Stephanie Leary wrote:
> Ran into a bizarre problem over the weekend. The goal of this page is to
> combine a nav menu with progress meters for a funding campaign. After much
> fiddling with stacking things, I realized the simplest way to do it would be
> to set a background image on each list item, and move it to the left by
> whatever amount has not been funded.
> 
> Example:
> http://stephanieleary.net/progress-test/
> 
> For goal #10, I tried
> background-position: -76% 50%;
> 
> ...but it doesn't work. If I do the math and specify the pixels, it does:
> background-position: -152px 50%;
> 
> (Actually, that doesn't work in Safari either, but I'm willing to deal with
> Safari later.)
> 
> Why won't the percentage work?
> 
> 


Image and box are approx. 200px wide in your example.


As per CSS spec, the percentage does not move the left top corner:

"With a value pair of '14% 84%', the point 14% across and 84% down the
image is to be placed at the point 14% across and 84% down the padding
area."

This means, percentage bg positioning uses two corresponding points, a 
point in the img and a point in the box, and matches them.

See fig1 in Zoe's article [1].

As long as your image is as wide as the box, no percentage value should 
move it, since the position of any percentage-related point is the same.

If both are 200px wide, 50% means: the middle of the img is matched to 
the middle of the box: no move.
0%: no move. -76%: no move.
Because both are equal in width.

With percentages, you can move a smaller or an wider image in relation 
to the box.


To cover 100% of the box with the image: The image should have a width 
of 400px, and the half of it should get the color [+], the other half 
should be white [=].

img: 0%++++====100% 400px

box: 0%####100% 200px

turned by -90° here

imgpos:0%, boxcovered:100%

=
=
=
=
+#
+#
+#
+#_0%


pos 25%, covered 75%
=
=
=
=#
+#
+#_25%
+#
+

pos 50%, covered 50%
=
=
=#
=#_50%
+#
+#
+
+

pos 75%, covered 25%
=
=#_75%
=#
=#
+#
+
+
+

Or is it the other way round? Hmm.

And Safari will probably have
problems with %-positioning bg-images that are bigger than the box anyway.

Ingo


[1] http://www.communitymx.com/content/article.cfm?page=1&cid=AFC58
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