How to estimate the size of JPEG image which will be scaled down

imageimage processingjpeg

For example, I have an 1024*768 JPEG image. I want to estimate the size of the image which will be scaled down to 800*600 or 640*480. Is there any algorithm to calculate the size without generating the scaled image?

I took a look in the resize dialog in Photoshop. The size they show is basically (width pixels * height pixels * bit/pixel) which shows a huge gap between the actual file size.

I have mobile image browser application which allow user to send image through email with options to scale down the image. We provide check boxes for the user to choose down-scale resolution with the estimate size. For large image (> 10MB), we have 3 down scale size to choose from. If we generate a cached image for each option, it may hurt the memory. We are trying to find the best solution which avoid memory consumption.

Best Answer

I have successfully estimated the scaled size based on the DQT - the quality factor.

I conducted some experiments and find out if we use the same quality factor as in the original JPEG image, the scaled image will have size roughly equal to (scale factor * scale factor) proportion of the original image size. The quality factor can be estimate based on the DQT defined in the every JPEG image. Algorithm has be defined to estimate the quality factor based on the standard quantization table shown in Annex K in JPEG spec.

Although other factors like color subsampling, different compression algorithm and the image itself will contribute to error, the estimation is pretty accurate.

P.S. By examining JPEGSnoop and it source code, it helps me a lot :-)

Cheers!

Related Solutions

Javascript – How to get image size (height & width) using JavaScript

You can programmatically get the image and check the dimensions using Javascript...

const img = new Image();
img.onload = function() {
  alert(this.width + 'x' + this.height);
}
img.src = 'http://www.google.com/intl/en_ALL/images/logo.gif';

This can be useful if the image is not a part of the markup.

Getting Image size of JPEG from its binary

According to the Syntax and structure section of the JPEG page on wikipedia, the width and height of the image don't seem to be stored in the image itself -- or, at least, not in a way that's quite easy to find.

Still, quoting from JPEG image compression FAQ, part 1/2 :

Subject: [22] How can my program extract image dimensions from a JPEG file?

The header of a JPEG file consists of a series of blocks, called "markers". The image height and width are stored in a marker of type SOFn (Start Of Frame, type N).
To find the SOFn you must skip over the preceding markers; you don't have to know what's in the other types of markers, just use their length words to skip over them.
The minimum logic needed is perhaps a page of C code.
(Some people have recommended just searching for the byte pair representing SOFn, without paying attention to the marker block structure. This is unsafe because a prior marker might contain the SOFn pattern, either by chance or because it contains a JPEG-compressed thumbnail image. If you don't follow the marker structure you will retrieve the thumbnail's size instead of the main image size.)
A profusely commented example in C can be found in rdjpgcom.c in the IJG distribution (see part 2, item 15).
Perl code can be found in wwwis, from http://www.tardis.ed.ac.uk/~ark/wwwis/.

(Ergh, that link seems broken...)

Here's a portion of C code that could help you, though : Decoding the width and height of a JPEG (JFIF) file

Best Answer

Related Solutions

Javascript – How to get image size (height & width) using JavaScript

Getting Image size of JPEG from its binary

Related Topic