03-082.qxd 1/11/05 5:17 PM Page 157 Effects of JPEG2000 on the Information and Geometry Content of Aerial Photo Compression Jung-Kuan Liu, Houn-Chien Wu, and Tian-Yuan Shih Abstract evaluated the effects of compression on geometric accuracy. Li The standardization effort of the next ISO standard for et al., (2002) indicated that when compression ratios are less compression of the still image, JPEG2000, has recently than a factor of 10, the compressed image is near-lossless with reached International Standard (IS) status. This wavelet- JPEG. In other words, the visual quality of JPEG compressed based standard outperforms the Discrete Cosine Transform images remains excellent and the accuracy of manual image (DCT) based JPEG in terms of compression ratio, as well mensuration is, essentially, not influenced. Paola et al. (1995) as, quality. In this study, the performance of JPEG2000 is and Schmanske and Loew (2001) concentrated on the classifi- evaluated for aerial image compressions. Different com- cation accuracies of compressed images. Paola et al. (1995) pression ratios are applied to scanned aerial photos at the revealed that high quality classifications could be obtained for 1:5 000 scale. Both the image quality measurements and the images with JPEG compression ratios approaching 10:1 or even accuracy of photogrammetric point determination aspects higher. The classification result retains its overall appearance, are examined. The evaluation of image quality is based but the smoothing effect of high compression tends to elimi- on visual analysis of the objects in the scene and on the nate much of the pixel-to-pixel detail. Robinson et al. (1995) computation of numerical indices, including RMSE, entropy, and Rane & Sapiro (2001) evaluated the effect of image com- and Peak Signal-to-Noise Ratio (PSNR). The geometric quality pression on Digital Surface Model (DSM) generation. Maeder of JPEG2000 with different compression ratios is studied for (1998) investigated the compression effects on digital image some photogrammetric operations, including interior matching. Pal et al. (2002) measured the performance of orientation, relative orientation, absolute orientation, and JPEG2000 with various feature extractions and classifications of DSM generation. The objective of this study is to explore the hyperspectral images. Lee et al. (2002) reported on a combined possibility of JPEG2000 for replacing JPEG as a standard in JPEG2000 and spectral correlation method for hyperspectral photogrammetric operations. image compression. Santa-Cruz et al. (2002) compared JPEG2000 with JPEG-LS, MPEG-4 VTC, JPEG, PNG, and SPIHT. Currently, the most common form used for still image Introduction compressions is JPEG. This standard was developed by the A common characteristic of most images is that their neigh- Joint Photographic Expert Group in the late 1980s, and since boring pixels are correlated, and therefore, contain redundant then, has been the most successful and widely used image information. The foremost task then, is to find less-correlated compression technique. JPEG has been implemented by most representation of the image. Two fundamental components of the digital photogrammetric workstations (DPW) currently of compression are the reduction of both redundancy and available commercially (Li et al., 2002). However, in 1997, irrelevancy. Redundancy reduction aims at removing dupli- the JPEG Committee decided that the needs and requirements cation from the signal source (image/video). Irrelevancy of imagery applications in today’s world point to the need reduction omits parts of the signal that will be noticed by the for a new standard. This proposition brought forth the new signal receiver (Saha, 2000). Image compression research standard JPEG2000. aims at reducing the number of bits needed to represent an In a working environment, the performance of JPEG2000 image, which increases the compression ratio, by removing is evaluated for aerial image compression. Image quality the redundancies as much as possible. measurements and the accuracy of photogrammetric point Digital image compression is not only important for trans- determination (PPD) are two primary aspects to consider mission on the Internet, but also in the workflows of digital when evaluating the JPEG2000. Different compression ratios photogrammetry. For example, a standard 23 cm ϫ 23 cm are applied to scanned aerial photos of 1:5 000 scale. First, panchromatic aerial photo scanned at 20 ␮m resolution the evaluation of image quality is based on visual analysis produces a file size of about 140 MB. The total amount of of the objects in the scene, with several indices computed storage for imagery will reach several gigabytes for an individ- with different ratios for both JPEG2000 and JPEG, e.g., RMSE, ual photogrammetric mission. Image compression has been entropy, and Peak Signal to Noise Ratio (PSNR). Second, the one important issue in photogrammetry and remote sensing in geometric quality of JPEG2000 with different compression recent years. Schiewe (1998) investigated the effect of lossy ratios is