Background Neutralization

[Bob Hertel]

If for each color channel, I were to use the HistogramTransformation tool, and manually set the Midtones value to the median value for that channel, is the result equivalent to performing Background Neutralization?

[Philip de Louraille]

I don't think so. Background means just that: background sky. E.g.: no nebulae, galaxies, ... Just stars and "empty" space. Background neutralization makes that sky neutral in color and may remove color biases in the picture.

[Cosmick]

If for each color channel, I were to use the HistogramTransformation tool, and manually set the Midtones value to the median value for that channel, is the result equivalent to performing Background Neutralization?

I think this is closer to how Colour Calibration is done. Background Neutralization is a bit like setting the blackpoint of each channel to the same point on the histogram. As Philip said -

Background means just that: background sky. E.g.: no nebulae, galaxies, ... Just stars and "empty" space. Background neutralization makes that sky neutral in color and may remove color biases in the picture.

[Juan Conejero]

Hi Bob,

If for each color channel, I were to use the HistogramTransformation tool, and manually set the Midtones value to the median value for that channel, is the result equivalent to performing Background Neutralization?

Not at all. The background neutralization algorithm is as follows:

I_NR = I_R - B_IR + b
I_NG = I_G - B_IG + b
I_NB = I_B - B_IB + b

where I_NR, I_NG and I_NB are the color channels of the resulting (background-neutralized image); I_R, I_G and I_B are the original color channels; B_IR, B_IG and B_IB are the mean backgrounds measured on the original channels, and b is the target mean background. Typically, b is defined by:

b = min( B_RR, B_RG, B_RB )

where the B_R's are the mean backgrounds measured on the background reference image.

Each mean background is estimated as the median of the pixels in the background sampling range (defined by the lower and upper limit parameters in the BackgroundNeutralization tool) for the corresponding channel of the original or background reference image.