上传前调整图像大小 - 将画布转换为文件对象

How would you write (reasonably) cross browser function for most modern browsers to resize the File before upload, handling jpg,png and gifs with transparency:

/** 
 * @param {File} file 
 * @param int max_width
 * @param int max_height
 * @param float compression_ratio
 * @returns File
 */
function resize(file, max_width, max_height, compression_ratio){}

推荐答案

试试这样的:

function resize(file, max_width, max_height, compression_ratio, imageEncoding){
    var fileLoader = new FileReader(),
    canvas = document.createElement('canvas'),
    context = null,
    imageObj = new Image(),
    blob = null;            

    //create a hidden canvas object we can use to create the new resized image data
    canvas.id     = "hiddenCanvas";
    canvas.width  = max_width;
    canvas.height = max_height;
    canvas.style.visibility   = "hidden";   
    document.body.appendChild(canvas);  

    //get the context to use 
    context = canvas.getContext('2d');  

    // check for an image then
    //trigger the file loader to get the data from the image         
    if (file.type.match('image.*')) {
        fileLoader.readAsDataURL(file);
    } else {
        alert('File is not an image');
    }

    // setup the file loader onload function
    // once the file loader has the data it passes it to the 
    // image object which, once the image has loaded, 
    // triggers the images onload function
    fileLoader.onload = function() {
        var data = this.result; 
        imageObj.src = data;
    };

    fileLoader.onabort = function() {
        alert("The upload was aborted.");
    };

    fileLoader.onerror = function() {
        alert("An error occured while reading the file.");
    };  


    // set up the images onload function which clears the hidden canvas context, 
    // draws the new image then gets the blob data from it
    imageObj.onload = function() {  

        // Check for empty images
        if(this.width == 0 || this.height == 0){
            alert('Image is empty');
        } else {                

            context.clearRect(0,0,max_width,max_height);
            context.drawImage(imageObj, 0, 0, this.width, this.height, 0, 0, max_width, max_height);


            //dataURItoBlob function available here:
            // http://stackoverflow.com/questions/12168909/blob-from-dataurl
            // add ')' at the end of this function SO dont allow to update it without a 6 character edit
            blob = dataURItoBlob(canvas.toDataURL(imageEncoding));

            //pass this blob to your upload function
            upload(blob);
        }       
    };

    imageObj.onabort = function() {
        alert("Image load was aborted.");
    };

    imageObj.onerror = function() {
        alert("An error occured while loading image.");
    };

}

请注意:

使用文件加载器和加载图像意味着存在一些延迟，因此该函数是异步的，因此试图简单地返回 blob 数据是行不通的.您需要等待加载发生，然后才能使用加载的数据并为每个文件触发对上传函数的调用.