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Section - [8] What is color quantization?
Many people don't have full-color (24 bit per pixel) display hardware.Inexpensive display hardware stores 8 bits per pixel, so it
can displayat most 256 distinct colors at a time. To display a full-color image, thecomputer must choose an appropriate set of
representative colors and map the image into these colors. This process is called "color quantization". (This is something of a
misnomer; "color selection" or "color reduction" would be a better term. But we're stuck with the standard usage.)
Clearly, color quantization is a lossy process. It turns out that for most images, the details of the color quantization algorithm
have *much* more impact on the final image quality than do any errors introduced by JPEG itself (except at the very lowest
JPEG quality settings). Making a good color quantization method is a black art, and no single algorithm is best for all images.
Since JPEG is a full-color format, displaying a color JPEG image on 8-bit-or-less hardware requires color quantization. The
speed and image quality of a JPEG viewer running on such hardware are largely determined by its quantization algorithm.
Depending on whether a quick-and-dirty or good-but-slow method is used, you'll see great variation in image quality among
viewers on 8-bit displays, much more than occurs on 24-bit displays.
On the other hand, a GIF image has already been quantized to 256 or fewer colors. (A GIF always has a specific number of
colors in its palette, and the format doesn't allow more than 256 palette entries.) GIF has the advantage that the image maker
precomputes the color quantization, so viewers don't have to; this is one of the things that make GIF viewers faster than JPEG
viewers. But this is also the *disadvantage* of GIF: you're stuck with the image maker's quantization. If the maker quantized
to a different number of colors than what you can display, you'll either waste display capability or else have to requantize to
reduce the number of colors (which usually results in much poorer image quality than quantizing once from a full-color image).
Furthermore, if the maker didn't use a high-quality color quantization algorithm, you're out of luck --- the image is ruined.
For this reason, JPEG promises significantly better image quality than GIF for all users whose machines don't match the image
maker's display hardware. JPEG's full color image can be quantized to precisely match the viewer's display hardware.
Furthermore, you will be able to take advantage of future improvements in quantization algorithms, or purchase better display
hardware, to get a better view of JPEG images you already have. With a GIF, you're stuck forevermore with what was sent.
A closely related problem is seen in many current World Wide Web browsers: when running on an 8-bit display, they force all
images into a pre-chosen palette. (They do this to avoid having to worry about how to allocate the limited number of available
color slots among the various items on a Web page.) A GIF version of a photo usually degrades very badly in this situation,
because it's effectively being forced through a second quantization step. A JPEG photo won't look wonderful either, but it will
look less bad than the GIF equivalent because it's been quantized only once.
A growing number of people have better-than-8-bit display hardware already: 15- or 16-bit/pixel "high color" displays are
now quite common, and true 24-bit/pixel displays are no longer rare. For these people, GIF is already obsolete, as it cannot
represent an image to the full capabilities of their display. JPEG images can drive these displays much more effectively.
In short, JPEG is an all-around better choice than GIF for representing photographic images in a machine-independent fashion.
It's sometimes thought that a JPEG converted from a GIF shouldn't require color quantization. That is false; even when you
feed a 256-or-less-color GIF into JPEG, what comes out of the decompressor is not 256 colors, but thousands of colors. This
happens because JPEG's lossiness affects each pixel a little differently, so two pixels that started with identical colors will
usually come out with slightly different colors. Considering the whole image, each original color gets "smeared" into a cluster
of nearby colors. Therefore quantization is always required to display a color JPEG on a colormapped display, regardless of
the image source.
The same effect makes it nearly meaningless to talk about the number of colors used by a JPEG image. Even if you tried to
count the number of distinct pixel values, different JPEG decoders would give you different results because of roundoff
error differences. I occasionally see posted images described as "256-color JPEG". This tells me that the poster (a) hasn't
read this FAQ and (b) probably converted the JPEG from a GIF. JPEGs can be classified as color or gray-scale, but
number of colors just isn't a useful concept for JPEG, any more than it is for a real photograph. |
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发表于 2015-4-18 17:17:03
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