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HTTP: What are the best headers for rarely changed images

Answer 1

+1 A:

In this case you should use a "far future expires" to maximize cache hits. This means if you change an image, you need to give it a new name, but I can't imagine you're changing them often given that there are so many.

You really have 100s of images on the page, and you should sprite them. Perhaps there are some more common ones (your recommended) that you can start with to see whether it helps. Or maybe you could do a sprite of those "least likely to change".

ndp 2009-08-30 23:22:16

By sprite them you mean convert them to an image map. That way you only have one large image to download which will minimise the number of http requests sent to the server and speed up the download. That would work well for the link you sent but may not always be appropriate.

Matt H 2009-08-30 23:30:12

Here's a link for the far future expires header: http://www.askapache.com/htaccess/apache-speed-expires.html

ndp 2009-08-30 23:34:27

@Matt Yes. Put all the images into one file, and then "tile" it. Let's say you have tiles 100x100. If you make it a background image, CSS for row 1, column 2 looks something like "background-position: -100, 0; width: 100; height: 100; overflow:hidden".

ndp 2009-08-30 23:37:39

Well, I have 14,000 images which are 48x48. Each page will only use around 500 of them, but which 500 changes for each person (I have 300,000 people). And more people and images are added programatically. Think I should still sprite them?

Paul Tarjan 2009-08-31 00:24:16

Yes. You'll probably never get the loading of 500 images to work smoothly. The browser is just going to struggle with that (at least in the near term).I don't know your data. You're going to have to figure out a logical strategy. Ideas: (a) maybe the order doesn't matter and you can sprite some common ones and always show those at the top of the page. The page will pop up faster initially. (b) make a custom image for each user with just their sprites (you can test this by making a screen shot of an existing page)

ndp 2009-08-31 14:37:04

Answer 2

A:

Caching might help a little when a visitor moves from profile to profile, and serving the images from different subdomains with persistent connections could help speed delivery but the real performance killer is simply the number of HTTP requests. If you can identify two or more images that appear together more than 50% of the time, it seems it would be worthwhile to combine them into a sprite in a regularly-scheduled offline process. If resources permit, it might even be possible to use a image manipulation library e.g. php-gd to combine and serve blocks of images at runtime.

Dave 2009-08-31 12:49:00

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