Changing image sizes of scanned images

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grandad
Posts: 13
Joined: Wed Jul 05, 2006 1:14 pm

Changing image sizes of scanned images

Post by grandad »

Help please,
I have several images that I obtained by scanning, the result is of course is they are too big how can I convert them to bytes from megabytes I apologise if this is covered somewhere I really am still learning if it is covered in help could you point me to it
thanks ever grateful
thanks always grateful
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xnview
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Posts: 37892
Joined: Mon Oct 13, 2003 7:31 am
Location: France

Re: changing image sizes

Post by xnview »

grandad wrote:Help please,
I have several images that I obtained by scanning, the result is of course is they are too big how can I convert them to bytes from megabytes I apologise if this is covered somewhere I really am still learning if it is covered in help could you point me to it
thanks ever grateful
You convert them in jpeg format, and changing output quality.
Pierre.
grandad
Posts: 13
Joined: Wed Jul 05, 2006 1:14 pm

image size

Post by grandad »

Thanks very much
thanks always grateful
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helmut
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Joined: Sun Oct 12, 2003 6:47 pm
Location: Frankfurt, Germany

Post by helmut »

It's difficult to give the ultimative tip how to reduce a scanned image's size. Below please find a list of ideas and options to optimize your scanned images.

Optimizing when scanning
Rather than trying to find a way to reduce the scanned image's file size, it might be a good idea to let the scanner do the job. Nowadays scanner software is smart and works well. Perhaps your scanner comes with a manual which might provide valuable tipps.

You shouldn't scan in a document which is black & white in colour scan mode. Also you may want to try out various scan resolutions. Low resolutions will result in smaller file size but image processing will be limited. Higher resolutions will allow for image processing after scanning, e.g. denoising might be more effective.

Some scanners allow for scanning in a resolution which is much higher than the physical scan resolution. The scans will not have more details in it, but file size will be larger. Try to find out the physical scan resolution your scanner supports and don't scan in in higher resolutions than that.

___

Below please find various ways to reduce the size of images. This list applies for scanned images, but also for any other images. Basically these approaches can be divided in two categories: "Lossless" and "Lossy".

Lossless (Image data remains unchanged)

A1 - Reducing image size (width & height)
An easy way to reduce file size is by reducing width & height of an image. In XnView this can be done using "Image > Resize...".

A2 - Saving in a graphic format that compresses the image data well
The image formats are not just different in their file headers, but also in the way image data is compressed. Some graphic formats like TIFF even support several compression algorithms.

Lossy (Image data is changed)

The following approaches will change the image data. I recommend to keep the original image file as backup for later use - in change you find a better way to optimize image quality vs. file size.

B1 - Saving in a graphic format that is lossy.
Apart from lossless graphic formats like PNG, there are lossy graphic formats. When saving an image, the compression algorithms will apply minor or major changes to the image so that the image compresses better. With the "JPG quality" the user can control the compression and the quality of the saved image.

B2 - De-noising images
When scanning image, often the scanned document has a noise on it which you can see when zooming the image very much. Each single pixel and colour information which is different to it's neighbour pixels is some sort of extra information which needs to be saved and which will blow up the image file. So it's a good.

B3 - Reducing colour depth
Reducing the colour depth (number of different colours used in document) is often forgotten as a means to reduce image size, although this might have big impact on image size. Especially when scanning in documents this should be tried out since the scans often have "noise" which blow up the image size.
A greyscale scan of a document for example can be easily reduced to 16 grey tones and quality will be still very reasonable. Depending on the document you scan this might apply for your colour scan, too. Reducing colour depth (=number of colour used) of photographs will not work well, whereas reducing colours of documents with only few colours in it will work very well.

B4 - Improving image quality using filters (e.g. denoise).
There are graphic filters for de-noising images. Best is a graphic program which allows to set options before de-noising (e.g. pixel size, ...). XnView has an filter in menu "Filter > Effects... > Reduce noise".


Related links
- How to prepare an image for the internet
- Common graphic formats
- What format should I save this image in?
- Lossy? Lossless? 256 colors? This is confusing!
- How to convert several images at once
grandad
Posts: 13
Joined: Wed Jul 05, 2006 1:14 pm

changing image sizes

Post by grandad »

Thanks
your reply is very much appreciated I certainly understand a lot more now
I hope I don't have to bother you again
thanks always grateful