问题
I have been trying to draw gaussin-like function using bezierCurveTo
find the code below
<canvas id="thisCan" width="0px" height="0px" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
(function() {
var
// Obtain a reference to the canvas element
// using its id.
htmlCanvas = document.getElementById('thisCan'),
// Obtain a graphics context on the
// canvas element for drawing.
ctx = htmlCanvas.getContext('2d');
var width = 0;
var height = 0;
// Start listening to resize events and
// draw canvas.
initialize();
function initialize()
{
// Register an event listener to
// call the resizeCanvas() function each time
// the window is resized.
window.addEventListener('resize', resizeCanvas, false);
// Draw canvas border for the first time.
resizeCanvas();
}
// Display custom canvas.
// In this case it's a blue, 5 pixel border that
// resizes along with the browser window.
function redraw()
{
ctx.beginPath();
ctx.moveTo(width/2, 0);
ctx.bezierCurveTo(150, 119, 186, 121, 66, 185);
ctx.moveTo(width/2 + width * 0.08 , 0);
ctx.bezierCurveTo(344, 119, 344, 121, 504, 185);
ctx.stroke();
}
// Runs each time the DOM window resize event fires.
// Resets the canvas dimensions to match window,
// then draws the new borders accordingly.
function resizeCanvas()
{
var contentElement = document.getElementsByClassName("content-box post")[0]
width = htmlCanvas.width = (contentElement.clientWidth * 0.75)
height = htmlCanvas.height = (contentElement.offsetWidth*0.75 * 0.5);
redraw();
}
})();
</script>
I am planning to draw many curves in between as well. But how do I make it parametric, based on width and height variables?
I need to specify the control points using width and height parameters, so that it becomes window-size invariant.
Is there a way?
回答1:
Don't use Bezier curves if you want an exponent curve, they're different functions. Instead, just plot the function you actually need, with something like http://jsbin.com/nubutodosu/edit?js,output, where you define a Gaussian object:
// Gaussian distribution generator
var Gaussian = function(mean, std) {
this.mean = mean;
this.std = std;
this.a = 1/Math.sqrt(2*Math.PI);
};
Gaussian.prototype = {
addStd: function(v) {
this.std += v;
},
get: function(x) {
var f = this.a / this.std;
var p = -1/2;
var c = (x-this.mean)/this.std;
c *= c;
p *= c;
return f * Math.pow(Math.E, p);
},
generateValues: function(start, end) {
var LUT = [];
var step = (Math.abs(start)+Math.abs(end)) / 100;
for(var i=start; i<end; i+=step) {
LUT.push(this.get(i));
}
return LUT;
}
};
And then you can give that a draw routine so that it can plot itself over the interval that you need:
...
draw: function(ctx) {
var points = this.generateValues(-10,10);
var len = points.length;
ctx.strokeStyle = "black";
ctx.beginPath();
var p0 = points[0];
ctx.moveTo(0, height - (height*p0));
points.forEach(function(p,i) {
if(i===0) {
return;
}
ctx.lineTo(width * i/len, height - (height*p));
p0 = p;
});
ctx.stroke();
}
...
So you build your array of values over the interval, then draw them on the canvas by "connecting the dots".
回答2:
I managed to resolve it.
To be specific, I was looking out for half-gaussian curve.
I managed to figure out that guassian function has a very special property with respect to bezier curves. It might be elementary, but I could not find it on google. So this might be informative finding.
If each control points of the cubic bezier curve reside on a "line parallel to X axis and passing through start point/end point", the resulting curve will be of a half guassian shape.
e.g.
On top of this finding, I have written the following code:
<canvas id="thisCan" width="0px" height="0px">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
(function() {
var
// Obtain a reference to the canvas element
// using its id.
htmlCanvas = document.getElementById('thisCan'),
// Obtain a graphics context on the
// canvas element for drawing.
ctx = htmlCanvas.getContext('2d');
// Start listening to resize events and
// draw canvas.
initialize();
function initialize()
{
// Register an event listener to
// call the resizeCanvas() function each time
// the window is resized.
window.addEventListener('resize', resizeCanvas, false);
// Draw canvas border for the first time.
resizeCanvas();
}
// Display custom canvas.
// In this case it's a blue, 5 pixel border that
// resizes along with the browser window.
function redraw(width, height)
{
var start = width/2;
var margin = width * 0.01;
var height = (width / 3) - (margin * 4);
var end_step = width/4
ctx.beginPath();
ctx.moveTo(start - margin, 0);
ctx.bezierCurveTo((start - height/3), 0 , (start - height/3), height , end_step*1, height);
ctx.moveTo(start + margin, 0);
ctx.bezierCurveTo((start + height/3), 0 , (start + height/3), height , end_step*3, height);
ctx.moveTo(start - margin, 0);
ctx.bezierCurveTo((start - height*0.33), 0 , (start - height*0.16), height , end_step*1.5, height);
ctx.moveTo(start + margin, 0);
ctx.bezierCurveTo((start + height*0.33), 0 , (start + height*0.16), height , end_step*2.5, height);
ctx.moveTo(start, 0);
ctx.bezierCurveTo((start ), 0 , (start ), height , end_step*2, height);
ctx.stroke();
}
// Runs each time the DOM window resize event fires.
// Resets the canvas dimensions to match window,
// then draws the new borders accordingly.
function resizeCanvas()
{
var width = 0;
var height = 0;
var contentElement = document.getElementsByClassName("content-box post")[0]
width = htmlCanvas.width = (contentElement.clientWidth * 0.85)
height = htmlCanvas.height = (contentElement.offsetWidth*0.85 * 0.33);
redraw(width, height);
}
})();
</script>
and the output:
来源：https://stackoverflow.com/questions/35356343/html5-draw-gaussian-function-using-beziercurveto