output images using OCR accuracy. We also take into account main remarks and conclusions characterizing the performance the PSNR measure to serve as an additional quality metric. of these codecs for different contexts in accordance with OCR The main goal of this study is to make a clear statement performance and PSNR metric. about performance of JPEG standard and its competitors on Keywords— , document encoding, trans- document images in terms of compression rate, coding quality, form coding, coding artifacts. artifact distortion and processing time. The rest of this paper has been oganized as follows. I.INTRODUCTION In Section II, we provide a review of the most potential With fast increasing 3G-/4G-based markets and infrastruc- competitors of the JPEG standard. Next, Section III details tures, online retrieval of document images and exploiting the evaluation protocol, experimental settings and evaluation related applications are becoming a crucial need for users with metrics. Section IV presents the detailed results along with an handheld-devices. Customers want to access and retrieve good open discussion of all the studied codecs. Finally, Section V quality images while expecting a low bandwidth consumption. concludes the paper and gives several directions of potential Additional needs are fast time response and memory-efficient future researches. usage. These constraints imply that document images must be encoded and decoded in a very efficient manner. Loss- II.REVIEWOF JPEG’SCOMPETITORS less compression algorithms allow the encoded images to be A. (BPG) correctly reconstructed but the gain of compression ratio is not satisfactory. In contrary, lossy coding schemes provide BPG [12] is a new lossy image compression technique. very high compression rate at the cost of losing some certain It is based on the latest video compression standard High degree of image quality. Besides, coding quality can be easily Efficient Video Coding (HEVC [9]) which has been developed controlled by a set of parameters. For these reasons, most by the Joint Collaborative Team on Video Coding (JCTVC). multimedia data in practice employ a lossy coding algorithm. BPG uses a subset of JCTVC (default) and HEVC en- In this work we move out of the box and the standards coders. Both share the same workflow for encoding/decoding schemes and run experiments on some of the famous coding and implement the essential tools of HEVC. However x265 schemes (BPG1, Mozjpeg2, WebP3 and JPEG-XR4), where all is considered as a fast implementation of HEVC, it offers of them are competing with the JPEG5 standard, and might different encoding settings and reflects a fair trade-off between be fair successors. Several attempts were done [17] [18] and compression rate and encoding speed. Also, x265 is developed showed that these codecs have beaten the standard JPEG based on single instruction multiple data (SIMD) and multi- for natural images. However, the tests were conducted on media extensions (MMX) technologies [19]. Consequently, it small datasets and the results were evaluated by computing could be much faster than JCTVC but with lower compression different similarity quality metrics (Y-SSIM [2], RGB-SSIM quality. [2], IW-SSIM [8] and PSNR-HVS-M [4]). A relevant work Figure 1 shows the subset of HEVC encoder and decoder used in BPG [10]. Essentially, HEVC is a video compression 1 http://bellard.org/bpg/ technique based on a hybrid coding scheme which involves 2https://github.com/mozilla/mozjpeg 3https://developers.google.com/speed/webp/download inter/intra predictions, transform and entropy coding after 4https://jxrlib.codeplex.com/ decomposing input pictures into varying size coding tree 5http://www.ijg.org/ blocks (CTB). The inter prediction mode explores the spatial redundancy between successive frames, while the intra pre- nonzero transform coefficient levels or not, the last nonzero diction mode exploits the spatial redundancy within a single level index, the presence of nonzero levels in the subblocks frame. Consequently, for still picture coding, it is possible and some other syntax elements. to use HEVC with intra prediction mode as BPG does. As shown in Figure 1, the encoder is an iterative process Encoded signal Entropy Prediction mode Intra Prediction composing of three main blocks: intra prediction, encoding Decoding

(i.e., transform/quantization), and decoding (i.e., inverse trans- form/quantization). All of these blocks are processed on the Predicted signal variable size CTB representation. That means the size of CTB Decoded + can be flexible (i.e., 16 × 16, 32 × 32, 64 × 64). By using Inverse (Transform + signal &Quantization) Filtering a flexible and adaptive size, HEVC enables the encoder to exploit better spatial redundancy, especially when working with high resolution images. Fig. 2. Subset of HEVC decoder used by BPG.

