International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
S444, Blocks C2 and H3. This fact indicates that it is difficult to
estimate the compression ratio achieved by a certain value of
the quality factor such as the IJG quality setting in JPEG
compression.
Figures 7, 8 and 9 demonstrate that Type J2K provided the best
quality images in color features. Type $444 is seen to be
superior to both Types S411 and RGB in color features as well.
However, the differences of PSNR of Saturation between Types
J2k and S444, and the differences of PSNR of Value between
Types S444 and S411 become smaller with an increase in the
compression ratio as shown in Figures 8 and 9 respectively.
Figures 7, 8 and 9 present that the degradation of quality of the
reconstructed images of Block H3 of coarse texture features is
less than that of Block C3 of finer texture features in color
features.
As for texture features shown in Figures 10 to 13, there are
some discrepancies of the effect of image compression among
various texture measures. The inclinations of the effect of
image compression in the compression ratio below
approximately 6 differ form those in the compression ratio
range above approximately 6. Quality of reconstructed images
of all types in texture features degrades with increasing the
compression ratio range above approximately 6.
On the whole, Type RGB is inferior to the other types, and
Type S411 is superior to Type S444 in the compression ratio
range above approximately 6.
The degradation of quality of reconstructed images of Block H3
of coarse texture features can be said to be less than that of
Block C3 of finer texture features in texture features as well as
in color features.
4. CONCLUSION
From the experiment results, it may be concluded that color
space conversion and downsampling in JPEG compression have
an effect on quality of reconstructed images. While Type S444
with color space conversion and no downsampling may provide
an image of good quality in color features, Type S411 with
color space conversion and downsampling may provide an
image of good quality in texture features.
As for comparison between lossy JPEG compression and lossy
JPEG 2000 compression in performance, it was confirmed that
lossy JPEG 2000 compression is superior to lossy JPEG
compression in color features. However, lossy JPEG 2000
compression does not necessarily provide an image of good
quality in texture features.
Moreover, comparison of the experiment results between
Blocks C2 and H3 indicates that an image of finer texture
features is less compressible, and quality of reconstructed
images is worse in both color and texture features.
In JPEG compression, the compression ratio of an image is
controlled by a constant (Q-factor) such as the IJG quality
setting. The experiment results confirmed that it is difficult to
set an appropriate Q-factor, because the optimal setting of Q-
factor varies from one image to another.
We plan in the future to conduct further experiments by using
another source images such as high resolution satellite images
652
and images obtained by an aerial digital camera. Effects of land
cover and ground resolution, and capabilities of various image
quality measures are going to be investigated.
ACKNOWLEDGEMENTS
The authors thank the Geographical Survey Institute of Japan
and Kokusai Kogyo Co., Ltd. for allowing us to utilize the
digitized color aerial photographs.
REFERENCES
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