write text using an equation Well like the batman equations and equations for heart, I once saw a site that draws equation for whatever text you type....but now I can't find it.
Does anybody know such a site?
Also a general explanation how do they do it...I think they have to write equations for all characters from $a$ to $z$ manually then do some coordinate shift etc.
PS: If somebody has some matlab code to help then it will be awesome.
 A: Perhaps you meant that link: http://www.xamuel.com/inverse-graphing-calculator.php?phrase=Hello+World 
[update, concerning your request for explanation]: Once you have a solution to define a plotting formula in the form $\small f(x,y)=0 $ and define the curve to be painted black if that equation is true, then you may combine many such formulas $\small f(x,y) \cdot g(x,y) \cdot h(x,y)=0 $ to draw combined pictures. (the multiplication implements the boolean OR: if any of the components is zero, draw a black point).  The key is then to define some primitives, combine them to a letter (shifting coordinates appropriately), and combine that combinations to words, and ...      
(Another example is at http://go.helms-net.de/math/fun/pdf/PdFdiscovery_ED.html ; I'd made some fun with an enthusiastic "fermatist" in the newsgroup sci.math one time )    
(A bit more explanative is the following; here you can see the composition of the graph by its elements, although the logic is a bit different: click the top-left icon (that with th scribbled face) on http://go.helms-net.de/math/index.htm to see my first version of such a picture, when I was a schoolboy)
A: This can generally solved as an interpolation problem. Find points on the outline of the "image" you want to draw with a formula and use for example piecewise interpolation. Then you will get a formula that approximates the surface of your image.
