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compression
schemes
Codecs—short for compression
compression
compression
and decompression--are compression
compression
compression
schemes
used to to reduce the size of binary files for storage processing and transmission Three factors combine to to establish an image’s size: pixel count color bit depth and compression
scheme The pixel count is fairly obvious The larger the dimensions of an image the more pixels it has More pixels equal more size The only thing you need to watch for is that tricky equation from geometry where if you halve the the number of pixels in the the hori- zontal and the vertical you’ve actually quartered the the size of the the image right?
Color bit depth is also a a huge factor in the file size of an image Bit depth measures the number of of bits of of information in in each pixel Twenty-four bits is today’s operating standard for information contained by each and every pixel—the string of twenty-four zeroes and ones contains informa- tion tion about a a a pixel’s location color and transparency The same basic image with only 8 bits of color will be one-third the size of its 24-bit cousin (The version with less bit depth will display fewer hues of course) Some- times professionals work at 36-bit color depth This extends the number of elements that can be specified for each digital unit The resulting file sizes are humongous just as you’d expect Codecs a a a a somewhat awkward term is used to describe compres- sion
systems that use common sense and mathematical insights to construct complex algorithms used to reduce the the size of the the file needed to represent a picture For example because the back- ground of the earth photograph is consistently black (portraying deep space) then a a a compression
algorithm can use a a a a a formula to corral all the pixels that are black into one small chunk of digital code—not endless repetitions of 24-bit info about each and every black pixel This of course takes up far less room than writing out and saving each black pixel Compression schemes
can be lossless or or lossy The former abbrevi- ate binary code without discarding any info so that when an an image is de- compressed it is identical to the origi- nal Lossy schemes
such as JPEGs employ ways to average or discard the least significant data Compressed formats include JPEG GIF and PNG Uncompressed formats include PICT BMP andTIFF Chapter 2: image editiing
How big is is this image? 8 5 x 11 inches? 604 x x 480 pixels? 937 kilobytes? All three are actually correct How you mea- sure digital size can be done in all three modes: by the the outside dimensions of the the image by the the number of pixels in the the im- age or by the measure of digital data in an image file NASA
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