[英]HTML5 canvas - drawImage not always drawing the image
每次用户按下按钮时,我都会在画布上绘图,但有时图像没有绘制在画布上。 我认为这可能是在 context.drawimage 函数运行之前没有及时加载图像,因为有时会绘制一些较小的文件。 我已经使用了控制台并检查了资源,所以这是我能想到的唯一问题。
我如何避免这个问题?
这是我的 Javascript 代码。
var canvas = document.getElementById("myCanvas");
var context = canvas.getContext("2d");
var questionbg = new Image();
var answerbg = new Image();
//this code is inside a function that is called each time a user presses a button
if(questiontype == "text"){
questionbg.src = "./resources/textquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
//if image question
else if(questiontype == "image"){
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
//if audio question
else if(questiontype == "audio"){
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
//else it is a video question
else{
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
您应该检查图像是否已加载。 如果没有,则侦听加载事件。
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
if (questionbg.complete) {
context.drawImage(questionbg, 0, 0);
} else {
questionbg.onload = function () {
context.drawImage(questionbg, 0, 0);
};
}
function draw() { var ctx = document.getElementById('canvas').getContext('2d'); var img = new Image(); img.onload = function(){ ctx.drawImage(img,0,0); ctx.beginPath(); ctx.moveTo(30,96); ctx.lineTo(70,66); ctx.lineTo(103,76); ctx.lineTo(170,15); ctx.stroke(); }; img.src = '/files/4531/backdrop.png'; }
显然,您不想应用描边或填充。 然而,想法是一样的。
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