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 David Virgil Hobbs 10-26-2004 10:36 AM

increased color saturation solves hyper-contrast problems

EXCESSIVE CONTRAST PROBLEMS IN DIGITAL PHOTOGRAPHY CAN BE SOLVED VIA
PHOTO EDITORS IN COMPUTERS APPLYING INCREASED COLOR SATURATION AND
THEN CORRECTION OF HUE, BETTER THAN THEY CAN BE SOLVED USING THE
CONTRAST ADJUSTMENT CONTROLS IN SUCH PHOTO EDITORS.

Sometimes in digital photography, you have the problem of getting
exaggerated contrast in portraits. This problem is compounded by
imperfections in the setting of white balance. Photos featuring excess
contrast can be improved by increasing color saturation levels and
then adjusting hue to compensate for the color changes produced by the
changes in color saturation level. This approach can solve problems
that you would not be able to solve using contrast adjustment controls
that are specifically designed to adjust contrast.

You can read plenty of attempts to describe what happens when the
color saturation of a photograph is discussed on the internet. I
present to you the simple truths it seems are too hard to find at
these sites. A point of color in a digital image is determined by the
luminosity of red, green, and blue at a point in the image. Various
brightnesses of red green and blue combine to produce all the colors
you see in digital images. The red green and blue levels for any point
of color are measured from 0-255. When color saturation is increased,
generally speaking, of these three color components in a color point,
color components that are above the average luminosity of the three
components in terms of luminosity, are increased in luminosity, while
color components that are below the average luminosity of the color
point are decreased in luminosity, with the rule being that the
greater the divergence of the R G or B color element's luminosity from
the average luminosity of the three R red G green and B blue elements
in the color point, the greater the change in luminosity produced in
the R G or B element in the color point by an increase in color
saturation level.

Increasing color saturation in an image, when two different colors are
increased in saturation by an equal amount, can decrease the
difference or contrast between the two colors, because the dominant
brightest color (R red or G green or B blue) in the RGB spec of the
two different colors is pushed to near the 255 maximum in both colors,
as a result of which differences in luminosity of, for example, the R
specs found in the different colors in the image decrease resulting in
decreased contrast.

When there is an increase in the luminosity of the dominant color in
an RGB color point, if the color points vary from each other less in
terms of the R G or B color element in the color point that increases
in luminosity, than they do in terms of the R G or B elements in the
color point that decrease in luminosity, as a result of the increase
in color saturation, this can result in decreased contrast because the
R G or B elements in the color points that contrast less from point to
point in the image become more dominant, while the R G or B elements
that contrast more from point to point become less dominant.

Since the increase in color saturation pushes the (R G or B) colors in
a color point that are relatively weak compared to other R G or B
elements in the color point, in terms of luminosity, to even lower
levels, this can result in a decrease in contrast when the major
source of the excessive contrast is found in RGB elements in a color
point that are relatively low in luminosity compared to the other RGB
elements in the color point.

You cannot just add up the differences in the numerical RGB specs in
two different colors to determine how much those colors vary, because
the sub-dominant colors (lower luminosity than the dominant color in
the color point) in the rgb color points (points which are composed of
red green and blue in various luminosities) have less impact on the
color presented by the color point, the dimmer they are.

There is less difference between RGB 255-70-0 and 255-0-0 than there
is between 255-255-0 and 255-220-0; at lower levels of brightness in
an RGB element in a color point, changes in the luminosity of the RGB
element in the color point, change the color less than at higher
levels of RGB element brightness.

Thus it would appear that problems when a camera is exaggerating
contrast, could be: the camera is producing excess differentiation
from color point to color point in terms of the brightness of the
brightest r or g or b component of the color point; the camera is not
letting the most dominant components of the rgb points, which vary
from point to point in the image less than other components of the rgb
points, be bright enough compared to the other R G or B elements in
the color point; and the camera is letting the subdominant elements of
the RGB points in the photos, the colors that vary from each other
more from point to point than the dominant R or G or B color in the
color points, manifest themselves relatively speaking too brightly and
impact the points of color excessively.