Original For the decoding side as shown in Figure 2, the encoded Signal Residual Transform & Inverse (Transform + signal is first processed by the entropy decoder to yield Quantization &Quantization) - the quantized transform coefficients (QTC) of the residuals RDO accompanying with the prediction mode. Next, dequantization + + is applied to these coefficients followed by inverse transforma-

Intra Prediction Encoded tion to reconstruct the residual in spatial space. The resulting Predicted Entropy signal residuals are then added with the predicted signal to form Signal Prediction mode Coding the complete signal which is eventually undergone a filtering

process (i.e, deblocking and sample adaptive offset filters) to Fig. 1. Subset of HEVC encoder used by BPG. reduce coding artifacts.

Each CTB is then divided into coding units (CU), each B. Mozjpeg of which can be further partitioned forming a quad-tree Mozjpeg is an open license project established by Mozilla decomposition that minimizes the rate distortion (RD) cost community [13]. Its main target is to provide a web encoding (known as recursive quad-tree splitting process). Every CU workflow. Applying one instruction on array/vector of data consists of a coding block (CB) for luminence and two CBs with multiple processors is determinedly faster than running for chrominance along with syntax elements. Each CU will the instruction on each data value ( in case of image), this be the root of one or multiple prediction units (PU) and approach is well known and used commonly for image/video transform units (TU). PU and TU have the same components encoders. Mozjpeg applies during the compression the Huff- as CU (i.e., one luma block, two chroma blocks and syntax man trees and progressive scans, also the Trellis quantization elements). PU is computed based on intra-picture prediction technique which is based on carefully determining the details which provides up to 35 prediction modes (33 angular, planar that could be discarded (reduced the losses during quantization and direct current (DC)) applied to the neighbors of the current stage). CU. Each prediction mode indicates a different direction to compute the block predictor, making it efficiently fitting to One of the most important features that is included in the the block content characteristics. TU holds the residual which latest release of Mozjpeg library (3.0) is the reduction of is computed as the difference of the prediction and the original the ringing artifacts that appears in black and white JPEG blocks. The size of the square transform block (TB) can be images at low bit rate compression (Figure 3 (a) shows the varied from 4×4 to 32×32. Except for the smallest TB (4×4) waveform distorted) [14]. This is addressed by increasing which uses the discrete sine transform (DST), the others are the minimum and maximum of the waveform in the DCT encoded using discrete cosine transform (DCT). domain for sharp edges before applying the quantization. This approach takes care of the clipping of the overshooting Originally, HEVC performs rate distortion optimization caused by the quantization. It extrapolates the waveform with (RDO) by using the Lagrangian encoding technique [11] splines to produce rounded minimum and maximum waves. applied in 3 modes: mode decision, motion estimation and Consequently, it reduces the sharpness of the edges and quantization. However for BPG, the motion estimation is corners (Figure 3 (b) depicts the waveform after clipping). ignored. Mode decision emerges primarily in the selection Nevertheless, this feature is still limited and only deals with of the intra prediction modes (i.e., the best one out of 35 artifacts in black-on-white document images. directions), the selection of transform size of CU, and the decision whether to split a CU into more smaller CUs. The . WebP best combination coding mode is selected so as to minimize the RD cost. As for the quantization and for each TB, it WebP is a new lossy and technique computes several flags to indicate whether the TB holds found by with its main role to speed up the image adaptive , DC and AC coefficient prediction scheme, and coefficients reordering. Additionally, JPEG-XR breaks the image into rectangular tiles before encoding, thus decompression of a specific area in the image can be done easily by decoding the needed tiles without the need of (a) (b) decoding the whole image. Fig. 3. Deringing process in Mozjpeg [14]: (a) wzaveform Overshoot (cyan) and undershoot (red); (b) waveform after clipping. III.EXPERIMENTAL SETTINGS

