2024年4月22日发(作者：)

1、 Baseline vs. progressive JPEGs

JPEG格式有一个特殊的变种，名为“高级JPEG”。在创建高级JPEG文件时，数据是这

样安排的：在装入图像时，开始只显示一个模糊的图像，随着数据的装入，图像逐步变得

清晰。它相当于交织的GIF格式的图片。高级JPEG主要是考虑到使用调制解调器的慢速

网络而设计的，快速网络的使用者通常不会体会到它和正常JPEG格式图片的区别。

本文来自: 中科软件园() 详细出处参考：

/whatis/sort03164/

Baseline are the “normal” JPEGs, the type of JPEG that all image programs

write by default. The browsers load them top-to-bottom as more of the image

information comes down the wire.

Loading a baseline JPEG, click to enlarge

Progressive JPEGs are another type of JPEGs, they are rendered, as the name

suggests, progressively. First you see a low quality version of the whole image.

Then, as more of the image information arrives over the network, the quality

gradually improves.

Loading a progressive JPEG, click to enlarge

From usability perspective, progressive is usually good, because the user gets

feedback that something is going on. Also if you’re on a slow connection,

progressive JPEG is preferable because you don’t need to wait for the whole

image to arrive in order to get an idea if it is what you wanted. If not, you can click

away from the page or hit the back button, without waiting for the (potentially

large) high quality image.

A reason against progressive JPEGs I’ve heard is that they look a bit old

school and that users might be underimpressed, if not irritated, by the progressive

rendering. I am not aware of a user study that focuses on this issue, please

comment if you have heard or conducted such a experiment.

There is controversial information in blogs and books whether progressive

JPEGs are bigger or smaller than the baseline JPEGs in terms of file size. So, as part

of the never-ending quest for smaler file sizes and lossless optimization, here is an

experiment that attempts to answer this question.

The experiment

One of the many free APIs that Yahoo! provides is the image search API. I used