javascript - HTML5 Canvas - 如何在图像背景上画一条线？

Question

我想在图像背景上画一条线 - 在 HTML5 Canvas 中。然而，这条线总是画在图像后面。实际上先画线然后画图——不管我如何调用函数。

如何将线条置于图像顶部？

var canvas = document.getElementById("myCanvas");
        var context = canvas.getContext("2d");
            context.clearRect(0, 0, canvas.width, canvas.height);
drawbackground(canvas, context); 
drawlines(canvas, context); 


function drawbackground(canvas, context){

    var imagePaper = new Image();


        imagePaper.onload = function(){


            context.drawImage(imagePaper,100, 20, 500,500);
        };

      imagePaper.src = "images/main_timerand3papers.png";
}



function drawlines(canvas, context){


            context.beginPath();
            context.moveTo(188, 130);
            context.bezierCurveTo(140, 10, 388, 10, 388, 170);
            context.lineWidth = 10;
            // line color
            context.strokeStyle = "black";
            context.stroke();
}

Answer 1

完全未经测试的代码，但您尝试过这样的代码吗？

function drawbackground(canvas, context, onload){

    var imagePaper = new Image();


        imagePaper.onload = function(){


            context.drawImage(imagePaper,100, 20, 500,500);
            onload(canvas, context);
        };

      imagePaper.src = "images/main_timerand3papers.png";
}

然后像这样调用方法...

 drawbackground(canvas, context, drawlines);