(^112) PART 2 HOW DIGITAL CAMERAS CAPTURE IMAGES
How Software Compresses
Pictures for Storage
Everyone agrees the more megapixels, the better. That’s what gives digital
photos detail and depth. But mega-megapixels come with a megaproblem—
how to store all those millions of pixels. Each pixel may take as many as 24
binary bits to accurately represent the exact shade and intensity of the color
that the pixel is recording. When a camera records a photo to a memory
card—and the same is true when a computer is writing an image to a hard
drive—it creates a bitmap, a map of the location and color or each pixel
that make up the photo. There are three ways to
record the picture, depending on the sophistication of
the camera: RAWformat, uncompressed, and
lossy compressed.
RAW is simply an unaltered record of the light
readings from each of the pixels in the cam-
era’s image sensor. This is the most graphic
information the camera can provide, and
though it consumes a lot of storage, it’s the
format of choice for photographers who trust
themselves more than their cameras to interpret
that image data.
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Uncompressed files, such as those in the
.TIFF format, have been through the camera’s
processing to correct such attributes as white
balance, sharpness, and exposure. In the
process some of the raw data is lost, but no data
is discarded simply for the purpose of making the
photo file smaller.
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Lossy compression, the most common of which is
JPEG (also known as .jpg), discards some of the
information gathered by the image sensor in the
quest for ever smaller files. JPEG files can be saved
with more or less compression, meaning more or
fewer bits are thrown away. It is the most common
file format for photos on the Internet because small
file sizes lend themselves to traveling faster from
someone else’s computer to yours and the graphic’s
losses are not readily noticeable within the limits of
a computer screen.
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