在画布上调整照片大小而不丢失纵横比 [英] Resizing photo on a canvas without losing the aspect ratio

问题描述

大家好，感谢您提前提供的任何帮助.我正在编写一个 phonegap 应用程序，并且无法在不丢失外观或错误裁剪图像的情况下缩小照片.在下面的函数中，imageData"是相机拍摄的照片的 64 位字符串.我可以在我的页面上将图像绘制到画布上，但如果图片是横向拍摄的，那么它会被粉碎在一起.如果我不使用 scale 函数或 drawimage 函数对其进行缩放，那么我只会得到照片的顶角.一个例子:我拍了一张照片，onPhotoDataSuccess 函数中的图像宽度显示它是 1296px 宽和 968px 高.但我在 iPhone 上的画布是 272px 宽(纵向模式).我在网上尝试了许多不同的缩放方法，这似乎不是最接近的.我做错了什么?

Hi all thanks for any help in advance. I am writing a phonegap app and can not get the photo to shrink without either losing the aspect or cropping the image by mistake. In the function below "imageData" is a 64bit string of the photo that the camera took. I can draw the image onto the canvas on my page, but if the picture was taken in landscape then it is smashed together. If i do not scale it either by using the scale function or the drawimage function then i get only the top corner of the photo. A example: i took a photo the image width in the onPhotoDataSuccess function shows it to be 1296px wide, and 968px height. But my canvas on the iPhone is 272px wide (portrait mode). I have tried many different scaling methods on the web and this appears to be the closest by not quite. What am i doing wrong?

 function onPhotoDataSuccess(imageData) 
{
var myImage = new Image();
var thecanvas = document.getElementById("queImg");
var ctx = thecanvas.getContext("2d"); 
myImage.onload = function() 
  { 
    thecanvas.setAttribute('width', '100%');
    thecanvas.setAttribute('height', 'auto');
    ctx.scale((myImage.width * 0.15), (myImage.height * 0.15));  
    ctx.drawImage(myImage, 0, 0);   
    }     

    myImage.src = "data:image/jpeg;base64," + imageData;      
}

推荐答案

drawImage 内置了缩放功能.

Scaling is built in to drawImage.

在处理画布时不要使用 setAttribute，而是调用 canvas.width = 272，并且只能处理整个像素值.

Don't use setAttribute when dealing with canvas, call canvas.width = 272 instead, and only with whole pixel values.

这里有一些工作代码可以帮助您入门:

Here is some working code to get you started:

 function onPhotoDataSuccess(imageData)
{
var myImage = new Image();
var thecanvas = document.getElementById("queImg");
var ctx = thecanvas.getContext("2d");

    myImage.onload = function() {
    ctx.drawImage(myImage, 0, 0, myImage.width * 0.15, myImage.height * 0.15);
    }     

    myImage.src = "http://placekitten.com/1296/968";
}


onPhotoDataSuccess(null);

活生生的例子:

http://jsfiddle.net/QBTrg/6/

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