相关疑难解决方法(0)

我在HMTL 5 2D画布上绘制了5条水平线：

var canvas_ctx = my_canvas.getContext("2d");
    canvas_ctx.lineWidth = 0.5;
    canvas_ctx.strokeStyle = "black";

    {
        let line_x = 0;
        let line_length = canvas_ctx.width;
        let offset = 5;
        let numLines = 5;
        let numYincrement = 10;
        for (let i=0;i<numLines * numYincrement;i+=numYincrement) {
            //canvas_ctx.beginPath();
            canvas_ctx.moveTo(line_x,i + offset);
            canvas_ctx.lineTo(line_length,i + offset);
            canvas_ctx.stroke();
            //canvas_ctx.closePath();
        }
    }

理想情况下，这应导致出现5条黑线。取而代之的是，线条的颜色似乎随着每条新线条而逐渐褪色（好像是渐变！），因此第5行是灰色。如果我取消注释canvas_ctx.beginPath();和canvas_ctx.closePath();，则所有行变为灰色。为什么会这样？