Image comparison - How to read an image so you can compare pixels?

I am writing a program in C that compares two images taken by a camera to see if they are different. I have an idea of how to do this, by putting the images into 2 dimensional arrays and comparing the RGB value at corresponding array positions, however I cannot figure out how to get the image into an array.

I've found a ton of example of how to do this in C#, but that's not so helpful. The only bit of C code I've been able to find uses fread() to read the images but that doesn't seem to work.

I've also heard some suggestions that converting the images to greyscale is helpful in comparing the pixels. Agree? Disagree?

Thanks in advance.

The issues of how to read the data from the disk using fread() and whether you convert to grayscale are extremely minor in comparison to handling the image file format. What kind of images will you be processing (I mean file format: TIFF, JPEG, BMP, PNG, what?)

I have been working with jpegs but I'm open to changing if there's a better format to suit my needs.

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Originally Posted by dearbear

I have been working with jpegs but I'm open to changing if there's a better format to suit my needs.

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Reading a JPEG without the help of a library like libjpeg is extremely difficult. Without knowing more about your application and constraints, I would recommend converting the images to PPM format using some utility like ImageMagick. PPM is extremely easy to parse without the aid of specialized image libraries.

More information would be helpful, though. How big are the images, how many of them will you be processing, what are your disk space constraints, etc

Are you looking for bit identical, or just similar pictures? If you look for an identical match my guess is that you could create a checksum of the entire data section of the files and compare them.

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Originally Posted by Subsonics

Are you looking for bit identical, or just similar pictures? If you look for an identical match my guess is that you could create a checksum of the entire data section of the files and compare them.

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Might be worth a try, but for complex, compressed formats it's not necessarily a given that identical images will produce identical data blocks. It wouldn't be a reliable method.

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Originally Posted by brewbuck

Might be worth a try, but for complex, compressed formats it's not necessarily a given that identical images will produce identical data blocks. It wouldn't be a reliable method.

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Mm, yeah I guess not. It would definately not work for identical images with different compression ratios for example.

I can't tell you how big the images are because I don't have access right now to the webcam I've been using. This is a small scale project so there won't be that many images, maybe a dozen. If I use PPM, how do I read it into the array?

The two images are taken within seconds of each other, so maybe the checksum method would be more reliable in this case?

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Originally Posted by dearbear

The two images are taken within seconds of each other, so maybe the checksum method would be more reliable in this case?

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If they are two different images, not a original and a copy it won't work, it relies on them being bit by bit identical.

Thanks for introducing me to PPM. It looks to be very easy to read in. I plan to start experimenting with it tomorrow. Thanks again to both of you for all your help!

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Originally Posted by brewbuck

Reading a JPEG without the help of a library like libjpeg is extremely difficult.

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???
2 lines of code on Windows...

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Originally Posted by Alex31

2 lines of code on Windows...

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If you have to specify "Windows", then you have a library in mind ;)

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Originally Posted by Alex31

???
2 lines of code on Windows...

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Not everyone uses Windows.

I've been working with the PPM file format and it is awesome. It is so easy to read and I am so grateful to you, brewbuck, for telling me about it. Thanks to everyone else as well for your input.