Ch. 5 - Viewing, Editing, and Annotating Images
Edit - Image Quality
User's Guide to the SPOT Cooled Color Digital Camera, 6/9/98
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Hue
- Hue is the designation for the actual color, as distinguished from others in
the color spectrum. Hue is derived from a color wheel and is expressed in the
color's angular location (in degrees) on the wheel. (i.e., +180° to -180° for the
SPOT software).
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Saturation
- Saturation is the proportion of perceived pure hue in the color.
Saturation (in the SPOT software) is measured on a numerical scale of 0 to 100,
where zero equals pure gray and 100 equals pure hue (i.e., day glow).
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Luminance
- Luminance is the relative brightness or darkness of a hue, or how
white or black a given color is. Luminance (in the SPOT software) is measured
on a numerical scale of 0 to 100, where zero equals pure black and 100 equals
pure white.
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Note: A saturated color starts out as black at a luminance of 0, reaches full
brightness as a saturated color at a luminance of 50, and then transitions through
lighter pastel shade to white at a luminance of 100.
Adjust RGB or Adjust HSL - Which One Should I Use?
Because the Adjust RGB and Adjust HSL options perform the same types of editing tasks
but in different ways, they are suited to different image types. The following table
compares the methodologies and lists some situations in which either the RGB or the
HSL color space might be preferable to use in image editing.
RGB
HSL
Color Translation
None.
Editing in the RGB color space operates
directly on the red, green, and blue values read
by the camera.
1. RGB values are translated into the HSL
color space.
2. Edits are made.
3. HSL values are translated back to the RGB
color space.
Usage
RGB editing functions are less intuitive than
those done in the HSL color space.
Using hue, saturation, and luminance is a more
intuitive way to edit colors than working directly
with RGB values.
Guidelines
Use RGB:
§
When the colors in the bright areas of an image
need to remain saturated. HSL editing can
cause bright areas in images with simple red,
green, and blue shades to turn pastel.
§
With images that are composed mainly of simple
primary colors, such as fluorescence samples.
§
With monochrome images. Monochrome
images can only be edited in RGB mode.
Use HSL:
§
With editing functions that work solely
on the luminance value. This includes
contrast, gamma, and stretch. Editing
luminance does not affect the hue or
saturation of darker colors. RGB
editing tends to give darker colors a
"washed out" appearance.
§
With brightfield and macro images
that have many subtle shades of
color.
