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Section - [10] Does loss accumulate
with repeated compression/decompression?
It would be nice if, having compressed an image with JPEG, you could decompress it, manipulate it (crop off a border,
say), and recompress it without any further image degradation beyond what you lost initially. Unfortunately THIS IS
NOT THE CASE. In general, recompressing an altered image loses more information. Hence it's important to minimize
the number of generations of JPEG compression between initial and final versions of an image.
There are a few specialized operations that can be done on a JPEG file without decompressing it, and thus without
incurring the generational loss that you'd normally get from loading and re-saving the image in a regular image editor.
In particular it is possible to do 90-degree rotations and flips losslessly, if the image dimensions are a multiple of the
file's block size (typically 16x16, 16x8, or 8x8 pixels for color JPEGs). This fact used to be just an academic curiosity,
but it has assumed practical importance recently because many users of digital cameras would like to be able to rotate
their images from landscape to portrait format without incurring loss --- and practically all digicams that produce JPEG
files produce images of the right dimensions for these operations to work. So software that can do lossless JPEG
transforms has started to pop up. But you do need special software; rotating the image in a regular image editor won't
be lossless.
It turns out that if you decompress and recompress an image at the same quality setting first used, relatively little
further degradation occurs. This means that you can make local modifications to a JPEG image without material
degradation of other areas of the image. (The areas you change will still degrade, however.) Counterintuitively, this
works better the lower the quality setting. But you must use *exactly* the same setting, or all bets are off. Also, the
decompressed image must be saved in a full-color format; if you do something like JPEG=>GIF=>JPEG, the color
quantization step loses lots of information.
Unfortunately, cropping doesn't count as a local change! JPEG processes the image in small blocks, and cropping
usually moves the block boundaries, so that the image looks completely different to JPEG. You can take advantage
of the low-degradation behavior if you are careful to crop the top and left margins only by a multiple of the block
size (typically 16 pixels), so that the remaining blocks start in the same places. (True lossless cropping is possible
under the same restrictions about where to crop, but again this requires specialized software.)
The bottom line is that JPEG is a useful format for compact storage and transmission of images, but you don't want
to use it as an intermediate format for sequences of image manipulation steps. Use a lossless 24-bit format (PNG,
TIFF, PPM, etc) while working on the image, then JPEG it when you are ready to file it away or send it out on the
net. If you expect to edit your image again in the future, keep a lossless master copy to work from. The JPEG you
put up on your Web site should be a derived copy, not your editing master. |
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发表于 2015-4-25 15:26:39
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