Graphics Reference
In-Depth Information
Fig. 11.10 There is a light falloff artifact in this image, causing the upper center section to be
brighter than the lower right corner. Depending on which part of the image is sampled, a wide
range of colors may be selected, but none of them are correct because of the strong hue shift in
the photo
changes from one end of the picture to the other, even if you know the color should
be completely even all the way across.
If, for instance, you are looking at a wall that has been painted white, and there
is a diagonal gradient falloff from one corner of the wall to the opposite corner, you
will have a number of colors to choose from. You know the entire wall is a fl at white
because a fresh coat of paint has just been evenly applied, and it has dried perfectly.
What do you do?
If you take a photo of the scene with a reference grey card , you can calibrate the
color on the card in the photo, which is a known grey of exactly 50 % intensity, 0 %
saturation, with the other colors in your scene. This will correct many of the color
problems, but will not eliminate the gradient.
For a fl at white wall, the best solution may be to pick what appears to be the most
accurate color and then fl ood fi ll the entire region with it. If the region has a highly
complex variety of colors, as in a graffi ti covered wall, this will not work very well. In
that case, you are best off either using a gradient mask to reverse the gradient in the
shot, or repaint the scene with fl at colors, based on information in your reference.
11.5.3.7
Atmosphere/Fog
Any environment, even an interior one, contains small specks of particulate matter
suspended in the ambient atmosphere. Light collides with these particles, and is
affected by them. At near distances in a clean environment, the effect is nearly invis-
ible. At greater distances, or those with higher concentration of airborne pollutants
(natural or otherwise) like pollen, dust, smoke, etc., the change in color values can
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