07-3 7.3 Image Compression.pptx

第1页：7.3 生词表

第2页：7.3 生词表（续）

第3页：7.3 常用短语

第4页：为什么图像压缩如此重要

Whyisimage compression so important? Imagefiles, in an uncompressed form, are very large. And the Internet, especially for people using a 56 kdialupmodem, can be pretty slow.[1]This combination could seriously limit one of the Web'smost appreciated aspects—its ability to present images easily.

语言点分析：简单句。句中its ability to present images easily为oneoftheweb’s most appreciated aspects的同位语。7.3图像压缩

为什么图像压缩如此重要？非压缩格式的图像文件非常大。而Internet，尤其对使用56k调制解调器拨号上网的用户来说，是相当慢的。巨大的图像和慢速上网的结合很大程度上限制了网络最令人欣赏的方面，即易于显示图像的能力。

第5页：联合图像专家组）压缩

JPEG（联合图像专家组）压缩是当前Web压缩图像的最好方法，其他图像压缩格式，包括GIF和PNG，更多地用于Web的其他目的。JPEG(Joint Photographic Experts Group)compression is currently the best way to compress PHOTOGRAPHIC IMAGE Sfortheweb. Otherformsof image compression,including GIFand PNG,are best used for other purposes on the web.

第6页：可交换的图像文件格式）最适合于色彩较少并

GIF（可交换的图像文件格式）最适合于色彩较少并且有大面积的均匀色调的图像，如卡通或旗帜。尽管它有几个明显的特点，譬如透明以及显示动态图像的能力，但是由于256色的限制，它不适于显示照片之类色调连续的图像。GIF(Graphics Interchange Format)is best used for graphics that have a limited color pallet and large areas of flat tone,like cartoons or banners. Although it has several remarkable features,such as transparency and the ability to present animated images,it is not well suited for the presentation of continuous tone images,such as photographs,due to its limit of 256 colors.

第7页：便携式网络图像）

JPEG,orJPG,is an evolving format that is universal in its use as a means of compressing continuous tone photographs for speedy transmission over the Internet. Photographs compressed using the JPEGformat look good because JPEGsupports millions of colors,so you can see the gradation of tones. PNG（便携式网络图像）是一种相对较新的有很大潜力的格式。但是，在所有浏览器可以浏览PNG格式的图像之前，使用该格式并不是个好主意。JPEG或JPG格式是一个发展中的通用格式，用于压缩连续色调图像以提高网络传输速度。采用JPEG格式压缩的照片看起来很好，因为JPEG支持数百万种色彩，所以你可以看到色调的分级。PNG(Portable Network Graphics)is a relatively new format with alot of potential but,until all browsers can see images compressed in PNGform,it is not a good idea to use it.

第8页：7.3 图像压缩 课文要点

ABitmapisa simple series of pixels all stacked up. But the same image saved in GIFor JPEGformatus eslessbytes to makeup the file. How?Compression."Compression"is a computer term that represents a variety of mathematical formats used to compress an image 'sbytesize. Let'ssayyou have an image where the upper right-hand corner has four pixels all the same color. Whynotfind away to make those four pixels into one?[2]Thatwouldcut down the number of bytes by three-fourths,atleast in the one corner. That's a compression factor.分析：简单句。句中by意为“到……程度”，例如：Thecarpetis too short by three feet.地毯短了3英尺。位图文件是一系列的像素堆积起来的。但是同一幅图片用GIF或JPEG格式保存可以用较少的字节形成文件。怎样实现？就是用压缩的方法。压缩是一个计算机术语，表示用于压缩图像字节数的数学格式。假设有一幅图像的右上角有四个同色的像素，为什么不找一个方法将四个像素放在一起？那样将会至少在该区域将字节的数目减少3/4。这就是一个压缩因子。

第9页：比较级，the +比较级”以省略形式出现

Bitmapscanbe compressed to a point. The process is called"run-length encoding."Runsofpixels that are all the same color are all combined into one pixel.[3]The longer the run of pixels,the more compression. Bitmapswith little detail or color variance will really compress. Those with a great deal of detail don't offer much in the way of compression. Bitmapsthat use the run-length encoding can carry either the common".bmp"extensionor".rle".Another difference between the two files is that the common Bitmapcan accept 16 million different colors per pixel. Saving the same image in run-length encoding knocks the bits-per-pixel down to 8. Thatlocksthe level of color in at no more than 256.分析：本句是“the +比较级，the +比较级”以省略形式出现的主从复合句，前面部分是从句，后面部分是主句，意为“越……，越……”。位图可以压缩成一个点，这个过程叫做“游程长度编码”。连续同色的像素作为一个游程被结合成一个点。游程越长，压缩幅度越大。细节和色彩变化较少的位图的确可以压缩，那些有很多细节的位图用这种方法没有太大的压缩空间。使用游程长度编码的位图文件扩展名可以为“.bmp”或“.rle”。这两种文件的另一区别是普通位图文件可以接收每像素1,600万种不同的颜色，采用游程长度编码保存同一个图像将每像素字节数减少到8，这限制颜色分级不超过256。

