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Odd Pixels when Scanning Pictures

Posted on 1998-01-13
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Last Modified: 2013-11-08
I purchased an Acer Prisa 310P (300x600 dpi, 30 bit, parallel) scanner. I am very happy with it, however when I scan images at 300 dpi (color photographs) I often get stray pixels of the wrong color. It is barely noticeable when you view 1:1, but when you zoom in, you can see that there is one pixel here and there that are way off, e.g.,
pink on white.

If I resample the image in Paint Shop Pro, most of the problem disappears, but I assume that this is due to the calculation of pixel color used in resampling.

I've cleaned the glass with a special cleaner for glass screens and filters, and a lint free cloth.

Is it possible that I have a setting wrong, or is it the hardware?
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Question by:nomad2
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by:stevespencer
ID: 1122911
If this is happening directly after you have taken the scan then I dont have an answer. However I suspect you are finding these odd pixels on images you have saved and re-loaded at some later time. If this is the case then I bet you are using jpeg for storing your files, if this is the case then it is the compression algorithum that is damaging your scans, not the scanner.

If this is close, I will give you some more info, 'else reject my answer and let somebody else have a go.

Stephen.
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by:nomad2
ID: 1122912
This happens immediately upon scanning. Once resampled, cleaned up and saved as JPEG, the images look fine.
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magigraf earned 100 total points
ID: 1122913
nomad2...

The behavior you just described could be the results of few things.  You do realize that when scanning the optical lens reads the images and digitize the colors.  Every scanner when scanning refer to some color table to process the colors.  Now due to some reflections from the picture (when it's lamintated not mat) this reflection confuses the lens while reading the colors, and results in these WHITE pixels.

When you resample the image (what we call also interpolation in size and resolution) the programs goes and read every pixels and resamples the closest color to it and create a new one.

This same behavior happens when you have a 1" X 1" image scanned at 300 DPI and you want to increase it to 2" X 2".  The correct settings would result in an final image resolution of 150 DPI.
(Size go UP, resolution go DOWN) Now if you force the resolution to stay at 300 DPI at 2" X 2", this is when the INTERPOLATION would take place.  Every neighbooring pixels will get sampled to the closest match and recreate a new one to fill the gap.

Now the question here would be:
1) Does that happens in every picture you scan??
2) Does it only happen randomly??

This will help me to give you even more details about that problem.

Regards
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Author Comment

by:nomad2
ID: 1122914
The problems appear on the "raw" scan, before any image manipulation. I tried to recreate the problem, and changed my video driver from 16 to 24 bit.

What I found was that the first scans that I did after the scanner warmed up (15 seconds) had pixels of the wrong color showing up throughout the image. Later scans were OK, regardless of the video driver settings or TWAIN driver settings.

Is it possible that the scanner requires more than the minimum warm up time to be able to correctly read full color images?

Also, for the first couple of scans, the "wrong" color pixels did not show up in the same place if the same picture was scanned twice without moving the picture on the scanner or changing any TWAIN driver settings.
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Expert Comment

by:magigraf
ID: 1122915
nomad2...

Indeed the warm up the lamp before scanning is always BETTER than the regular procedure.  These lamps when warmed would read better the color tables.  So it might help greatly in your case to use that feature.

We always use it for our scans to make sure that we have to best possible reading.

When you said that the pixels or the line change from one place to another is the proof that nothing is wrong with the scanner.  If something was wrong it would ALWAYS appear in the same place.

Hope this clarified more, but if you require more info, just ask
Regards
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