画布drawImage失败 [英] canvas drawImage fails

问题描述

我是Canvas的新手，如果这很简单，请原谅。如果浏览器支持使用画布，我想在上传之前调整图像大小。但是，这段代码

I'm fairly new to Canvas so please excuse if this is to simple. I want to resize images prior to upload if the browser supports this using canvas. However, this code

var img = document.createElement("img");
var reader = new FileReader();  
    reader.onload = function(e) {img.src = e.target.result};
var files = event.target.files;
    reader.readAsDataURL(files[0]);
var canvas = document.createElement('canvas');
    canvas.width = 100;
    canvas.height = 100;
var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0);

失败

NS_ERROR_NOT_AVAILABLE：组件不可用
slice.call（docElem.childNodes，0）[0] .nodeType;

NS_ERROR_NOT_AVAILABLE: Component is not available slice.call( docElem.childNodes, 0 )[0].nodeType;

<在我的测试浏览器Firefox中。

in my test-browser Firefox. What could be wrong?

更新

很奇怪：如果我添加一个警报（画布）；作为最后一行的第三行（有时我会进行调试，我知道我可以并且应该使用console.log），该错误不会出现，但是，我的映像中仍然看不到任何东西...

Very strange: If I add a "alert(canvas);" as the 3rd last line (I do that sometimes for debugging, I know I could and should use console.log), the error does not appear, however, I still do not see anything from my image...

推荐答案

需要加载图像才能在画布上绘制：

image need to be loaded to be drawn on canvas :

var img = new Image()
img.onload = function () {
     ctx.drawImage(img, 0, 0)
}
img.src="image.png"

在您的示例中：

var img = document.createElement("img")
var reader = new FileReader()

var canvas = document.createElement('canvas')
canvas.width = 100
canvas.height = 100
var ctx = canvas.getContext("2d")

reader.onload = function(e) {
    var img = new Image()
    img.onload = function () {
        ctx.drawImage(img, 0, 0)
    }
    img.src = e.target.result
}

var files = event.target.files
reader.readAsDataURL(files[0])

这篇关于画布drawImage失败的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！