[thelist] Sprite images and performance

Bill Moseley moseley at hank.org
Sun Aug 3 12:10:40 CDT 2008

Sorry, this is a bit long.  It will only be interesting to those that
are combining multiple background images into sprites to improve web
site performance.  Hopefully, there's a few developers out there
with experience in working with sprites.

Under development we want the css to reference individual background
images -- basically, we don't want the designers to have to worry
about building sprite files.

Instead, I have a production build script that parses the css,
extracts out the background image links from "url()" settings, builds
a sprite file from all the images and then rewrites the css with the
new image location and a background-position css rule.

I'm using ImageMagick to build the sprite images along with some
additional minification/stripping tools.  (Yes, I'm aware of existing
tools such as SmartSprites).

Making the sprite generation automatic has a few challenges.

Here's my concerns and questions:

1) I've seen a number or recommendations to arrange the icons in the
sprite file horizontally[1], and indeed in my tests that does result
in smaller files.  But, I think vertically aligned might be safer.

My concern is that it will be more likely that adjacent images in the
sprite will show up with horizontally aligned images than vertically
aligned images because a block element with a background image could
end up very wide in a fluid layout if a user has a very wide screen.

For this reason I'm thinking vertical arrangement is better.  Any
one have experience with this?

2) On the sprite page I'm adding 100px transparent space between each
image (to avoid showing adjacent images).

Does this seem like a reasonable amount of space?

2) Assume a sprite is arranged vertically -- images below the previous
one. And assume they are small icons (say 16x16).

If I also include a very wide (but small) image (say 1000x20) the
resulting image size is much more than the size of the icons along
plus the size of the bar alone (in one test the sum of the two was 50K
but the resulting image was 90K).  I guess all that empty transparent
space created by adding the wide image takes up more file space than I

Is that extra 40K worth a separate HTTP request?  Perhaps.

I guess my question is when automatically processing a long list of
background images what can I use to determine when to exclude an image
from the sprite?  (I'm currently using an exclude list in my
configuration file and also by checking if the total "area" of an
image exceeds some threshold.)

Those are my main concerns.  Anyone have any experience working with
sprites they can share?

FWIW -- I'm using PerlMagick to generate the sprites.  I have not
looked into optimizing with ImageMagick yet, but here's my simple
approach so far:

    my $im = Image::Magick->new;
    $im->Read( @image_list );

    my $montage = $im->Montage(
        mode        => 'Concatenate',
        tile        => '1x',
        background  => 'none',
        border      => 50,
        bordercolor => 'none',

    $montage->Write( $sprite_path );

[1] See Optimizing CSS Sprites at:

Bill Moseley
moseley at hank.org
Sent from my iMutt

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