Instruction manual

34
A digital image is basically a mosaic of square tiles or "pixels" of
uniform color which are so tiny that it appears uniform and smooth.
Instead of sorting them by color, we could sort these pixels into 256 levels
of brightness from black (value 0) to white (value 255) with 254 gray
levels in between. Just as we did manually for the mosaic, imaging
software automatically sorted the pixels of the image below into 256
groups (levels) of "brightness" and stacked them up accordingly. The
height of each "stack" or vertical "bar" tells you how many pixels there are
for that particular brightness. "0" and "255" are the darkest and brightest
values, corresponding to black and white respectively.
On this histogram each "stack" or "bar" is one pixel wide. Unlike the
mosaic histograms, the 256 bars are stacked side by side without any space
between them, which is why for educational purposes, the vertical bars are
shown in alternating shades of gray, allowing you to distinguish the
individual bars. There are no blank spaces between bars to avoid confusion
with blank spaces caused by missing tones in the image. Normally all bars
will be black as indicated in the second histogram.