很多网站需要在上传的图片上面打水印。
通常在配置apache之前，可以使用ImageMagic的命令工具composite来测试水印的效果，composite主要用于overlap one image over another.
composite -gravity SouthEast -geometry '+5+5' watermark-small.png t3.png t31.png
composite -gravity SouthEast -geometry '+10+10' watermark.png \( t1.png -resize '550x450<' \) t11.png
apache配置：使用ext_filter module调用ImageMagic包里面的composite命令对响应的图片加水印，类似下面的流程图。
<IfModule mod_ext_filter.c>
## ExtFilterDefine watermark mode=output intype=image/jpeg cmd="/usr/bin/composite -gravity SouthEast -geometry '+8+8' /home/media/watermark.gif - -"
ExtFilterDefine watermark mode=output intype=image/jpeg cmd="/usr/bin/composite -gravity SouthEast -geometry '+8+8' /home/media/watermark.gif \( - -resize '550x450<' \) -"
ExtFilterDefine watermark-small mode=output intype=image/jpeg cmd="/usr/bin/composite -gravity SouthEast -geometry '+4+4' /home/media/watermark-small.gif - -"
</IfModule>
<Location ~ "/.*photo-s">
SetOutputFilter watermark
</Location>
<Location ~ "/.*photo-f">
SetOutputFilter watermark-small
</Location>
<Location ~ "/.*photo-l">
SetOutputFilter watermark-small
</Location>
注意
1.ExtFilterDefine中最后的- - 分别表示输入与输出的图片。通常我们在测试的时候，是需要指定这个文件的。比如源文件是一个表达式,表示将t1.png文件放大后的文件做为源文件: \( t1.png -resize '550x450<' \)， t11.png表示生成的目标文件。
2.规范命名上传文件的路径。使用正则式匹配大小图片的路径并使用不同的Filter处理。
3.mode=output 表示mod_ext_filter处理的是输出的内容，也仅支持这一种模式。 intype=image/jpeg表示处理的内容类型须是图片。
4.mod_ext_filter定义及使用场合
At the most basic level, mod_ext_filter is simply a way to run content through an external program before it is outputted to the client. Think of it as a filter sitting between Apache and the user’s web browser.
However, practically speaking, it would be horribly inefficent and probably too slow to be usable.
• Dynamically add a copyright footer, even on plain ‘ole static HTML pages. (Though mod_include would be better for this unless you’re doing something tricky that requires the extra logic of an external program. I’ll be covering this in an upcoming blog entry.)
• A profanity filter that can easily be used with any blog or messageboard software.
• Dynamically adding a watermark to images using “composite” (Step-by-step HOWTO).
• And my favorite… Automatically add a wrapper around external links for tracking purposes. This could be done by piping the output through search-and-replace regex using Perl, sed, or awk. This is great when used with a CMS and end-users will be self-publishing content, possibly adding their own links.
参考
http://www.rightbrainnetworks.com/blog/using-apaches-mod_ext_filter-to-automatically-add-watermarks-to-website-images/
http://tda7088.blog.163.com/blog/static/294411542009112033151468/