Comparative methods: Our evaluation will mainly focus transmission over the network [15]. The on the aforementioned codecs (BPG1, MozJPEG2, WebP3 and algorithm of WebP is based on block prediction for image JPEG-XR4). In addition, the JPEG standard5 will be also encoding, the same one used in VP8 ( found by included to serve as a baseline method. For BPG we will Google as well). In the same spirit as the BPG codec, the run the tests using the default encoder JCTVC and then x265 idea is to compute the difference between actual values in a encoder. It is worthy pointing out that x265 encoder supports block and the predicted ones based on the neighbor (intra) only binary images. blocks; this residual will be encoded during the compression. It is worthy to mention that WebP compression also supports Evaluation metrics: One of the important aspects of docu- variable block sizes to efficiently exploit the spatial redun- ment image quality assessment (IQA) concerns the readability dancy of local content characteristics. The residuals computed of the inspecting documents. Subjective IQA addressed by based on block prediction will be transformed using large human perception is prone to error and is too time-consuming. DCT or ADCT (asymmetric DCT). Next, the transformed It is, therefore, preferred to using a machine-based readability coefficients are quantized as usual. Due to the use of residuals measure to assess the document quality. To this aim, OCR in the encoding, the quantized image will contain many zero- performance seems to be the most appropriate measure as valued elements which will be encoded efficiently with entropy most core activities in the field of document image analysis encoding. At the end WebP file uses the resource interchange (DIA) concern the development of robust reading systems. file format (RIFF) for image and encoded stream rep- In fact, this point was mentioned in [1], that an important resentation. WebP supports in both lossless and measure to reflect machine readability is OCR accuracy. In 6 lossy compressions, which could be considered an advantage our experiments, the latest tool ABBYY FineReader 12.04 over JPEG and PNG [15]. is selected to serve as an OCR system. At the same time, we take into consideration the PNSR measure, as it is the D. JPEG-XR most commonly used to evaluate the quality of decompressed images. JPEG-XR is an image compression format developed by to compete with the standard JPEG and to provide Datasets: We apply our tests on the Medical Archive Records (MAR) dataset for OCR recognition from U.S. Na- a high compression ratio with less artifacts. It supports both 7 lossy and lossless compression in addition to the new features tional Library of Medecine . This dataset consists of 293 of transparency (alpha channel), fixed and floating point color real documents, scanned at 300dpi, from different types of values and progressive decoding levels [5], [7], [16]. biomedical journals and thus could be useful for our purpose of evaluating the coding distortion on real scenarios. Each doc- Similar to the JPEG codec, JPEG-XR encoder will sub- ument contains several zones accompanying the corresponding sample the image after converting it to an internal colorspace groundtruth. Hence, These zones are cropped and saved into (luma-chroma colorspace), divide it into blocks, then transform seperate images, resulting in 296 images in total. These text them into the frequency domain and finally apply quantization images will serve to run OCR performance, while PSNR and entropy coding. However it treats each step differently. measures are computed directly on the original 293 documents. First, for the it uses 4x4 blocks (it might vary To stress all the codecs, we also created another dataset which depending on the used) and applies PCT is the low resolution version (170 dpi) of the original text (Photo Core Transform) which is an integer transformation images and documents. Consequently, OCR and PSNR results with lifting scheme. The rationale of using PCT is to avoid are done on both high resolution (300 dpi) and low resolution the losses caused by DCT. Furthermore, it introduces a filtering (170 dpi) images/documents. OCR accuracy is traditionally step called POT (Photo Overlap Transform) before running the computed as the ratio of the number of properly recognized PCT on each 4x4 block where POT is only presented at low characters to the total characters in the groundtruth [6]. The bit-rate compression since its role is to reduce the blocking final OCR accuracy and PSNR are averaged, according to the artifacts. Subsequently, it groups the blocks into macroblocks bit-rate, from the results of all the images/documents in the of size 16x16 for direct and alternating currents (DC/AC) datasets. computations (DC and AC are computed after the DCT transformation). Finally, an adaptive entropy coding will code 6http://finereader.abbyy.com/professional/features/ the DC and AC coefficients, using three techniques including 7http://marg.nlm.nih.gov/ OCR result for high resolution images OCR result for low resolution images 100 100

95 90 BPG−JTCVC BPG−JTCVC BPGx265 80 BPGx265 90 WebP WebP MozJpeg MozJpeg 70 JPEG baseline JPEG baseline 85 60

