Author Topic: Resizing large image files without losing image quality  (Read 38124 times)

Offline PrueM

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Resizing large image files without losing image quality
« on: Thursday 29 January 09 05:36 GMT (UK) »
Here are some very basic instructions for re-sizing scanned images to get them below the 500kb file size limit on Rootschat.  To resize scanned images, you will need some graphics/photo editing software.  A good free download is Irfanview:  http://www.irfanview.com/ (Instructions specific to IRFANVIEW are included below - please scroll down!)

Basically, the aim when posting an image for restoration or identification is to strike a balance between:
physical file size (the height and width of the picture as you see it on the screen),
digital file size (the amount of space the picture takes up on your computer, measured in megabytes - mb, or kilobytes - kb),
resolution of the image (measured in dots or pixels per inch - dpi or ppi), and
retaining colour information (i.e. scanning the image in full colour/RGB colour mode, NOT black and white, even if your photograph is black and white!)

Note:  These instructions are generic and every software package is different, but the general principles should be the same.  Some package-specific instructions are included where people have provided them.

1.  Scan your photos at 600dpi (dots per inch - may also be ppi: pixels per inch) and in full colour mode - even if it's a black and white picture.   Do a preview scan first, then select only the photo, or section of photo, that you want - make sure you select the photo so as not to include lots of 'white space' i.e. the scanner lid!   See "Cazza's Scanning Tutorial" on this board if you need more help with how to scan.

2.  Open the scan in your editing software.

3.  To resize a 600dpi scan, you want to change the actual dimensions of the photo (i.e. its physical size) rather than compressing it (e.g. changing the jpeg quality or reducing the dpi/ppi).  The first method will keep the original high resolution while reducing the file size (the kbs); the second method will reduce the resolution as well, which is what you don't want.  See pictures following this posting.

     Change the photo size a little at a time, and check the file size after each change, until you get it just under 500kb.

     Note for Microsoft Office Picture Manager:  Save the changes after resizing. If you don't, reducing the file size and/or the dimensions of the photo won't help as the numbers will reflect the original size.  You should do this using "Save as" so you don't alter the original scan.

4.  If you want to you could reduce the resolution to 300dpi, and then start resizing the photo - you might find you end up with a physically larger product for the same kbs, and it's still plenty for the restorers to work with.

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Offline PrueM

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Re: Resizing large image files without losing image quality
« Reply #1 on: Thursday 02 April 09 04:57 BST (UK) »
I've tried to get some images together that will show you what we mean when we ask for you to save images at a high dpi with no compression, and how to resize big images.

Instructions specific to IRFANVIEW are included below - please scroll down!

There are also some examples of what images look like when scanned/saved at certain dpi, and what restorers see when they enlarge the images on their screens in order to work on them.  Hopefully this will help get the message across about how important it is to scan at a high resolution and not reduce the resolution in order to reduce the file size!

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Offline PrueM

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Re: Resizing large image files without losing image quality
« Reply #2 on: Thursday 02 April 09 05:02 BST (UK) »
Here are some scans of a photo of my grandad Harry.  One was scanned at 600 dpi, the other at 75 dpi (the same effects would apply if you had resized an image to these dpi from a higher dpi).

The first set of images shows the photos side by side - even resized to 50% of their original dimensions, you can see a real difference in quality between the two images.

The second set shows the images enlarged to 400% of the original dimensions - this is the kind of view a restorer would get when working to clean up a photo.  You can see from the lack of information and big blocks of pixels on the image at right (the 75 dpi image) how hard it would be to get a good result from this file. 

Offline PrueM

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Re: Resizing large image files without losing image quality
« Reply #3 on: Monday 19 April 10 04:28 BST (UK) »
Here is another example...sometimes the restorers will ask  you to save an image "without JPG (or jpeg) compression".   Most imaging/digitising/photoshopping programs come with an option to save an image with a variety of "levels" of jpeg quality.  The higher the quality, the better the image will end up (and the larger the file size). 

The screen shot below is taken from Photoshop Elements for Windows, and shows part of the "Save For Web" dialogue box.  Your software may have a different name for this, but the concept of changing the jpeg quality should be the same.


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Re: Resizing large image files without losing image quality
« Reply #4 on: Monday 24 January 11 22:35 GMT (UK) »
Here are some instructions for resizing using Irfanview software, thanks to jim1  :)

  • Open image in Irfanview. 
  • Select "image" menu and in the dropdown list select "resize/resample".  You will get a box that looks like the one below.
  • Put a tick in "preserve aspect ratio"
  • Put a bullet in "set new size" and put in the dimensions you want (you only have to reduce the width or height as the other will change automatically). 
  • At the bottom of the main window you will see the original size the file was (in this case 8.81Mb) and when you click OK you will see the new size.  In this case the new size was 1.22Mb - keep resizing until you get the file size just below 500Kb.