Why I should not compress images in HTTP headers?

Question

I read some articles about HTTP headers compression. Today I installed YSlow and it recommends that I compress the resources (text/html, javascript, css and images). Now I'm reading the documentation for Apache mod_deflate but in the example don't compress images.

Should I or should I not compress images in my site?

Contradictory

YSlow recommends that images be compressed? Are you sure? That sounds like odd advice. – Hydric

Answer 1

Your images should already be compressed - any extra compression won't have any noticeable effect on filesize, but will increase processing time.

• .png files use DEFLATE compression already.
• .jpg files generally use lossy compression.
• .gif files use LZW compression.

Compressing files that have already been compressed very rarely results in a reduction in filesize, and can often increase filesize.

Interjoin

@Agusti: if your images happen to be uncompressed, like say, .bmp files, you should convert them to .png or .jpg, as appropriate, and then serve the converted version, of course :) – Plywood

@Martinho Fernandes thank you but my images are .png files, btw it's very good to know this tip :-D – Contradictory

@Martinho Good point - that was what I meant by "Your images should already be compressed", but it's good to make it explicit. – Interjoin

Answer 2

Images such as GIF, JPEG or PNG are already compressed via highly specialized algorithms that achieve better result than general purpose algorithms such as deflate.

Therefore, re-compressing them yields little to no gain in size, and can even make files bigger, with the added cost of server-side processing.

So, in other words... do not compress images.

Igbo

Answer 3

SVGs should be compressed by Apache mod_deflate as

AddType image/svg+xml svg

AddOutputFilterByType DEFLATE image/svg+xml

More information at https://httpd.apache.org/docs/2.4/mod/mod_deflate.html.

Unknowing