26-05-2012, 10:19 AM
Enhancement and Manipulation 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).
Adjustment of Local Background
Illumination
In adjusting the local background illumination, the DC
coefficient of a block is used. The DC value gives the mean of
the brightness distribution of the block. This adjustment may be
performed by mapping the brightness values to a value in the
desired range. This function should be monotonic in the given
range. Let us denote the maximum brightness value of the image
as (which may be available from the header of the compressed
stream). Let the DCT coefficients of a 8 x 8 block of the
luminance component be denoted by. Then is the DC coefficient
and the rest are the AC coefficients. As before we denote the
normalized DC and AC coefficients by . In adjusting the local
brightness, this DC coefficient is mapped to by using a
monotonically increasing function
Preservation of Colors
As described earlier, we have performed a restricted scaling
operation according to (8) and (9) for preserving the colors.
These techniques only change the luminance component and
keep the chrominance components ( and , respectively) unaltered.
Though in the Y-Cb-Cr color space the chrominance
components are de-correlated better than that in the R-G-B color
space, the increasing values in the component usually tend to desaturate
the colors. Typically one may observe from the
conversion matrix for going from the Y-Cb-Cr space to the R-GB
space, for G > R and G > B increasing Y while keeping Cb
and Cr unchanged reduces both the (R/G) and (G/B)factors.
CONCLUSION
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 scheme 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 & Color image
manipulation with RGB color space.