| Image | Size | Number of representative pixels | Time of One-iteration method (sec) | Time of Local-optimal strategy (sec) |
| 05 | $ 384 \times 256 $ | 546 | 6.8 | 18.3 |
| 23 | $ 768 \times 512 $ | 1404 | 15.9 | 62.5 |
Image compression is an important technology in digital image processing. In this paper, a novel colorization-based codec for vector-valued images is proposed. In compression, we first define the concept of "structure image", which contains rich geometric structure information of the vector-valued image. Then, to extract representative pixels from the original vector-valued image, a "one-iteration method" is proposed. It can tremendously improve compression efficiency. In decompression, starting from colorizing the structure image, an anisotropic variational model is proposed. The existence and uniqueness of minimizers for the proposed variational model are established. Besides, we develop a fast and efficient algorithm for solving the model numerically by employing the scaled form of the alternating direction method of multipliers (ADMM). Numerical experiments on natural color images demonstrate that the proposed method outperforms the state-of-art colorization-based image compression method. Compared with the transform-based approaches, experiments on satellite multispectral images illustrate that the proposed method is superior to the JPEG and JPEG2000 standards.
| Citation: |
Figure 1. $ \rm (a) $ Original images in the $ \rm RGB $ color space. $ \rm (b) $ Structure images obtained by taking $ (\alpha_1, \alpha_2, \alpha_3) = (0.299, 0.587, 0.114) $. $ \rm (c) $ Structure images obtained by taking a random set, $ \left(\alpha_1, \alpha_2, \alpha_3 \right) = (0.05, 0.3, 0.65) $. $ \rm (d) $ The difference between structure images (b) and (c). Note that the pixel values have been rearranged to [0, 255]
Figure 2. $ \rm (a) $ When $ (\alpha_1, \alpha_2, \alpha_3) = (0.299, 0.587, 0.114) $, the geometric structure information in the structure image. $ \rm (b) $ The geometric structure information contained in the structure image, when $ \left(\alpha_1, \alpha_2, \alpha_3 \right) = (0.05, 0.3, 0.65) $. $ \rm (c) $ The difference of geometric structure information between (a) and (b). Pixel values have been rearranged into $ [0, 255] $ for better visibility
Figure 3. Results of image change rate for Kodak images [19] $ (07, 23, 24) $ calculated by different methods
Figure 9. $ \rm (a) $ Original false-color image for the Los Angeles multispectral image set, which is generated by assigning the spectral band $ \rm B4 $ to red, $ \rm B3 $ to green, and $ \rm B2 $ to blue. $ \rm (b)–(f) $ Original images for the spectral bands $ \rm B1-B5 $. The size of these images is $ 512\times512 $ $ 8 $ bit depth. $ \rm (g)–(h) $ Original images for the thermal bands $ \rm B61 $ and $ \rm B62 $. The size of these two images is $ 512\times512 $ $ 8 $ bit depth
Figure 10. $ \rm (a) $ Structure image for the Los Angeles image thermal bands (B61 and $ \rm B62) $. $ \rm (b) $ Structure image for the Los Angeles image spectral bands $ \rm B1-B5 $ with $ \alpha $ = (0.2040, 0.2028, 0.2012, 0.2097, 0.1823). $ \rm (c) $ Structure image for the Los Angeles image spectral bands $ \rm B1-B5 $ with $ \alpha = (0.1912, 0.3681, 0.1305, 0.2244, 0.0858) $
Figure 11. Decompression results for the Los Angeles images. {(a), (c) and (e)}: Decompressed thermal band $ \rm B62 $ using the JPEG standard, the JPEG2000 standard and the proposed method, respectively. {(b), (d) and (f)}: Decompressed spectral band $ \rm B3 $ using the JPEG standard, the JPEG2000 standard and the proposed method, respectively
Table 1. The computational time (sec) of the proposed "one-iteration method" is compared with the local-optimal strategy in [43] for extracting representative pixels
| Image | Size | Number of representative pixels | Time of One-iteration method (sec) | Time of Local-optimal strategy (sec) |
| 05 | $ 384 \times 256 $ | 546 | 6.8 | 18.3 |
| 23 | $ 768 \times 512 $ | 1404 | 15.9 | 62.5 |
Table 2. The computational time (sec) taken by different methods
| Number of representative pixels | Time of reaction-diffusion equations (21) (sec) | Time of Algorithm 2 (sec)) |
| 184 | 15.7 | 3.3 |
| 394 | 21.6 | 3.6 |
Table 3.
Decompression results of different methods. The PSNR values are shown in Table 3(a). Table 3(b) gives the SSIM values of the decompressed images. For the image to be tested, we first calculate the index value of each channel and then average the values of all channels to obtain the PSNR or SSIM values of the decompressed image. Here the notation
| (a) PSNR | |||||
| Image | CR | JPEG | JPEG2000 | Shi et al. | Proposed |
| 06 | 20:1 | 29.5755 | 31.5652 | 30.5245 | 30.6496 |
| 09 | 41:1 | 28.0773 | 30.3243 | 29.2732 | 29.5357 |
| 15 | 61:1 | 25.7227 | 28.3352 | 26.8622 | 27.4383 |
| (b) SSIM | |||||
| Image | CR | JPEG | JPEG2000 | Shi et al. | Proposed |
| 06 | 20:1 | 0.8387 | 0.8702 | 0.8556 | 0.8566 |
| 09 | 41:1 | 0.8173 | 0.8816 | 0.8704 | 0.8751 |
| 15 | 61:1 | 0.7247 | 0.8086 | 0.7985 | 0.8112 |
Table 4.
PSNR values of the two thermal bands (B61 and B62) using different decompression methods. Here the notation
| Thermal bands | CR | JPEG | JPEG2000 | Proposed |
| B61 | 10:1 | 44.5696 | 44.7990 | 47.0500 |
| B62 | 10:1 | 40.8826 | 41.3790 | 42.9845 |
| Avg. | 42.7261 | 43.0890 | 45.0172 |
Table 5.
PSNR values of the spectral bands
| Spectral bands | CR | JPEG | JPEG2000 | Proposed ($\alpha$ _rand) | Proposed |
| B1 | 27:1 | 32.5552 | 32.8792 | 33.5227 | 33.4983 |
| B2 | 27:1 | 31.6115 | 31.8138 | 34.4564 | 33.8015 |
| B3 | 27:1 | 28.9824 | 29.4833 | 31.1467 | 31.4802 |
| B4 | 27:1 | 30.5670 | 31.1113 | 28.8501 | 29.5729 |
| B5 | 27:1 | 27.7357 | 28.1368 | 27.6438 | 28.6967 |
| Avg. | 30.2903 | 30.6848 | 31.1239 | 31.4103 |
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Results of image change rate for Kodak images [19]
Extracting representative pixels from the
The Kodak images
Decompression results for the Kodak images
Decompression results for Kodak image
Plots of energy and PSNR values for decompressing the Kodak image
Decompression results for the Los Angeles images. {(a), (c) and (e)}: Decompressed thermal band