Generating images: Alternate font by User Agent (OS)

I need to generate small images for certain parts of text. Those will have to fit into the text seamlessly. I know I can not make up for every browser font settings available, but how do I have to check for Linux, Mac and Windows users depending on the user agent? I want to use the right font (of the three "Verdana, Arial, Helvetiva") for the right user agent OS.

So:

1. How do I check for the OS? What do I have to compare to in the user agent?
2. Where can I get the other two fonts. Does the Windows-included Verdana look the same as the one used on Linux (or, is it free anyway? I don't know much about fonts)? Where do I get Helvetica? Is 开发者_如何学运维it Mac OS licensed?

Any code examples can be in either c# or VB.NET. I can read both.

Thank you in advance.

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For client side, have a look at http://www.w3schools.com/js/js_browser.asp

For server side, look at Request.Browser.Platform.

As for the fonts, I can't tell you much, except to make sure that you have a (virtual) machine that matches each platform you want to target. In my opinion, it's the only way to be really sure that whatever you end up doing actually works.

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If you use @font-face in your css and specify the font-size in pixels, you will get the same font on all platforms at a predictable size. Then you can create one set of images for all platforms.