/**
This library rewrites the Canvas2D "measureText" function
so that it returns a more complete metrics object.
Author: Mike "Pomax" Kamermans
**/
(function(){
var NAME = "FontMetrics Library"
var VERSION = "1-2011.0927.1431";
var debug = true;
// if there is no getComputedStyle, this library won't work.
if(!document.defaultView.getComputedStyle) {
throw("ERROR: 'document.defaultView.getComputedStyle' not found. This library only works in browsers that can report computed CSS values.");
}
// store the old text metrics function on the Canvas2D prototype
CanvasRenderingContext2D.prototype.measureTextWidth = CanvasRenderingContext2D.prototype.measureText;
/**
* shortcut function for getting computed CSS values
*/
var getCSSValue = function(element, property) {
return document.defaultView.getComputedStyle(element,null).getPropertyValue(property);
};
// debug function
var show = function(canvas, ctx, xstart, w, h, metrics)
{
document.body.appendChild(canvas);
ctx.strokeStyle = 'rgba(0, 0, 0, 0.5)';
ctx.beginPath();
ctx.moveTo(xstart,0);
ctx.lineTo(xstart,h);
ctx.closePath();
ctx.stroke();
ctx.beginPath();
ctx.moveTo(xstart+metrics.bounds.maxx,0);
ctx.lineTo(xstart+metrics.bounds.maxx,h);
ctx.closePath();
ctx.stroke();
ctx.beginPath();
ctx.moveTo(0,h/2-metrics.ascent);
ctx.lineTo(w,h/2-metrics.ascent);
ctx.closePath();
ctx.stroke();
ctx.beginPath();
ctx.moveTo(0,h/2+metrics.descent);
ctx.lineTo(w,h/2+metrics.descent);
ctx.closePath();
ctx.stroke();
}
/**
* The new text metrics function
*/
CanvasRenderingContext2D.prototype.measureText = function(textstring) {
var metrics = this.measureTextWidth(textstring);
fontFamily = getCSSValue(this.canvas,"font-family"),
fontSize = getCSSValue(this.canvas,"font-size").replace("px","");
metrics.fontsize = fontSize;
var canvas = document.createElement("canvas");
var padding = 100;
canvas.width = metrics.width + padding;
canvas.height = 3*fontSize;
canvas.style.opacity = 1;
canvas.style.fontFamily = fontFamily;
canvas.style.fontSize = fontSize;
var ctx = canvas.getContext("2d");
ctx.font = fontSize + "px " + fontFamily;
// for text lead values, we meaure a multiline text container.
var leadDiv = document.createElement("div");
leadDiv.style.position = "absolute";
leadDiv.style.opacity = 0;
leadDiv.style.font = fontSize + "px " + fontFamily;
leadDiv.innerHTML = textstring + "
" + textstring;
document.body.appendChild(leadDiv);
var w = canvas.width,
h = canvas.height,
baseline = h/2;
// Set all canvas pixeldata values to 255, with all the content
// data being 0. This lets us scan for data[i] != 255.
ctx.fillStyle = "white";
ctx.fillRect(-1, -1, w+2, h+2);
ctx.fillStyle = "black";
ctx.fillText(textstring, padding/2, baseline);
var pixelData = ctx.getImageData(0, 0, w, h).data;
// canvas pixel data is w*4 by h*4, because R, G, B and A are separate,
// consecutive values in the array, rather than stored as 32 bit ints.
var i = 0,
w4 = w * 4,
len = pixelData.length;
// Finding the ascent uses a normal, forward scanline
while (++i < len && pixelData[i] === 255) {}
var ascent = (i/w4)|0;
// Finding the descent uses a reverse scanline
i = len - 1;
while (--i > 0 && pixelData[i] === 255) {}
var descent = (i/w4)|0;
// find the min-x coordinate
for(i = 0; i=len) { i = (i-len) + 4; }}
var minx = ((i%w4)/4) | 0;
// find the max-x coordinate
var step = 1;
for(i = len-3; i>=0 && pixelData[i] === 255; ) {
i -= w4;
if(i<0) { i = (len - 3) - (step++)*4; }}
var maxx = ((i%w4)/4) + 1 | 0;
// set font metrics
metrics.ascent = (baseline - ascent);
metrics.descent = (descent - baseline);
metrics.bounds = { minx: minx - (padding/2),
maxx: maxx - (padding/2),
miny: 0,
maxy: descent-ascent };
metrics.height = 1+(descent - ascent);
// make some initial guess at the text leading (using the standard TeX ratio)
metrics.leading = 1.2 * fontSize;
// then we try to get the real value from the browser
var leadDivHeight = getCSSValue(leadDiv,"height");
leadDivHeight = leadDivHeight.replace("px","");
if (leadDivHeight >= fontSize * 2) { metrics.leading = (leadDivHeight/2) | 0; }
document.body.removeChild(leadDiv);
// show the canvas and bounds if required
if(debug){show(canvas, ctx, 50, w, h, metrics);}
return metrics;
};
}());