2. Compression of aerial images for reduced-color devices UNIVERSITY OF JOENSUU DEPARTMENT OF COMPUTER SCIENCE FINLAND Pasi Fränti and Ville Hautamäki http://cs.joensuu.fi/pages/franti/comp/

3. Images in mobile navigation Locating the user by GPS or network. Location is shown in a meaningful context such as map or aerial image. Compression for reducing the amount of data. Number of colors limited.

4. Methods in comparison Compression methods: Image types: 8-bit (gray-scale) images 2-bit images 2-bit dithered images

5. System diagram: two possibilities Scheme 1: Color quantization Compression Scheme 2: Compression Quantization after decompression

6. Set of test images Turku1 Turku4 Turku2 Turku3 600x450

7. Visual examples 8-bit image2-bit image2-bit dithered

8. Software used (All pages were still operational in 2002)

9. Lossless compression 8-bit images Best result

10. Lossy compression 8-bit images JPEG-2000 better and steady Images were originally compressed by JPEG once !!! Fractal compressor good with low bit rates

11. Lossy versus lossless 8-bit images Lossy results selected as the lowest bit rates with visually lossless quality

12. Visual comparison JPEGJPEG-2000FIASCO 8-bit images

13. Lossless comparison 2-bit images JBIG gives best results Dithering adds complexity to images

14. Lossy versus lossless Visually lossless LosslessLossy

15. Visual comparison (1/2) JBIG (dithered) 1.34 bpp JPEG-2000 1.00 bpp JPEG-2000 0.25 bpp 2-bit images

16. Visual comparison (2/2) JBIG (dithered) 1.58 bpp JPEG-2000 1.00 bpp JPEG-2000 0.25 bpp 2-bit images

17. Conclusions JPEG-2000 outperformed JPEG and FIASCO by wide margin. In the case of 2-bit images, the following scheme is recommended:  Compress 8-bit image by JPEG-2000.  Color quantization after decompression.