The color photographs are almost always required if we are to be calibrated a good color balance across the image. Due to the characteristics of the capture devices, either digital or CCD , the answer is usually not uniform throughout the color spectrum. It is therefore necessary to adjust the weight of each RGB channel . And of course this setting must be linear, because otherwise we can not get a perfect color balance throughout the image.
I always performed the color calibration in PixInsight with the processing module ChannelMatch . My argument is based primarily on the statistical values \u200b\u200bof an object which I know their color. This object reference is usually the sky background, which is assumed to be completely neutral, with no color casts.
First visualize raw images helping a linear transfer function display ( ScreenTransferFunction ) and thus easily locate the regions of sky. Then generated from the initial data from a separate image a preview , which usually call ' background. " Reading and histogram representation of the background shows me clearly indicates that the distribution of pixel values \u200b\u200bof each color channel differ greatly.
The idea is to overlap the bells for each channel of the histogram from ChannelMatch modifying linear correction factors. This way I get a correct color balance, since the main peak of the distribution is often a good indicator of one's own sky.
As you can see above, by varying the correction factors, the bells of the histogram of each color channel are perfectly overlapped, thus obtaining a sample of completely neutral background.
finally obtained correction factors (in this case R = 0.85, G = 0.66 and B = 1.0) are applied over the entire image.
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