How to determine resolution on an image without using tag or property info?

Hello experts,

We do some process on images that customers send us on a regular basis. They scan the documents with their own scanners and software. Our only requirements are that the images must be scanned with minimum of 200 DPI.
We are now dealing with a new customer, the TIFF images we received from him are 100 DPI. I just spoke with him and he told me that their scanning software has an option to change the image resolution, he change it to 200, the image size has increased but the tag still remains on 100 DPI. Also when I right click the image and select properties, the resolution remains 100.

My question is:
We are developing in .Net, is there a way to know the true resolution without looking the image tag or image property? The solution we are looking for is either by .net programming or a utility that will show us the true resolution.

I tried to use irfanview to check the resolution but it shows the same info that I wrote above, just like property window of the image (Shows only the the resolution written in the tag)

Sorry for the long story, just wanted to avoid misunderstanding :)

Thank you in advance
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ste5anSenior DeveloperCommented:
No, there is no way. The DPI resultion used for scanning is a hardware setting/issue. Either it is correctly embedded in the image properties or not. The only way to determine it from the raw data is also scanning an test image like WG-18:

WG-18 resolution test image
But the problem is: you get only information about the resulting resolution after the entire process, not about the physical resolution.
oferzAuthor Commented:

Thank you ste5an for the response.
Sorry for asking again, but I didn't understand your answer regarding the test image.
For example, I received an image from a customer, when I right click the file and choose properties, the resolution property is 100 DPI. The customer told me he changed the scanner software to scan the document with 200 DPI.

How can verify the true DPI?

Thanks again
ste5anSenior DeveloperCommented:
You can only determine it, when you know the original source image. Otherwise it's not possible.

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oferzAuthor Commented:
Thank you
Joe Winograd, Fellow&MVEDeveloperCommented:
Hi oferz,
I see that you already closed the question, but I would offer a different conclusion. You said that when you right-click the image and select Properties (and I assume the Details tab after that), the resolution is 100 (I assume you mean both horizontal and vertical resolution — the dialog box shows both — and it also shows the width and height in pixels). That information should be correct. You then asked for "a utility that will show us the true resolution." It would not be difficult to create a command line executable (a stand-alone EXE file) that you could call from your .NET program to retrieve those values. Regards, Joe
oferzAuthor Commented:
Hello Joe,

I can receive ths info via .Net, that is not my problem. My problem is to verify that the info I receive is true.
Anyway, Stan answered a resonable answer so the question is indeed closed.

Thank you anyway
Joe Winograd, Fellow&MVEDeveloperCommented:
I'm not concerned that the question is closed (this is not about getting points). I'd like to understand what's going on, as I do a lot of work with TIFF files in my document imaging projects. I think the situation is that you're receiving a TIFF file and using a .NET program to look at the Horizontal and Vertical Resolutions (which are the same values that you would see via Properties>Details). If the TIFF file says that the Horizontal and Vertical Resolutions are 100 dpi, then that's what they are. I suspect that the customer did not change the resolution to 200 dpi when scanning. Phrased another way, I don't know how the resolution could have been changed to 200 dpi in the scanning software and yet the TIFF file produced shows 100 dpi. I do a lot of scanning to PDF and TIFF files and don't recollect ever seeing that. If it really is happening in this case, I'd like to understand why. Regards, Joe
oferzAuthor Commented:
Hi Joe,

The customer showed me today that he is changing the resolution option in the scanning software to 200, but yet, the Horizontal and Verical Resolution remain 100.
The file size is almost 3 times larger, so that for me indicates it is a bug in the software.
This is the situation I asked on the first post, how can I know if the bug is only in the Tiff info although the resolution seems OK?

Thanks a lot
ste5anSenior DeveloperCommented:
Well, a more simple explanation: 100 DPI means that 100 dots are generated by scanning an inch. Now you take your image. Do you know its original width in inch? You don't. Thus you have to rely on the information in the TIFF. So you can take the data and print 100 dots per inch. Then you have an image or the original size. When this information is wrong as in your case and you print 100 DPI inch whilst it was scanned with 200 DPI, you will get an image which is larger then the original.

A image of the size 1000x1000 pixels printed with 100 DPI is an 10in x 10in image on paper. Printed with 200 DPI you will only get a print of 5in x 5in.
So you can only know, whether the DPI information is wrong, when you know the original size of the image which is scanned.

And as long as the DPI value is incorrect, you cannot trust any size information in the image, as long as it is also generated by the scanning process.
Joe Winograd, Fellow&MVEDeveloperCommented:
Interesting that the DPI is wrong in the TIFF file. Don't recollect ever seeing that. To learn more about DPI, pixels, etc., I recommend Wayne Fulton's excellent site, "A few scanning tips":

In this case, specifically the sections Pixels, Printers, Video - What's With That? and What is a digital image anyway?. Regards, Joe
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