Digital Image capture
The human eye has color receptor
"cone" cells which are sensitive to red, green, and blue
wavelengths of light. The brain combines the signals from these three
receptors to form the perception of all colors. For example, the
perception of a bright yellow is actually caused by equal stimulation
of both the red and green cone cells.
Digital image capture devices such as cameras and scanners
follow the model of human vision and record the original scene through
red, green, and blue filters, recording it as three separate red,
green, and blue channels of information. Most digital still cameras
divide the brightness of each channel into 256 discrete steps numbered
from 0 (darkest) to 255 (lightest). This 256 step brightness level
information can be stored in 8 binary "bits" (e.g.
10101111 ) which a computer can easily process. In most cameras the
recording device is a Charged Coupled Device (CCD) about the size of
your thumbnail which is divided, chessboard fashion, into cells
overlaid with either a red, green, or blue filter gel. Each cell
records 8 bits of data representing on a scale of 0 to 255, how much
light was detected. The combined signals from three adjacent red (R),
green(G), and blue(B) cells are combined into a single 24 bit binary
data string (e.g., 111111110000000011111111). This 24
bit / 3 byte data point is called a pixel, and is the basic unit of digital
image storage and manipulation.
Image resolution: Pixels = Detail
Because the image is broken into a chessboard pattern
of pixels, the amount of detail which is recorded is limited by the number of
pixels which can be packed onto the tiny CCD recording chip. Entry level
digital cameras typically have a CCD array of 1600 x 1000 pixels, or 1.6
megapixel resolution. The high-end consumer models now have 2500 x 1500 pixels,
or 3.45 megapixels. 5 megapixel cameras will hit the market 2001. More pixels is
better, and each new generation of cameras raises resolution of the CCD array.
Many digital cameras offer the option of several image
resolutions. For example my DC290 offers: High (1792x1200), Medium
(1440x960), Standard (720x480), and Ultra (2240x1500). The CCD image
sensor in this camera is actually 1792x1200. For the smaller sizes
software in the camera scans or "resamples" the recorded
image data and then stores it in fewer pixels. For the Ultra
(2240x1500) format it resamples the recorded data adding new
"interpolated" pixels containing values computed from the adjacent
pixels recorded when the picture was taken.
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