Saturday, January 8, 2011

How To Draw a Circle in HTML <5 (without Canvas/SVG)

The following tiny script allows to draw circles in "regular" pre-HTML 5 document, it is so fast that it really allows for animations in real time.

Example:

   drawCircle(500, 500, 150, "red", 50, 1);

This will draw a  red circle, 50% transparent, with 150 px radius at the point (500,500). The circle will be drawn using 1px-fine pen (granularity).
The wider is the "pen" the faster is the routine. I usually increase granularity to 4px while dragging/sizing for speed and then re-render with 1px granularity upon drag grip release.


Here is the self-sufficient(does not rely on any libs) procedural source code:


//classic one
function isIE()
{
 return (navigator.appName.indexOf("Microsoft")!=-1);
}

function drawCircle(cx, cy, r, clr, op, gran)
{
 var x = 0;
 var div; 
 
 var dc = document.createElement("DIV");//aka "Device Context" wich is a DIV in doc tree
 dc.style.position="absolute";
 dc.innerHTML = "&nbsp;";
 dc.style.left = cx - r; 
 dc.style.top =  cy - r; 
 
 cx = r; cy = r;
 
 if (gran<1) gran = 1;
 
 if ((r/gran)> 280) gran = Math.round(r / 280);
  
 for(var y=0; y<=r; y+=gran)
 {
  x = Math.sqrt((r*r) - (y*y));
  
  div = document.createElement("DIV");
    
  div.style.position="absolute";
  div.style.background = clr;
  div.style.left = cx - x;
  div.style.top = cy+y;
  div.style.width= x*2;
  div.style.height=gran;
  div.style.fontSize=1;
  div.innerHTML = "&nbsp;";
  setObjectOpacity(div, op);
  dc.appendChild(div);
  
  div = document.createElement("DIV");
  div.style.position="absolute";
  div.style.background = clr;
  div.style.left = cx - x;
  div.style.top = cy-y-gran;
  div.style.width= x*2;
  div.style.height=gran;
  div.style.fontSize=1;
  div.innerHTML = "&nbsp;";
  setObjectOpacity(div, op);
  dc.appendChild(div);
  
 }
 
 return dc;
}//drawCircle

//semi-transparent through CSS/filter
function setObjectOpacity(obj, opacity) 
{
  //IE
  if (isIE())
   obj.style.filter = "alpha(opacity:"+opacity+")";
  else
  {
    opacity = (opacity == 100)?99.999:opacity;//99.9 for Firefox flicker bug
  
    // Konqueror, Safari
    obj.style.KHTMLOpacity = opacity / 100;
    // Old Mozilla and Firefox
    obj.style.MozOpacity = opacity / 100;
    // CSS3,  Safari , new Firefox (Gecko)
    obj.style.opacity = opacity / 100;
  }//else
}//setObjectOpacity


As you can see, this approach uses DIVs as a vehicle for graphics delivery. Also, one may ask why I did not use squares to reduce
 the number of DIVs - the answer is - flicker - squares would have overlapped and would have worked slower than interlaced DIVs.

Use as you please!


19 comments:

Anonymous said...

Cool, HTML5 will not come for some time yet

Anonymous said...

you forgot

document.body.appendChild(drawCircle(...))

Anonymous said...

This is obsolete, but great post though. One of my clients uses ie6 and plans to keep it till Xp supporting hardware is available bummmmmmr

web design Perth said...

Thank you for sharing, I've been dabbling with CSS and now I think I seem to live HTML5 better.

Anonymous said...

I am new to this, would you mind showing where the function drawCircle() is called from? Presume it is html?

Web Design Birmingham AL said...

Thank you for the hard work and effort putting on the website. Your blog is great with more informative and useful ideas.

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