Introduction to Layers - From PHOTOS to ART with Photoshop - page 120

Histograms
Many of the adjustments in Photoshop work by altering the photographic histogram for the
image.
A histogram is a graphical representation — normally a bar chart —of how data is distrib-
uted in a particular situation. The infamous Bell Curve, sometime used in grading students,
is an example of a histogram.
In Photography, the distribution of data represented by the Histogram , is Tonal Values —
shades of gray. With color images, a more accurate term than Tonal Values would be Rel-
ative Brightness or Luminosity .
The Histogram displayed on the LCD Screen of digital cameras is often referred to as a Lu-
minosity Histogram .
Many digital cameras have the capability of displaying the histograms for Red, Green, and
Blue as well as the Luminosity Histogram . These color histograms are referred to as Color
Channel Histograms .
In Photography, Histograms are always displayed as 256 bars. Each bar represents how
many pixels in an image have that exact tonal value. Because of the number of bars, a pho-
tographic histogram often looks like a smooth curve rather than a bar chart.
The color of every pixel in a digital image can be recorded as shades of Red, Green, and
Blue. For each of these colors, a scale of Zero to 255 is used. R122, G142, B168, describes
a specific color which is a combination of a shade of Red that is bar 122 on the Red Histo-
gram , a shade of Green that is bar 142 on the Green Histogram , and a shade of Blue that
is bar 168 on the Blue Histogram . One pixel of this exact color will add one data point to
each of these bars on their respective Histograms . For the Red Histogram , pure Red is at
the far left — bar 0 — and white is on the extreme right — bar 255. The other 254 bars
make a gradual transition from Pure Red to Pure White. The same is true for the Green and
Blue Histograms .
The RGB Histogram combines the data from all the pixels in the image to determine how
many data points are recorded for each bar of each Color Histogram . This data is displayed
as shades of gray. For example, bar 122 on the RGB Histogram is R122, G122, B122.
For the next image, I created an array of nine shades of gray ranging from Pure Black to
Pure White .