OCR (%) 80 OCR (%) 50

75 40

70 30

65 20 0.1 0.2 0.3 0.4 0.5 0.6 0.1 0.2 0.3 0.4 0.5 0.6 Bit−Rate Bit−Rate (a) (b)

PSNR result for high resolution images 90 PSNR result for low resolution images 50 BPG−JTCVC BPGx265 BPG−JTCVC 80 MozJpeg BPGx265 WebP 45 MozJpeg JPEG−XR WebP 70 JPEG baseline JPEG−XR 40 JPEG baseline

60 35 50 PSNR (dB) PSNR (dB)

30 40

25 30

20 20 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Bit−Rate Bit−Rate (c) (d)

Fig. 4. OCR accuracy and PSNR results of all the codecs: high resolution (left) and low resolution (right) images.

PSNR distribution for 293 high resolution images of JPEG−XR codec PSNR distribution for 293 low resolution images of JPEG−XR codec 42 39

38 40 37

36 38 35

36 34 PSNR PSNR 33 34 32

31 32 30

30 29 0.2 0.4 0.6 0.8 1 1.2 0.2 0.4 0.6 0.8 1 1.2 Bit−rate Bit−rate (a) (b)

Fig. 5. PSNR distribution of the JPEG-XR codec: high resolution (a) and low resolution (b) images. Each line refers to an image, where each image is compressed at 6 quality parameters (points on the line) to obtain varying bit-rates/PSNR. IV. RESULTS AND DISCUSSION However, since WebP does not give lower bit-rates than 0.22 (bpp), it is difficult to judge its real performance at lower Since all the aforementioned codecs (BPG, MozJPEG, bit-rates. For Mozjpeg, because a simple computational pre- WebP and JPEG-XR) are lossy trasform-based coding meth- processing step was included to reduce the ringing artifacts, ods, so they suffer from common artifacts (i.e., blocking and it gives excellent results only for high quality images. When ringing artifcats) at low bit-rate. Blocking artifacts are due to this is not the case, it is less effective, yet still much better the heavily quantization of the transformed coefficients (i.e., than the JPEG standard. fat distribution of pixel values, high discontinuities between the inter-block boundaries). While ringing artifact refers to B. Evaluation of the codecs based on PSNR the adding of spurious detail along the sharp transitions of the image (i.e., edge locations), this resulted by rough Figure 4 (c) and (d) show the trade-off between PSNR approximation of the high frequency components at low bit- measures and the bit-rates for high and low resolution images, rate. In our testing, we would like to highlight the OCR/PSNR respectively, of all the codecs. Special care is taken for JPEG- behaviors at low bit-rate range (between 0.1 and 0.6) where XR in this experiment. Figure 5 shows the PSNR performance blocking and ringing artifacts will be introduced, since all the for JPEG-XR with high (a) and low (b) resolution datasets codecs will give close and constant measures (OCR accuracy for each image (represented by a line). As we can see in and PSNR) at higher bit-rates (> 0.6). Figure 5 (a) and (b), the PSNR distributions for all images in both the datasets show a drop. Consequently, computing A. Evaluation of the codecs based on OCR accuracy the average PNSR values will show the JPEG-XR curve decreasing when moving from left to right (increasing the bit- Figure 4 (a) and (b) illustrate the OCR accuracy behavior at rate). For that reason, JPEG-XR is represented by a linear line low bit-rates for high and low resolution images, respectively, drawn between the highest and lowest PSNR values obtained with text contents for all the codecs except JPEG-XR. The (from Figure 5). This will mainly show, among the others, reason why JPEG-XR is excluded, is because it does not where JPEG-XR is, to make it comparable with the other provide low bit-rate images (< 0.6), even after compressing codecs (Figure 4 (c) and (d)). In the following, a detailed at the smallest quality parameter (q = 0). analysis of all the codecs will be given for both high and low 1) High resolution images: Based on the graph and at very resolution datasets. low bit-rate (< 0.2), both BPG’s encoders (i.e., JCTVC and 1) High resolution images: BPG-JCTVC encoder appears x265) perform very efficiently and their performance are very to give an important and excellent coding quality at low bit- close to each other. We can also see Mozjpeg and WebP have rates. While BPG with x265 encoder and WebP look a bit close similar actions at that range but both are still a bit less efficient to each other, but the former is better at the bit-rate greater when compared with BPG’s encoders. On the other hand, as than 0.3 and the latter is better for the bit-rate less than that. long as the bit-rate is increasing, the top codecs (i.e., BPG, As for JPEG-XR, it is far outperformed by BPG encoders Mozjpeg and WebP) seem to be very close and provide very and WebP. On the other hand, JPEG baseline and Mozjpeg high OCR accuracy (> 96%). In contrast, the JPEG baseline is apparently behave poorly at low bit-rates where their PSNR by far outperformed by the other codes at low bit-rates (< 0.3), values are too low. In contrast to the OCR performance, BPG- but it starts to give, more or less, the same performance when JCTVC clearly shows its substantial improvement in terms of increasing the bit-rates. image quality when compared with all the other codecs. 2) Low resolution images: While with the high resolution 2) Low resolution images: For low resolutions, basically dataset, all the top codecs (BPG, Mozjpeg and WebP) seem BPG’s encoders and WebP perform in a small PSNR interval, to share quite similar performances, the situation starts to with preference of BPG-JCTVC, BPG-x265 then WebP. Yet, change when working at low resolution images and at low bit- PSNR values are very low for Mozjpeg and the JPEG standard. rates. The first remark that can be extracted is the significant A good point to mention here is that the gap between the drop of OCR accuracy for all the codecs when the bit-rate codecs curves is reduced for low resolution images. This is is less than 0.25 (bpp). Also, we can realize that BPG and probably due to the lack of details in the images, thus the WebP still keep their same ranks as at high resolution, but codecs produce very close output (compressed images). Mozjpeg’s performance is considerably degraded. As can be The difference between BPG-JCTVC and BPG-x265 is that seen, Mozjpeg could not provide a good accuracy as BPG’s the former is not computationally optimized and takes all encoders and WebP do. The same for JPEG baseline, where the time needed to find the optimized setting having the the accuracy is quite low and far from the other codecs. lowest RD cost. While the latter is more optimized for faster All these results draw some interesting conclusions. BPG’s encoding (it includes different modes/settings). Hence, there is encoders (JCTVC and x265) are clearly quite robust to com- a compromise between the quality (here the PSNR and OCR pression artifacts in terms of OCR accuracy even for low measures) and the speed of compression. WebP gives superior bit-rates. WebP also works as efficiently as BPG’s encoders results comparing to Mozjpeg, JPEG-XR and JPEG baseline, do, probably because of the similar coding principles between and it is quite close to BPG-x265 encoder. WebP uses the them (i.e., intra prediction, residual encoding, video codec). block prediction technique and coding of the signal residuals, Table 1. Compression and decompression time (ms) for each codec

