CODEWITHSUNDEEP
c++qt
paulm
paulm

Reputation: 5892

Qt reading many images optimization - how to only read the size?

The title sums this one up. If I'm loading ~200 images of various size. How can I load just the header so I can know the size of each image?

Currently I find it takes a lot of cpu/memory and IO to load them all in to memory just for the size (I'm trying to generate an atlas from them).

QImage doesn't seem to have a way to do this. QImageReader sounded like it was what I wanted, yet this still seems to just go ahead and read the whole image, so not really sure what its purpose is. Is there another class or some way to use either of the class I've mentioned to only grab the image size from header?

Upvotes: 4

Views: 632

Answers (1)

Mantosh Kumar
Mantosh Kumar

Reputation: 5731

How can I load just the header so I can know the size of each image?

Apparently it looks like you have assumed that image file header(first few bytes of) contains the size of the image. This does not hold true(at least not for all image format type). I checked it for few of formats(PNG).

Currently I find it takes a lot of cpu/memory and IO to load them all in to memory just for the size

As you have mentioned that you are trying to load around ~200 image at one time just to find the size. This design does not looks good and we should try to decompose our problem into the smaller one. So here the efficient approach might be to open one file and find the size store into some data structure and close the file. If there is other part of your program which needs that ~200 image should be loaded into the memory then we should try to think on how can we avoid it.

QImage doesn't seem to have a way to do this?

It does not have as there seem to be no portable/consistent way to do it for all type of image format. However if you are aware about any file format which contains the header you may write small helper function which can open the file and read the header and find the size. But this helper function would be very specific to a particular type of image format and we may need to write different logic to read the header(all image formats have different header size and information).

Upvotes: 3

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