Generate image checksum

[bunjee]

Here is what I'm doing to calculate an Image checksum :

quint16 ZePicturePool::generateChecksum(const QPixmap & pixmap)
{
	QImage image = pixmap.toImage();
	quint16 checksum = qChecksum((char *) (image.bits()), image.numBytes());
 
	return checksum;
}

quint16 ZePicturePool::generateChecksum(const QPixmap & pixmap)
{
	QImage image = pixmap.toImage();
	quint16 checksum = qChecksum((char *) (image.bits()), image.numBytes());
	
	return checksum;
}

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Is it the good approach ?

[jacek]

What do you need that checksum for?

[bunjee]

To check wether a Pixmap is already in my cache or not.

[jacek]

How many pixmaps are you going to operate on? Also take a look at QPixmapCache.

[bunjee]

I just want to validate if my checksum calculation is good .

And yes I'm using pixmap cache, in my case pixmap's path is not enough to define a pixmap.

[jacek]

I just want to validate if my checksum calculation is good .

It's OK, if you won't have a lot of pixmaps.

[bunjee]

Unfortunately I get some glitches using that approach.

Since I'm using a huge number of pixmap, sometimes it picks the same one.

How can I refine this ?

[spud]

How about a combination of file path and creation date?

[bunjee]

Not good in my case I want to rely on QImage's data and that's all.

I've seen there is a 32bit checksum in Zlib, has anyone ever used that ? I don't want to add one dependency just for a checksum.

[jacek]

Since I'm using a huge number of pixmap, sometimes it picks the same one.

How can I refine this ?

You need a longer checksum, like MD5 hash. Of course it will take more time to calculate it and still there is some probability of a collision.

Maybe there is some other way to assign an unique identifier to those pixmaps? Where do they come from?

[bunjee]

Well I'm developping an instant messenging app.

Basically this is all the contact avatars.

BUT cached and displayed in various size, so I can have the same base pixmap but resized AND cached.

I think I might use a combination of checksum and pixmap size to match them.

[jacek]

Basically this is all the contact avatars.

What is the maximum size of such pixmap?

BUT cached and displayed in various size, so I can have the same base pixmap but resized AND cached.

Is the checksum valid only during a single session or you want to keep it after user closes the application? Is it important for you that two exactly the same pixmaps that come from two different sources have the same checksum?

[bunjee]

What is the maximum size of such pixmap?

No maximum size.

Is the checksum valid only during a single session or you want to keep it after user closes the application?

Valid during a given session.

Is it important for you that two exactly the same pixmaps that come from two different sources have the same checksum?

Yes it is, it speeds up the display that I don't regenerate the pixmap.