01-12-2012, 01:22 PM
Enhancement of Color Images by Scaling the DCT Coefficients
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Abstract
This paper presents a new technique for color enhancement
in the compressed domain. The proposed technique
is simple but more effective than some of the existing techniques
reported earlier. The novelty lies in this case in its treatment of
the chromatic components, while previous techniques treated only
the luminance component. The results of all previous techniques
along with that of the proposed one are compared with respect to
those obtained by applying a spatial domain color enhancement
technique that appears to provide very good enhancement. The
proposed technique, computationally more efficient than the spatial
domain based method, is found to provide better enhancement
compared to other compressed domain based approaches.
INTRODUCTION
I MAGE enhancement is required mostly for better visualization
or rendering of images to aid our visual perception.
There are various reasons, why a raw image data requires
processing before display. The dynamic range of the intensity
values may be small due to the presence of strong background
illumination, as well as due to the insufficient lighting. It may
be the other way also. The dynamic range of the original image
may be too large to be accommodated by limited number of
bit-planes of a display device. The problem gets more complicated
when the illumination of the scene widely varies in the
space. In that case, in some places the scene appears to be too
dark while in some other places it is too bright. An example
of such an image is shown in Fig. 1(a). It can be seen that the
brightness of the blue sky affects the display of the reflection
on the glass window of the car, where many details are not visible.
In such images it is necessary to improve the local contrast.
The result of such a processing [1], [2] with improved display
is shown in Fig. 1(b).
THE PROPOSED ALGORITHM
The proposed algorithm performs the color image enhancement
operation in three steps. First, it adjusts the background
illumination. The next step preserves the local contrast of the
image and the last one preserves the colors of the image. Moreover,
in a block DCT space, the algorithm attempts to exploit the
advantage of having localized information from the DCT coefficients.
The algorithm is designed in such a way that each block
(of size 8 8) for all the components could be handled independently.
This makes it more suitable for parallel implementation.
COMPARISON METRICS
We have compared the performance of the proposed enhancement
technique with several other compressed domain methods.
To this end, we considered how the compressed domain techniques
perform compared to that of a very good spatial domain
enhancement technique. For the latter we chose the scheme proposed
in [1] which is based on a multiscale retinex processing. It
essentially involves multiscale Gaussian filtering and point-wise
center-surround subtraction in the logarithmic scale. Finally, the
pixel values are remapped to their original scale, followed by
an empirical adjustment of color components. The processing
is performed in the - - color space and is computationally
quite expensive. However, the algorithm appears to provide
excellent enhancement results as can be seen in the website
http://dragon.larc.nasa.govt/retinex/pao/news. Fig. 7 shows
some of the images from this website.
CONCLUDING REMARKS
In this paper, we have presented a simple approach for
enhancing color images in the block DCT domain by scaling
the transform coefficients. The unique feature of this algorithm
is that it also treats chromatic components in addition to the
processing of the luminance component improving the visual
quality of the images to a great extent. A comparative study
with different other schemes has been carried out on the basis of
different performance criteria. It has been found that proposed
schemes outperform the existing schemes in most cases.