Martha Graham's Google doodle is not a video, is an image (a CSS sprite)

Today's Google doodle is featuring Martha Graham, an American modern dancer. The new doodle looks like a movie, but in fact it is only a CSS sprite.

The sprite is this one: 

You may be wondering how is the animation/movie appearance achieved if it only uses this static image? 

The technique involves CSS sprites. A sprite is a composition of several images grouped together in order to reduce the number of required HTTP request to the server. This way a website with 50 images requires only one request for the large image composition. The entire doodle involves only one sprite, which speeds up loading a lot (compared to loading each frame separately). Google has used this technique to leverage data transfer from the servers and consequently make the animation load faster than a traditional video clip. 

So how is this sprite converted to an animation?

Pretend you are looking through pierced piece of paper, so that you can only look through the hole in the paper. Right behind the paper there is a film strip. If you move the film strip quickly from one side to the other you will think that there is actual motion happening. This is basically how movies are made.

In CSS all you have to do is set the width and height of the container (the hole in the paper) and change the background-position at every frame. Google has done it using many div tags. 

The first div tag looks like this:

<div>&nbsp;</div> <pre><div id="hplogo0" style="left: 307px; top: 48px; width: 88px; height: 89px; background-image: url(http://www.google.com/logos/2011/graham11-hp-sprite.png); background-attachment: initial; background-origin: initial; background-clip: initial; background-color: initial;  background-position: 0px 0px; background-repeat: no-repeat no-repeat;"> </div>

The second:

<div id="hplogo1" style="left: 307px; top: 48px; width: 89px; height: 89px; background-image: url(http://www.google.com/logos/2011/graham11-hp-sprite.png); background-attachment: initial; background-origin: initial; background-clip: initial; background-color: initial; background-position: -88px 0px; background-repeat: no-repeat no-repeat;"> </div>

Notice how the background position has been shifted, so that the next frame should look like this: 

Changing the background position of the sprite very quickly therefore creates the illusion of animation.

Because Google tries hard to optimize the sizes of these doodles (that's why they did not simply create a video file) they have not even maintained equal ratios between the sample frames which would make it very simple to write a javascript function to move the background at each new frame (every 83 milliseconds). 

Therefore they are storing an array of coordinates where the next images should be positioned. I have copied their code and added a few comment to make it a bit more readable.

//This is the array of coordinates of each individual frame var d=[[307,48,88,89],[307,48,89,89],[307,48,91,89],[305,49,93,89],[305,50,93,88],[305,50,93,88],[306,52,92,86],[305,53,93,84],[305,54,94,83],[306,54,93,83],[307,54,92,83],[307,54,92,83],[308,54,90,83],[308,54,90,83],[306,53,91,84],[306,53,91,84],[308,53,90,84],[308,53,90,84],[305,53,92,84],[305,52,92,85],[306,52,91,85],[308,51,88,87,1],[308,50,88,88],[308,49,88,88],[307,49,89,88],[307,50,89,87],[308,51,89,86],[307,54,90,83],[307,57,90,80],[306,58,92,79],[306,58,92,79],[305,60,92,77],[302,61,95,76],[302,63,95,74],[302,51,96,86],[302,66,98,71],[304,67,96,69],[301,63,96,74],[301,58,93,79],[291,52,94,85],[288,50,71,88],[285,43,76,95],[285,37,70,101],[281,29,55,109],[278,20,58,119],[278,20,55,119,1],[277,12,121,127],[271,2,122,138],[267,1,126,139],[264,0,136,140],[260,0,141,140],[255,0,148,140],[252,0,151,140],[249,2,121,138],[247,3,123,137],[246,3,123,137],[246,2,124,137],[258,2,112,137],[263,2,106,137],[263,2,106,137],[262,2,103,137],[260,2,104,136],[260,2,104,137,1],[268,2,98,137],[267,2,99,137],[266,2,97,137],[266,3,96,136],[264,3,99,136],[263,3,100,136],[261,3,100,136],[259,2,138,137],[254,2,126,137],[247,2,101,136],[240,2,108,136],[238,1,110,137],[230,1,118,138],[220,15,128,124],[211,18,137,121],[205,43,102,96],[202,45,104,93],[200,38,97,101],[198,38,104,101,1],[197,39,107,100],[197,39,112,100],[213,39,94,110],[212,40,95,111],[211,41,97,111],[209,42,99,112],[209,43,98,112],[213,43,87,112],[213,42,83,113],[211,40,86,109],[211,38,86,103],[211,37,88,112],[211,20,186,131],[213,27,167,122],[212,44,87,105],[210,44,88,98],[195,44,106,98],[189,44,110,98],[182,46,117,99],[173,44,118,96,1],[161,43,130,99],[154,42,137,97],[153,42,137,97],[153,42,137,97],[152,41,137,98],[151,41,137,97],[149,41,145,97],[148,25,144,114],[148,13,144,126],[141,12,153,127],[115,11,173,128],[108,7,180,133],[108,4,180,136],[108,3,176,137,1],[108,1,161,139],[105,1,235,138],[103,1,295,148],[103,0,277,149],[108,0,234,137],[101,0,232,137],[99,0,135,139],[95,0,244,139],[81,0,152,139],[69,0,164,139,1],[66,0,169,139],[65,0,170,139],[63,0,168,138],[61,0,159,138],[35,0,304,139],[19,0,189,140],[18,11,138,129],[18,11,137,129],[18,11,137,128],[18,6,135,133],[7,4,146,136],[6,4,147,136],[3,4,150,136,1],[3,5,150,135],[3,8,150,132],[4,6,394,145],[12,6,388,145],[11,8,389,144],[11,8,387,144],[11,8,387,143,1],[10,8,113,131],[11,8,111,131],[10,9,112,130],[12,9,116,130],[12,9,111,130],[12,9,111,130],[12,9,110,131],[12,34,113,106],[13,35,110,104]],   //this function created all the DIV tags and adds them to the page every 83ms. (approximaterly 12 FPS) l=function(){ var a=d[f], c=document.getElementById("hplogo");  if(c&&a[0]){  var b=document.createElement("div"); b.id="hplogo"+f; b.style.left=a[0]+"px"; b.style.top=a[1]+"px"; b.style.width=a[2]+"px";b.style.height=a[3]+"px"; b.style.background="url(logos/2011/graham11-hp-sprite.png) no-repeat "+-g+"px "+-h+"px"; b.onmousedown=k; a[3]>i&&(i=a[3]); a[4]?(g=0,h+=i,i=0):g+=a[2]; c.appendChild(b); ++f; f< e && ( j=window.setTimeout(l,83) ) } }

I hope I have succeded to shine some light on how this Google doodle was made.