相关疑难解决方法(0)

我试图使用适当的x = cos(theta),y = sin(theta)函数绘制圆的下半部分.如果我将theta从Math.PI迭代到2*Math.PI,我似乎得到了圆圈的上半部分:

我在这段代码中做错了什么:

    window.onload = function() 
    {
        var canvas = document.getElementById('circle-canvas');

        if (canvas && canvas.getContext) {
            var context = canvas.getContext('2d');
            if (context) {
                context.strokeStyle = "#369";
                context.lineWidth = 4;


                j = canvas.width / 2;
                k = canvas.height / 2;
                r = canvas.width / 4; 

                function computeX(theta, r, j, k){ return r * Math.cos(theta) + j; }
                function computeY(theta, r, j, k){ return r * Math.sin(theta) + k; }

                start = Math.PI;
                context.lineTo(computeX(start, r, j, k), computeY(start, r, …