How do you re-colour, or colourize, a background layer such that the output hue and tints are highly saturated with colour?

Question

When I re-colour things in the GNU is not Unix Image Manipulation Programme the result often has dark grey patches or unsaturated light whit-ish patches.

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Suppose that:

• the blue-colored image is our input image.

• the orange-colored image is out output image.

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The blue input image has rich highly saturated dark blues.

The mottled orange output image has some low-saturation, dark-grey, regions.

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In the input image, the dark blues are highly saturated (vivid blue, not grey).

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How can we make an output image such that the desired output colour is orang-ish and the light and darks have the same level of rich vivid saturation numbers as the input image>

For example, the output pixel in row 40, and column 79 would have the same saturation level as the input pixel in row 40 and column 49.

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What is the name of the computer programme we might use to prevent patches from turning grey?

Most famous algorithms in computer graphics have a name and a wikipedia page on them.

Answer 1

If you apply in GIMP the single hue colorization effect to the blue version you get very close to your orange version. The slider positions are found by trial and error:

Your original may contain several hues and you may want to keep the hue differences (not happened above, every pixel got the same hue). If you apply something like "hue shift" to the original image you may easily end to a version which contains unwanted greyish areas and also apparent brightnesses can vary. That's because RGB system cannot produce the original apparent colorfulness (see OBS.) for the new hue at the original brightness level. In addition changing the hue affects also the apparent brightness due the sensitivity vs. wavelength dependency of the eye. The next image shows the result of the most straightforward hue shift:

I guess you have got the same and that's the reason of your question.

GIMP fortunately has many advanced filters for making hue shifts and still keeping the colorfulness and apparent brightness variations acceptable:

The image above shows one of them. It somehow applies the LCH color system which is a serious attempt to keep hue, colorfulness and apparent brightness apart from each other better than in the ordinary hue-saturation-value version of the common RGB system. Unfortunately explaining properly the used color system math is beyond my possibilities.

OBS. it's the difference from neutral grey and that's absolutely NOT the same as saturation, no matter many of us say "saturation" when they talk of colorfulness. The saturation of HSL and HSV versions of the RGB color system means approximately the used percentage of the available maximum colorfulness at the given hue and brightness.