Calculate the dimensions of an image by its filesize

Using javascript is there any method to calculate the dimensions of an image just from knowing its filesize?

I am assuming that differen't formats (开发者_如何学PythonJPEG/PNG) require a different calculation but I am only looking to use JPEG or PNG.

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No, there's no way to do that reliably. The size of the compressed image depends a lot on the actual nature of an image. A 1600x1200 white rectangle compresses to almost nothing, while a color photo of a circus would be much much larger.

Now, image formats may contain easy-to-read headers (or trailers or whatever) that tell you the image dimensions directly. (I suspect in fact that virtually all image formats have such metadata, because it seems like a practical necessity, but I'm not an expert so I'll hedge :-)

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Not really. Because you can have a very high quality Jpg with x dimensions but a low quality with the same dimensions. The filesize is going to vary dramatically.

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Based on the file type, and also the nature of the images that you will be dealing with, you could calculate an approximate image size from an average bits/pixel value.

However, how good the approximation is depends on how varying the images are. JPEG images can be saved with different compression rate, so the file sizes can vary a lot.