Getting Image Dimensions using Javascript File API

I require to generate a thumbnail of an image in my Web Application. I make use of the Html 5 File API to generate the thumbnail.

I made use of the examples from the below URL to generate the thumbnails.

http://www.html5rocks.com/en/tutorials/file/dndfiles/

I am successfully able to generate the thumbnails. The problem that I have is I am able to generate thumbnail only by using a static size. Is there a way to get the file dimensions from the selected file and then create the Image object?

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最佳答案

Yes, read the file as a data URL and pass that data URL to the src of an Image: http://jsfiddle.net/pimvdb/eD2Ez/2/.

var fr = new FileReader;


fr.onload = function() { // file is loaded
var img = new Image;


img.onload = function() {
alert(img.width); // image is loaded; sizes are available
};


img.src = fr.result; // is the data URL because called with readAsDataURL
};


fr.readAsDataURL(this.files[0]); // I'm using a <input type="file"> for demonstrating
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Or use an object URL: http://jsfiddle.net/8C4UB/

var url = URL.createObjectURL(this.files[0]);
var img = new Image;


img.onload = function() {
alert(img.width);
URL.revokeObjectURL(img.src);
};


img.src = url;
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I have wrapped pimvdb answer in a function for general purpose in my project:

function checkImageSize(image, minW, minH, maxW, maxH, cbOK, cbKO){
//check whether browser fully supports all File API
if (window.File && window.FileReader && window.FileList && window.Blob) {
var fr = new FileReader;
fr.onload = function() { // file is loaded
var img = new Image;
img.onload = function() { // image is loaded; sizes are available
if(img.width < minW || img.height < minH || img.width > maxW || img.height > maxH){
cbKO();
}else{
cbOK();
}
};
img.src = fr.result; // is the data URL because called with readAsDataURL
};
fr.readAsDataURL(image.files[0]);
}else{
alert("Please upgrade your browser, because your current browser lacks some new features we need!");
}
}
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The existing answers helped me a lot. However, the odd order of events due to the img.onload event made things a little messy for me. So I adjusted the existing solutions and combined them with a promise based approach. Maybe this helps someone else.

Here is a function returning a promise with the dimensions as an object:

const getHeightAndWidthFromDataUrl = dataURL => new Promise(resolve => {
const img = new Image()
img.onload = () => {
resolve({
height: img.height,
width: img.width
})
}
img.src = dataURL
})

Here is how you could use it with an async function:

// Get a file from an input field
const file = document.querySelector('[type="file"]').files[0]


// Get the data URL of the image as a string
const fileAsDataURL = window.URL.createObjectURL(file)


// Get dimensions
const someFunction = async () => {
const dimensions = await getHeightAndWidthFromDataUrl(fileAsDataURL)
// Do something with dimensions ...
}

And here is how you could use it using the then() syntax:

// Get a file from an input field
const file = document.querySelector('[type="file"]').files[0]


// Get the data URL of the image as a string
const fileAsDataURL = window.URL.createObjectURL(file)


// Get the dimensions
getHeightAndWidthFromDataUrl(fileAsDataURL).then(dimensions => {
// Do something with dimensions
})

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