什么是渐进式JPG

http://nornor.net/progressive-jpg.htm

DEMO

baseline(普通JPG格式):
http://www.webkaka.com/blog/archives/demo/1_normal-4276_50k.jpg
interlace(渐进显示的JPG格式):
http://www.webkaka.com/blog/archives/demo/1_ps(lx)-5_006m.jpg

$ file baseline.jpg interlace.jpg 
baseline.jpg:  JPEG image data, JFIF standard 1.01, resolution (DPI), density 72x72, segment length 16, baseline, precision 8, 5472x3648, frames 3
interlace.jpg: JPEG image data, JFIF standard 1.02, aspect ratio, density 100x100, segment length 16, progressive, precision 8, 5472x3648, frames 3
$ ls -l baseline.jpg interlace.jpg  # 看起来渐进式的图片文件会更大一些？
-rw-r--r--  1 fine  staff  4276498 12  6 17:24 baseline.jpg
-rw-r--r--  1 fine  staff  5249193 12  6 17:24 interlace.jpg

阿里云OSS

https://help.aliyun.com/document_detail/44704.html
阿里云OSS支持将JPG图片转成渐进显示的。
baseline:
http://lazada-processed-image.oss-ap-southeast-1.aliyuncs.com/aaron-burden-183659.jpg
interlace:
http://lazada-processed-image.oss-ap-southeast-1.aliyuncs.com/aaron-burden-183659.jpg?x-oss-process=image/format,jpg/interlace,1