*4.4. Discussion*

The ranking of JPEG2000, JPEG and ECW algorithms with different compression ratios differs slightly according to the qualitative suitability for the high-resolution aerial images "img1", and "img2" (Table 3, Figure 7a,b) as both images have similar features of the content. The "img1" and "img2" have textures of the same RGB values and intensities, but rough textures dominate in the "img2", while smooth ones are more common in the "img1". Due to prevalence of rough textures, "img1" has the lower visually acceptable threshold of JPEG2000, JPEG and ECW lossy compression (Figure 8a) and the compression artifacts are barely visible in the compressed image, compared to other ones. The "img3" ranking, as well as the content, differs considerably comparing to the "img1" and "img2" (Table 3, Figure 7c). The "img3" contains more features, sensitive to the lossy compression, like textures with the high spatial frequencies (of intensity change) and a large number of small regions differing in size, color, and intensity.

The changes of the content in the images "img4", "img5", and "img6" due to the down sampling also influenced the ranking of lossy compression (Table 3, Figure 7a–c) and the visually acceptable threshold of lossy compression (Figure 8g–i). The low-resolution aerial images do not have the rich spatial textures of the high-resolution images, and their lossy compression artifacts are more noticeable. The "img6" has the highest visually acceptable threshold of JPEG2000, JPEG and ECW lossy compression (Figure 8i) compared to "img4" and "img5" (Figure 8g,h). After the image "img2" was down sampled to the "img5", the textured regions became smoother. As the textures in the "img1" are of dominating low spatial frequencies of intensity changes, then down sampling it to the "img4" exposes a smaller effect on the visual content of the image. Thus, the low-resolution images "img4" and "img5" were similarly affected by lossy compression, especially at higher compression ratios for JPEG (Figure 8g,h).

Each algorithm introduces unique artifacts and has a different effect on the aerial images of different resolutions and contents. The JPEG lossy compression has a high impact on the images' content with uniform and coarse (low spatial frequencies of intensity changes) textures. The "img1" and "img3" contains this type of textures, but "img3" was affected more by JPEG compression as it contains a large number of small, uniform regions. The JPEG2000 compression more affects images containing a large number of regions with textures of high spatial frequencies of intensity changes ("img2"), and especially with the small regions of different textures ("img3"). The ECW lossy compression has a higher effect on images rich with rough textures of high spatial frequencies of intensity changes ("img2"). The JPEG2000 and ECW compression have a similar effect for low-resolution images with textures of high spatial frequencies of intensity changes ("img5"), or with small regions with different textures ("img6"). The JPEG2000 has higher quality at higher compression ratios than JPEG as it does not expose visually distracting artifacts, especially in low frequency or uniform areas. The JPEG artifacts are more pronounced in the low-resolution images' content than they are in the high-resolution images. The smoothing artifacts are more noticeable in ECW high-resolution images than in JPEG2000, specifically in images with textures of high spatial frequencies of intensity changes. The ECW compression has a slightly lesser effect on the low-resolution images.
