Here is a simple function. No library needed.
I wrote this function to get the top and bottom bounds relative to baseline. If textBaseline is set to alphabetic. What it does is it creates another canvas, and then draws there, and then finds the top most and bottom most non blank pixel. And that is the top and bottom bounds. It returns it as relative, so if height is 20px, and there is nothing below the baseline, then the top bound is -20.
You must supply characters to it. Otherwise it will give you 0 height and 0 width, obviously.
Usage:
alert(measureHeight('40px serif', 40, 'rg').height)
Here is the function:
function measureHeight(aFont, aSize, aChars, aOptions={}) {
// if you do pass aOptions.ctx, keep in mind that the ctx properties will be changed and not set back. so you should have a devoted canvas for this
// if you dont pass in a width to aOptions, it will return it to you in the return object
// the returned width is Math.ceil'ed
console.error('aChars: "' + aChars + '"');
var defaultOptions = {
width: undefined, // if you specify a width then i wont have to use measureText to get the width
canAndCtx: undefined, // set it to object {can:,ctx:} // if not provided, i will make one
range: 3
};
aOptions.range = aOptions.range || 3; // multiples the aSize by this much
if (aChars === '') {
// no characters, so obviously everything is 0
return {
relativeBot: 0,
relativeTop: 0,
height: 0,
width: 0
};
// otherwise i will get IndexSizeError: Index or size is negative or greater than the allowed amount error somewhere below
}
// validateOptionsObj(aOptions, defaultOptions); // not needed because all defaults are undefined
var can;
var ctx;
if (!aOptions.canAndCtx) {
can = document.createElement('canvas');;
can.mozOpaque = 'true'; // improved performanceo on firefox i guess
ctx = can.getContext('2d');
// can.style.position = 'absolute';
// can.style.zIndex = 10000;
// can.style.left = 0;
// can.style.top = 0;
// document.body.appendChild(can);
} else {
can = aOptions.canAndCtx.can;
ctx = aOptions.canAndCtx.ctx;
}
var w = aOptions.width;
if (!w) {
ctx.textBaseline = 'alphabetic';
ctx.textAlign = 'left';
ctx.font = aFont;
w = ctx.measureText(aChars).width;
}
w = Math.ceil(w); // needed as i use w in the calc for the loop, it needs to be a whole number
// must set width/height, as it wont paint outside of the bounds
can.width = w;
can.height = aSize * aOptions.range;
ctx.font = aFont; // need to set the .font again, because after changing width/height it makes it forget for some reason
ctx.textBaseline = 'alphabetic';
ctx.textAlign = 'left';
ctx.fillStyle = 'white';
console.log('w:', w);
var avgOfRange = (aOptions.range + 1) / 2;
var yBaseline = Math.ceil(aSize * avgOfRange);
console.log('yBaseline:', yBaseline);
ctx.fillText(aChars, 0, yBaseline);
var yEnd = aSize * aOptions.range;
var data = ctx.getImageData(0, 0, w, yEnd).data;
// console.log('data:', data)
var botBound = -1;
var topBound = -1;
// measureHeightY:
for (y=0; y<=yEnd; y++) {
for (var x = 0; x < w; x += 1) {
var n = 4 * (w * y + x);
var r = data[n];
var g = data[n + 1];
var b = data[n + 2];
// var a = data[n + 3];
if (r+g+b > 0) { // non black px found
if (topBound == -1) {
topBound = y;
}
botBound = y; // break measureHeightY; // dont break measureHeightY ever, keep going, we till yEnd. so we get proper height for strings like "`." or ":" or "!"
break;
}
}
}
return {
relativeBot: botBound - yBaseline, // relative to baseline of 0 // bottom most row having non-black
relativeTop: topBound - yBaseline, // relative to baseline of 0 // top most row having non-black
height: (botBound - topBound) + 1,
width: w// EDIT: comma has been added to fix old broken code.
};
}
relativeBot, relativeTop, and height are the useful things in the return object.
Here is example usage:
Page Titlefunction measureHeight(aFont, aSize, aChars, aOptions={}) {
// if you do pass aOptions.ctx, keep in mind that the ctx properties will be changed and not set back. so you should have a devoted canvas for this
// if you dont pass in a width to aOptions, it will return it to you in the return object
// the returned width is Math.ceil'ed
console.error('aChars: "' + aChars + '"');
var defaultOptions = {
width: undefined, // if you specify a width then i wont have to use measureText to get the width
canAndCtx: undefined, // set it to object {can:,ctx:} // if not provided, i will make one
range: 3
};
aOptions.range = aOptions.range || 3; // multiples the aSize by this much
if (aChars === '') {
// no characters, so obviously everything is 0
return {
relativeBot: 0,
relativeTop: 0,
height: 0,
width: 0
};
// otherwise i will get IndexSizeError: Index or size is negative or greater than the allowed amount error somewhere below
}
// validateOptionsObj(aOptions, defaultOptions); // not needed because all defaults are undefined
var can;
var ctx;
if (!aOptions.canAndCtx) {
can = document.createElement('canvas');;
can.mozOpaque = 'true'; // improved performanceo on firefox i guess
ctx = can.getContext('2d');
// can.style.position = 'absolute';
// can.style.zIndex = 10000;
// can.style.left = 0;
// can.style.top = 0;
// document.body.appendChild(can);
} else {
can = aOptions.canAndCtx.can;
ctx = aOptions.canAndCtx.ctx;
}
var w = aOptions.width;
if (!w) {
ctx.textBaseline = 'alphabetic';
ctx.textAlign = 'left';
ctx.font = aFont;
w = ctx.measureText(aChars).width;
}
w = Math.ceil(w); // needed as i use w in the calc for the loop, it needs to be a whole number
// must set width/height, as it wont paint outside of the bounds
can.width = w;
can.height = aSize * aOptions.range;
ctx.font = aFont; // need to set the .font again, because after changing width/height it makes it forget for some reason
ctx.textBaseline = 'alphabetic';
ctx.textAlign = 'left';
ctx.fillStyle = 'white';
console.log('w:', w);
var avgOfRange = (aOptions.range + 1) / 2;
var yBaseline = Math.ceil(aSize * avgOfRange);
console.log('yBaseline:', yBaseline);
ctx.fillText(aChars, 0, yBaseline);
var yEnd = aSize * aOptions.range;
var data = ctx.getImageData(0, 0, w, yEnd).data;
// console.log('data:', data)
var botBound = -1;
var topBound = -1;
// measureHeightY:
for (y=0; y<=yEnd; y++) {
for (var x = 0; x < w; x += 1) {
var n = 4 * (w * y + x);
var r = data[n];
var g = data[n + 1];
var b = data[n + 2];
// var a = data[n + 3];
if (r+g+b > 0) { // non black px found
if (topBound == -1) {
topBound = y;
}
botBound = y; // break measureHeightY; // dont break measureHeightY ever, keep going, we till yEnd. so we get proper height for strings like "`." or ":" or "!"
break;
}
}
}
return {
relativeBot: botBound - yBaseline, // relative to baseline of 0 // bottom most row having non-black
relativeTop: topBound - yBaseline, // relative to baseline of 0 // top most row having non-black
height: (botBound - topBound) + 1,
width: w
};
}
This is a Heading
This is a paragraph.
The relativeBot and relativeTop are what you see in this image here:
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