Image Color and Tone Mapping

There are various ways that the tones and colors of images can be manipulated. To avoid confusion of terms we can identify the following three types of tone and color manipulation:
  • Tonal Manipulation includes Brightness/Contrast, Level and Curve controls. Tonal manipulation applies to B&W and color images.
  • Pseudo Color images are indexed mode images which use an artificial or manipulated color palette.
  • False Color images map scene spectrum to image spectrum in an unusual or non standard way.

Tonal Manipulation

The following table describes some of the tonal manipulation capabilities typically found in digital image editors.

Technique Sample Image
Contrast & Brightness - This is a normal gray scale image. Contrast and brightness of a grayscale image or a component of an RGB color image can be manipulated.
Level - A levels control allows setting the black and white points of an image, along with its mid gray point.
Curve - A curves control offers the greatest flexibility in manipulating image tonality by allowing direct manipulation of its tonality transfer curve.



Pseudo Color Pallets

The table below shows the wide range of effects that can be produced from the same image just by changing its pseudo color palette. Any portion of image tonality can be emphasized using the properly constructed pseudo color palette. When combined with the techniques of spectral selectivity, pseudo color palettes can bring out particular aspects of the image for either scientific or artistic purposes. Pseudo coloring might be used emphasize healthy foliage or pollution. It might be used to apply selective tonality to a B&W image. Most digital imaging programs allow easy creation of custom color palettes. Pseudo color images are always indexed mode images which use a color palette to map pixel valuators into RGB colors.

Color Palette Sample Image Description
This is a normal gray scale image. Palette index 0 is in the upper left corner and palette index 255 is in the lower right corner.
This is the same image using the Spectrum color palette that ships with Photoshop.
Same image again using Photoshop's Black Body color palette.
A custom palette which is similar to a reversed order Spectrum palette.
This custom color palette consists of a dark gray scale with color in the highlight portions.
A custom color palette with black highlights, blue mid tones and magenta shadows.
Another custom color palette with black highlights, blue darker mid tones and green shadows running to black.



False Color

The following illustration shows the remapping of spectrum that is typical of false color images. In this case the ultraviolet portion of the spectrum is being mapped to blue, the visual portion of the spectrum is being mapped to green, and the infrared portion is being mapped to red. False color images are typically three component or RGB images. (Note: The false color image is simulated and not the actual result of the combination of UV, visual and IR components as described).



Here is another illustration of a false color image. In this case, the visible portion of the spectrum is being mapped to blue, the 800 to 900 nanometer infrared portion of the spectrum is being mapped to green, and the 700 to 800 nanometer infrared portion is being mapped to red.



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A Spectral Selectivity Metric
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Spectral Selectivity
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Infrared Characteristics

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