【LibJPEG专题系列一】JPEG 图像压缩: 什么是JPEG

席萌0209

                             JPEG 图像压缩 FAQ, part 1

                            Section - [1] What is JPEG?

JPEG (pronounced "jay-peg") is a standardized image compression mechanism.JPEG stands for
Joint Photographic Experts Group, the original name of thecommittee that wrote the standard.


JPEG is designed for compressing either full-color or gray-scale imagesof natural, real-world
scenes.  It works well on photographs, naturalisticartwork, and similar material; not so well
on lettering, simple cartoons,or line drawings.  JPEG handles only still images, but there is a
related standard called MPEG for motion pictures.


JPEG is "lossy," meaning that the decompressed image isn't quite the same as the one you
started with.  (There are lossless image compression algorithms,but JPEG achieves much greater
compression than is possible with lossless methods.)  JPEG is designed to exploit known limitations
of the human eye,notably the fact that small color changes are perceived less accurately than small
changes in brightness.  Thus, JPEG is intended for compressing images that will be looked at by
humans.  If you plan to machine-analyze your images, the small errors introduced by JPEG may
be a problem for you, even if they are invisible to the eye.


A useful property of JPEG is that the degree of lossiness can be varied by adjusting compression
parameters.  This means that the image maker can trade off file size against output image quality.
  You can make *extremely* small files if you don't mind poor quality; this is useful for applications
such as indexing image archives.  Conversely, if you aren't happy with the output quality at the
default compression setting, you can jack up the quality until you are satisfied, and accept lesser
compression.


Another important aspect of JPEG is that decoders can trade off decoding speed against image quality,
by using fast but inaccurate approximations to the required calculations.  Some viewers obtain remarkable
speedups in this way.  (Encoders can also trade accuracy for speed, but there's usually less reason to make
such a sacrifice when writing a file.)

席萌0209

                    JPEG图像压缩常见问题解答 第一部分

              Section 1—什么是JPEG？

    JPEG (读音为"jay-peg") 是标准化的图像压缩机制。JPEG的全称是Joint Photographic Experts Group,
委员会写这个标准时的早期名字。


         JPEG用于压缩“自然、真实世界”的全彩或灰度图像。它会很好地应用于图像、自然艺术品以及类
似的材料，而对于刻字、简单的卡通或是线条图来说却不是强项。JPEG只处理静态图像，但仍有一个用于
处理动态图像的相关标准—即MPEG。


   JPEG会有损耗，也就是说解压缩后的图像和输入图像并不完全相同。（有无损图像压缩的算法，但是相
对于此，JPEG可以实现更大程度的压缩。）JPEG旨在利用人眼已知的限制，特别是一个事实，也就是相对
于亮度上的小变化来说，色调方面在精准性上的改变更少。因此，JPEG适用于压缩可以被人眼看到的图像。
如果打算用机器来分析图像，那么JPEG引入的小误差或许就是一个麻烦，即使这些小误差你用眼睛看不到。


  JPEG有一个很实用的性质，也就是图像损耗的程度可以通过调整压缩参数而变化。这也就意味着图像制
造者可以针对输出图像的质量来权衡图像的文件大小。如果你不介意低质量的图像，你可以做出“极”小的
文件；这对于一些应用来说是很有用的，比如索引图像的存档。相反的，如果你不满意默认压缩设置的图像
质量，你可以调整压缩参数来提高图像的质量，直到你满意，并且实现更少的压缩。


  JPEG另一个更重要的方面，解码器可以针对图像的质量，通过使用快速但又不精准的近似值来折衷解码
速度。有些查看器就是通过这种方法得到显著地速度提升。（编码器也可以精确折衷速度，但是通常写入一
个文件时，很少这样做。）