Universal Color to Gray Conversion

This function provides some methods to convert a color image to a gray-value image.

The implemented methods are:
1) 'normal', 'standard': rgb2gray of Matlab; ITU-R Recommendation BT.601.
2) 'avg', 'average': the output is the average of all three channels.
3) 'minavg', 'min_avg', 'min_average', 'minimum_average': The minimum-average method introduced in [1] for document image processing. it has less color dependency.
4) 'dual', 'dual_transform': The Dual Transform [2].
[1] R. Farrahi Moghaddam and M. Cheriet, A multi-scale framework for adaptive binarization of degraded document images, Pattern Recognition, 43, pp. 2186--2198, 2010, DOI: 10.1016/j.patcog.2009.12.024
[2] R. Farrahi Moghaddam, et.al., "A maximal-information color to gray conversion method
for document images: Toward an optimal grayscale representation for document image
binarization," 2013, [arXiv preprint http://arxiv.org/abs/1306.6058 arXiv:1306.6058, June 2013].
USAGE:
ugray0 = universal_color_to_gray_converter(u, method_flag, method_name);
where
u is the input color image.
method_flag is 'method'
method_name is one of above mentioned methods.
ugray0 is the output gray-level image.
OR
ugray0 = universal_color_to_gray_converter(u);
where
u is the input color image.
ugray0 is the output gray-level image; in this case, the method is 'min_avg';