Finding coordinates after canvas Rotation

问题:

0 to 0,-70 by this :

ctx.strokeStyle = "red";
ctx.lineWidth = 2;
ctx.rotate(Math.PI/-10;);
ctx.beginPath();
ctx.moveTo(0,0); 
ctx.lineTo(0,-70);
ctx.stroke();

And I can rotate that by 'PI/-10', and that works.

How i can get the x,y points of this after using rotate?

回答1:

Your x and y points will still be 0 and -70 as they are relative to the translation (rotation). It basically means you would need to "reverse engineer" the matrix to get the resulting position you see on the canvas.

If you want to calculate a line which goes 70 pixels at -10 degrees you can use simple trigonometry to calculate it yourself instead (which is easier than going sort of backwards in the matrix).

You can use a function like this that takes your context, the start position of the line (x, y) the length (in pixels) and angle (in degrees) of the line you want to draw. It draw the line and returns an object with x and y for the end point of that line:

function lineToAngle(ctx, x1, y1, length, angle) {

    angle *= Math.PI / 180;

    var x2 = x1 + length * Math.cos(angle),
        y2 = y1 + length * Math.sin(angle);

    ctx.moveTo(x1, y1);
    ctx.lineTo(x2, y2);
    ctx.stroke();

    return {x: x2, y: y2};
}

Then just call it as:

var pos = lineToAngle(ctx, 0, 0, 70, -10);

//show result of end point
console.log('x:', pos.x.toFixed(2), 'y:', pos.y.toFixed(2));

Result:

x: 68.94 y: -12.16

Or you can instead extend the canvas' context by doing this:

if (typeof CanvasRenderingContext2D !== 'undefined') {

    CanvasRenderingContext2D.prototype.lineToAngle = 
        function(x1, y1, length, angle) {

            angle *= Math.PI / 180;

            var x2 = x1 + length * Math.cos(angle),
                y2 = y1 + length * Math.sin(angle);

            this.moveTo(x1, y1);
            this.lineTo(x2, y2);

            return {x: x2, y: y2};
        }
}

And then use it directly on your context like this:

var pos = ctx.lineToAngle(100, 100, 70, -10);
ctx.stroke(); //we stroke separately to allow this being used in a path

console.log('x:', pos.x.toFixed(2), 'y:', pos.y.toFixed(2));

(0 degrees will point to the right).

回答2:

So you're asking "after I set a transform, how can I run points through that transform"?

In that case, see HTML5 Canvas get transform matrix? . The question and answers are somewhat old, but seem up-to-date. I can't find anything in the current HTML5 spec that lets you access and use the transform matrix. (I see that it's theoretically accessable through context.currentTransform, but I don't see any functionality to let you use the matrix - you'd have to multiply your point through the matrix yourself, or fake it by creating a full SVGMatrix for your point vector.)

The top answer shows a transform class you can use. Track your changes through that, and use their transformPoint function to get the point you want transformed to its endpoint.