Change file size of image

jdhackett
jdhackett used Ask the Experts™
on
Hi
I wish to shrink the file size of jpg images, using VB.Net 2015.
So the user would select an image, then a smaller image would be saved. These images are usually photographs of 2Mb, can be up to 8Mb.
The smaller photograph:
           
  • should be 200k or less.
  •             The width/height ratio remain the same.
  •             Save to the same folder, but with a new name

I've seen some code which resizes by a percentage. However I want to make sure the size is always under 200k, no matter the size of the original.
What would be the best way to do this in VB.Net?
Thanks
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AndyAinscowFreelance programmer / Consultant

Commented:
>>I've seen some code which resizes by a percentage.
Use that code and keep reducing the percentage until the file size is what you desire.

Logic.
percentreduction = 90   90 % of original is target
while filesize > 200000
  percentreduction = percentreduction - 10
  reducefile percentreduction
end while

Author

Commented:
Not a bad idea Andy, but won't it be slow to save the file multiple times and then get the file size?
I was hoping there was a setting within the vb.net image control that lets you specify the file size.
Freelance programmer / Consultant
Commented:
>> setting within the vb.net image control that lets you specify the file size.

Sorry, but there isn't because one doesn't know in advance how much a jpg image will compress.  You can improve that algorithm by making guesses instead of just decrementing the percentage by a regular amount.
When reducing JPEG files there are three settings: size, resolution and compression quality.  ALl will affect file size.

On a side note, when reducing an image, it helps to sharpen the image after the reduction. Unfortunately, I found the .NET Image library didn't do a good job. Also, for some reason I do not recall, the results were better when the image was rotated 180 degrees. Naturally, I would perform that step twice.

I used the excellent AForge.Imaging library for sharpening. Almost the entire library is available under LGPL v3 license.

Good luck!

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