how to open some region from a compressed image file not loading full image file into memory?

Question

We had an image loaded into open CV from some compressed JPG or PNG file. It was really big. We had some ROI on it say from (2000:2000) to (2100:2100) (square 100x100). We closed an image. Could we any how open that ROI again not loading full image file into memory? If yes what shall we do for it? Any metadata to save?

Answer 1

If you are working with a bitmap or other format that stores bits in a regular pattern, then yes you can do this by decoding the file yourself and only reading the offsets that you need.

Bitmap file format

Extracting part of a PNG image

I am not sure, but I think with jpeg, that is not possible since the image is stored as frequency domain information meaning you would need all of the data to produce any one part of the image in the original detail.

Or as I suggested in the comments, you could just save the ROI when you have it in memory if you have a limited number of ROIs and you know which ones you will need later (or even if you don't know which ones if there are not so many).

Which of those sounds like a good approach for you? I did the bmp reading at some point a long time ago, but don't have the code on hand. I could dig up the opencv code if you need that for saving ROI. It's pretty easy though.

Answer 2

We had the same problem and ended up using GDAL (http://www.gdal.org) for loading images and then converting them to IplImage.