Question

I am developing a graphics application using Qt 4.5 and am putting images in the QPixmapCache, I wanted to optimise this so that if a user inserts an image which is already in the cache it will use that.

Right now each image has a unique id which helps optimises itself on paint events. However I realise that if I could calculate a hash of the image I could lookup the cache to see if it already exists and use that (it would help more for duplicate objects of course).

My problem is that if its a large QPixmap will a hash calculation of it slow things down or is there a quicker way?

Answer 1

A couple of comments on this:

1. If you’re going to be generating a hash/cache key of a pixmap, then you may want to skip the QPixmapCache and use QCache directly. This would eliminate some overhead of using QStrings as keys (unless you also want to use the file path to locate the items)

2. As of Qt4.4, QPixmap has a ‘hash’ value associated with it (see QPixmap::cacheKey() ). The documentation claims ‘Distinct QPixmap objects can only have the same cache key if they refer to the same contents.’ However, since Qt uses shared-data copying, this may only apply to copied pixmaps and not to two distinct pixmaps loaded from the same image. A bit of testing would tell you if it works, and if it does, it would let you easily get a hash value.

3. If you really want to do a good, fairly quick cache with removing duplications, you might want to look at your own data structure that sorts according to sizes, color depths, image types, and things such as that. Then you would only need to hash the actual image data after you find the same type of image with the same dimensions, bit-depths, etc. Of course, if your users generally open a lot of images with those things the same, it wouldn’t help at all.

4. Performance: Don’t forget about the benchmarking stuff Qt added in 4.5, which would let you compare your various hashing ideas and see which one runs the fastest. I haven’t checked it out yet, but it looks pretty neat.