第10页：那么，当所有颜色相近的时候为什么不创建一

那么，当所有颜色相近的时候为什么不创建一个单一的像素呢？你甚至可以降低可用颜色的数量以使得像素有更好的机会得到相近的颜色。这是个好办法，CompuServe公司的人也是这样想的。So,why not create a single pixel when all of the colors are close?You could even lower the number of colors available so that you would have a better chance of the pixels being close in color. Goodidea. The people at Compu Servefeltthe same way.

第11页：即

GIF（即Graphic Interchange Format，可交换的图像文件格式）于1987年由CompuServe公司给出标准，而用来产生GIF压缩的算法（数学公式）的专利属于Unisys。用于Web的第一个GIF格式叫做GIF87a，表示年份和版本。这种格式以每像素8个二进制位存储图像，颜色分级的上限为256。这种8位分级允许图像工作于多种服务器风格，包括CompuServe、TCP/IP。GIF,whichstandsfor"Graphic Interchange Format,"was first standardized in 1987 by Compu Serve,although the patent for the algorithm(mathematical formula)used to create GIF compression actually belongs to Unisys. The first format of GIFusedonthe Webwascalled GIF 87 a,representing its year and version. Itsavedimage sat 8 bits-per-pixel,capping the color level at 256. That 8-bit level allowed the image to work across multiple server styles,including Compu Serve,TCP/IP.

第12页：1989年

1989年，CompuServe公司更新了GIF格式，使其可以包含动画、透明和交织。你可以猜测到：他们称其新格式为GIF89a。87和89格式的基本（即无交织的）GIF无明显不同。Compu Serveupdated the GIFformatin 1989 to include animation,transparency,and interlacing. They called the new format,youguessedit:GIF 89 a. There's no discernable difference between a basic(known as non-interlaced)GIFin 87 and 89 formats.

第13页：习题7.3 一 判断正误

习题7.3 一 判断正误：根据课文内容判断下列说法正误。

Put“true”or“false”in the brackets for the following statements according to the passage.

1. ()Image compression is important because uncompressed image files that are very large could limit one of the Web’s most appreciated aspects.
2. ()GIF (Graphics Interchange Format)is not used for graphics that have a limited color pallet and large areas of flat tone, like cartoons or banners.
3. ()Photographs compressed using the jpeg format look good because jpeg supports millions of colors.
4. ()ABitmapisa simple series of pixels all stacked up.
5. ()GIFstandsfor“Graphic Internet Format”.
6. ()GIF 87 a saved images at 8 pits-per-pixel, capping the color level at 256.
7. ()JPEG, or JPGstandsfor (Joint Photographic Experts Group).
8. ()“Compression”is a computer term that represents a variety of mathematical formats used to decompress an image’sbytesize.

第14页：习题7.3 二 练习

习题7.3 二 练习：根据课文内容完成下列练习。

Fillinthe blanks according to the passage.

1. Image compression is_________________.
2. Imagefiles, inan_________________form, are very large.
3. JPEG compression is currently the best way to_________________PHOTOGRAPHIC IMAGE Sfortheweb.
4. GIF (Graphics Interchange Format)is best used for graphics that have a_________color pallet and____________areas of flat tone, like cartoons or banners.
5. GIFisnotwell suited for the_________________of continuous tone images, such as photographs, due to its limit of 256 colors.
6. PNG (Portable Network Graphics)is a relatively new format with alot of___________.
7. JPEGsupports millions of_________________.
8. “Compression”is a computer term that represents a variety of mathematical formats used to_________________animage’sbyte_________________.

第15页：习题7.3 三 翻译

习题7.3 三 翻译：将下列单词和短语译成中文。

Translate the following words and phrases into Chinese.

1. compression
2. uncompress
3. combination
4. seriously
5. ability
6. format
7. byte
8. mathematical
9. extension
10. bits-per-pixel