studied JPEG compression techniques on geometry and information content of satellite imagery (MOMS-02). Li et al., (2002) Photogrammetric Engineering & Remote Sensing Vol. 71, No. 2, February 2005, pp. 157–167. Department of Civil Engineering, National Chiao-Tung 0099-1112/05/7102–0157/$3.00/0 University, 1001 Ta-Hsueh Road, Hsin-Chu, Taiwan © 2005 American Society for Photogrammetry ([email protected]). and Remote Sensing PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING February 2005 157 03-082.qxd 1/11/05 5:17 PM Page 158 Overview of JPEG and JPEG2000 Additionally, JPEG2000 offers higher compression ratios for lossy compression than JPEG. According to Christopopulos JPEG et al. (2000), JPEG2000 can typically compress images any- There are several modes defined for JPEG, including baseline, where from 20 percent to 200 percent more efficiently than lossless, progressive and hierarchical. The baseline and JPEG can based on the same PSNR values. Compression effi- progress encoding methods are Discrete Cosine Transform ciency for lossy compression is usually measured based on (DCT) based on a predictive method. The hierarchical mode signal-to-noise ratios, i.e., PSNR, which will be described in encodes the image at multiple spatial resolutions using the next section. Furthermore, JPEG2000 is also able to sup- either the DCT-based compression or the lossless mode port compression ratios of about 2.5:1 for lossless compression (Wallace 1992; Li et al., 2002). (Aboufadel, 2001). The baseline mode is the most popular one and sup- Moreover, JPEG2000 is able to display images at different ports lossy coding only. In the baseline mode, the image is resolutions and sizes from the same image file, while its ϫ divided into 8 8 blocks, and each of these is transformed predecessor JPEG was only able to display images at a set by the DCT. The transformed blocks’ coefficients are quan- resolution. Since JPEG2000 is based on wavelets, the wavelet tized with a uniform scalar quantizer, zig-zag scanned, and stream can be only partially decompressed if the user only entropy coded with the Huffman coding. The quantization wants a low-resolution image, while the full resolution step size for each of the 64 DCT coefficients is specified in a image can be viewed when desired (Aboufadel, 2001). One quantization table, which remains the same for all blocks. of the major benefits of being able to access an image at The DC coefficients of all blocks are coded separately using a different resolutions is the ability to use only the amount of predictive scheme. Hereafter, this mode is simply referred to bandwidth required for any given particular level of interest. as JPEG (Santa-Cruz et al., 2002). For example, when clicking on a JPEG2000 image that has been set up for progressive-by-resolution access, the viewer JPEG2000 will be able to see the low-resolution version as soon as it JPEG2000 is a newly approved image compression stan- downloads, and can then, immediately, decide whether or dard that is intended to replace the existing JPEG standard. not to wait for higher resolution. The JPEG2000 standard is comprised of a number of parts, Another advantage of JPEG2000 is its Region of Interest including the core coding system, extension, motion (ROI) capability. The wavelets can select a particular portion JPEG2000, conformance testing, and reference software of an image to view at a high quality while leaving the rest (Adams, 2001c). In this study, the experiments are res- of the image at a lower quality. The last aspect of JPEG2000 tricted to only Part 1 of the standard, which defines the that is superior to JPEG is in the area of error resilience core system. (Adams, 2001c). Error resilience measures the ability of a JPEG2000 is based on the Discrete Wavelet Transfor- compression method to avoid letting errors introduced into mation (DWT), scalar quantization, context modeling, the image file affect the quality of the image. JPEG2000 offers arithmetic coding, and post-compression rate allocation. It significantly higher error resilience than JPEG. Therefore, handles both lossy and lossless compression using the same there is less chance that the image will be somehow cor- transform-based framework, and borrows heavily on ideas rupted and its quality sacrificed in some way. from embedded block coding with an optimized truncation (EBCOT) scheme. Quantization allows greater compression to be achieved, by representing transform coefficients with Indices of Image Quality only the minimal precision required, to obtain the desired Gonzalez and Woods (1992) stated that there are two fidelity level of image quality. Transform coefficients are quantized criteria for image processing: the objective and the subjec- using scalar quantization with a deadzone. A different tive. The subjective fidelity criteria are established based quantizer is employed for the coefficient of each sub-band, on evaluation ratings from human observers. While visual and each quantizer has only one parameter: its step size inspection is the most prominent method of subjective eval- (Adams, 2001c).