Such hyper-contrast problems might be caused in part by incorrect
white balance settings. Imperfect white balance setting, common when
fluorescent tungsten and flash all combine to contribute to the
lighting of a photo, can contribute to the problem of the exaggeration
of the luminosity of for example the G or green element in RGB color
points, resulting in contrast exaggeration which can be solved through
color saturation enhancement such as I have been discussing.

Incandescent and fluorescent white balance used in flash conditions,
produce bluish tints; incandescent and flash white balance used in
fluorescent conditions produce yellowish and greenish tints; and flash
and fluorescent white balance used in tungsten conditions produces
orangish (red combined with yellow which is a mix of red and green)
tints.

Thus fluorescent light photographed using flash white balance will
produce yellow (a combo of red and green) and green tints; and
tungsten light photographed using flash white balance will produce
orangish (a combination of red and green) tints. So you can see how a
camera overemphasizing the luminosity of the G or green elements in
the RGB color points could lead to an exaggeration of contrast in
flesh tones in which the R is most dominant, followed by the G Green
and then the B blue; and you can see how such could be solved through
increases in color saturation decreasing the relative luminosity of
subdominant elements such as G Green elements in color points.

When brightening light is shed on R red dominated faces, the
brightening effect of the lights leads to an increase in the G green
and B blue elements in the color points. You can see this from the
way, if you take basic red, (R-255, B-0, G-0) pushing the B and G
values higher and higher will make the color look brighter and whiter,
make it look more and more like an overexposed photo or a photo
subjected to excessive light. Thus you can see that light, which makes
objects closer to white, and which is used to compensate for the
limited light sensitivity of a camera compared to the eye, will tend
to increase (in certain common R red dominated flesh color tones) the
brightness of the sub-dominant G green and B blue elements to the
point where they, mixing with the red elements of the color points,
exaggerate contrast; and you can see how increasing color saturation,
resulting in a decrease in the relative luminosity of the G and B
elements in the color points, could reduce unwanted exaggeration in
contrast.

To reproduce what you see with the eye in limited light, you often
have to, when using the camera, exaggerate the power of every light
source you see with your eye in the natural light, while keeping the
relative power of the various light sources compared to each other,
the same as found in the natural scene.

Thus, the camera is more sensitive to the blues and greens that
dominate light from light sources (at least in terms of the effect of
such light on color points that are R red dominated) than the human
eye is. Therefore, after all is said and done, one can end up with
portraiture that is unsatisfactory due to an excess of contrast,
because it is the B blue and G greens in the RGB color points, that
creat the excessive contrast, since the camera is compared to the
human eye hypersensitive to them, and records them too emphatically or
luminously or brightly; but one can recover from the problem by
increasing the color saturation in the image so that the contrast
producing G green and B blue elements are diminished in their ability
to influence the colors produced by RGB color points.

@2004 David Virgil Hobbs

http://www.angelfire.com/ma/vincemoon

 bob 10-26-2004 11:23 AM

Re: increased color saturation solves hyper-contrast problems

vincemoon@rcn.com (David Virgil Hobbs) wrote in

> Sometimes in digital photography, you have the problem of getting
> exaggerated contrast in portraits. This problem is compounded by
> imperfections in the setting of white balance. Photos featuring excess
> contrast can be improved by increasing color saturation levels and
> then adjusting hue to compensate for the color changes produced by the
> changes in color saturation level. This approach can solve problems
> that you would not be able to solve using contrast adjustment controls
> that are specifically designed to adjust contrast.
>

In "good" editing software, you can also convert the image to Lab mode and
adjust the luminosity seperatly from the color. It's also a handy mode for
adjusting noise and sharpness. You can blur the color chanel with the noise
(usually a OR b, but sometimes both), and you can sharpen the luminosity.
The resulting image is both sharper and smoother. Something very difficult
to do in RGB mode.

Bob

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