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Human being can perceive natural scenes very well under various illumination conditions. Partial reasons are due to the
contrast enhancement of center/surround networks and opponent analysis on the human retina. In this paper, we propose
an image enhancement model to simulate the color processes in the human retina. Specifically, there are two
center/surround layers, bipolar/horizontal and ganglion/amacrine; and four color opponents, red (R), green (G), blue (B),
and yellow (Y). The central cell (bipolar or ganglion) takes the surrounding information from one or several horizontal
or amacrine cells; and bipolar and ganglion both have ON and OFF sub-types. For example, a +R/-G bipolar (red-center-
ON/green-surround-OFF) will be excited if only the center is illuminated, or inhibited if only the surroundings (bipolars)
are illuminated, or stay neutral if both center and surroundings are illuminated. Likewise, other two color opponents with
ON-center/OFF-surround, +G/-R and +B/-Y, follow the same rules. The yellow (Y) channel can be obtained by
averaging red and green channels. On the other hand, OFF-center/ON-surround bipolars (i.e., -R/+G and -G/+R, but no -
B/+Y) are inhibited when the center is illuminated. An ON-bipolar (or OFF-bipolar) only transfers signals to an ONganglion
(or OFF-ganglion), where amacrines provide surrounding information. Ganglion cells have strong
spatiotemporal responses to moving objects. In our proposed enhancement model, the surrounding information is
obtained using weighted average of neighborhood; excited or inhibited can be implemented with pixel intensity increase
or decrease according to a linear or nonlinear response; and center/surround excitations are decided by comparing their
intensities. A difference of Gaussian (DOG) model is used to simulate the ganglion differential response. Experimental
results using natural scenery pictures proved that, the proposed image enhancement model by simulating the two-layer
center/surrounding retinal networks can effectively enhance color images in terms of color contrast and image details.
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