css sprite with negative background positions not clear

I am working on some existing code where they are using css sprites. There is big image and its been used for to display all the images.

Its basically a Ul and li elements. The each li element uses this image and uses a background position.

I understood the css sprites completly ,its for better performance. But only thing which is not clear is why the x and y posit开发者_运维知识库ions start with negative margins. Most of the images in the sprites are getting retrived by using something like

-540px -30px like this

I was watching that image on the browser and it shows up correcetly and they should with 0,0 right.

am i missing something here

Just think about the element as a peep-hole, with the sprite image being moved behind it.

0,0 is the top left corner of the peep-hole.

Depending on which part of the huge sprite image you want to see, you need to give it a negative offset.

-50, -20
|-----------------------------------------------|
|                                               |
|       0,0                                     |
|       |-----|                                 |
|       |     |  <--- Peep-hole                 |
|       |-----|                                 |
|                                               |
|                                               |
|-----------------------------------------------|

Said another way: How far away is your peep hole from the top left corner. If you have a background position of -50, -20 that indicates that if you were to move the peep hole to be in the top left corner it would have to move left 50px (hence negative) and up 20px (hence also negative). So positioning is measure from the perspective of the peep hole and how its position is from the top left corner.

http://www.alistapart.com/articles/sprites

Do you have firebug? http://getfirebug.com

If so, just inspect one of the elements with the sprite background and change the background position to 0 0. Then just start playing around with the positioning and you should be able to figure out how it works.

background-position: 0 0; means put the background image 0 pixels from the left and 0 pixels from the top.

background-position: 0 -50px; means put the background image 0 pixels from the left border, and 50 pixels above the top border, effectively moving the background image up 50 pixels. Try it out on your own using firebug or just changing the stylesheet and refreshing to see the effects.