Image reading and writing line by line

[Astrologer]

Hi there. Would you please, guys help me out? I have got an image and sometimes a huge one. In order to avoid memory problems I would like to read an image line by line and after having processed each line I would want to write it to an output image.
The problem is I need to use QImage::scaled() function (or similar which can provide efficient scaling procedure which I need use) that suits me fine.
First guess is to divide input image into number of QImages (let's say, number of lines of input image) and accomplish scaling procedure on each of them. But how will I write all of them into single output image? There may be another solution or more suitable Q-class I'm not aware of. Anyway you're welcome to give any input on this. Thank you so much in advance.