BPG-JCTVC BPG-x265 Mozjpeg WebP JPEG-XR JPEG baseline JPEG Turbo Image size Com. Dec. Com. Dec. Com. Dec. Com. Dec. Com. Dec. Com. Dec. Com. Dec. 1448×1880 41430 560 4510 650 2580 320 750 430 200 200 330 470 130 270 2544×3340 95950 910 11600 1480 5380 250 1550 550 450 350 910 1260 320 270

which is similar to the BPG technique (based on HEVC). For ACKNOWLEDGMENT that reason, WebP gives quite interesting results. This work has been supported by the DOD project co- Mozjpeg results are expected with OCR accuracy measure funded by La BPI and Yooz. where we see it competing with BPG’s encoders and WebP. However, in terms of PSNR measures, it is far away from the REFERENCES other codecs and close to JPEG baseline. The reason behind [1] Ye P., Doermann. Document Image Quality Assessment: A Brief Survey. this is that Mozjpeg is an improvement of JPEG baseline which In 12th International Conference on Document Analysis and Recognition include an effective way to deal with compression artifacts (ICDAR), proceedings, 2013, pp. 723-727. [2] Z. Wang, A. C. Bovik, H. R. Sheikh and E. P. Simoncelli. Image quality by re-shaping the signal amplitudes. So OCR accuracy is assessment: From error visibility to structural similarity. 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