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    <p>The simplest solution to your problem would be to use 64-bit address spaces - modern Macs support this out of the box, on Windows and Linux you will need to install the 64-bit version of the OS. I believe Qt can be used to build 64-bit apps quite nicely. 32-bit systems won't be able to support single allocations of the size you're talking about - even a Mac with 4 GB of address space available to applications won't be able to make a single 3.7 GB allocation as there will not be a contiguous space of that size available.</p> <p>For undo I would look at using memory-mapped files and copy-on-write to copy the block:</p> <p><a href="http://en.wikipedia.org/wiki/Copy-on-write" rel="noreferrer">http://en.wikipedia.org/wiki/Copy-on-write</a></p> <p>This means you don't actually have to copy all the original data, the system will make copies of pages as they are written to. This will greatly aid performance if your images are significantly bigger than real memory and you're not changing every part of the image. It looks like <a href="http://www.boost.org/doc/libs/1_44_0/libs/iostreams/doc/classes/mapped_file.html" rel="noreferrer">boost::map_file</a> with "private" access might be helpful for this.</p> <p>If you really, really need to support 32-bit systems, your only alternative is to break those big blocks down somehow, typically into planes or sub-volumes. Both are horrible to work with when it comes to applying 3D filters etc. though, so I really would avoid this if you can. </p> <p>If you do go the sub-volume route, one trick is to save all the sub-volumes in memory-mapped files, and map them into your address space only when you need them. When unmapped from the address space they should stay around in the unified buffer cache until purged, effectively this means you can utilise more RAM than you have address space (particularly on Windows where 32-bit applications only get 2 GB of address space by default).</p> <p>Finally, on 32-bit Windows you can also look at the /3GB switch in boot.ini. This allows you to allocate 3 GB of address space to applications rather than the normal 2 GB. From the problem you describe I don't think this will give you enough address space, however it may help you with some smaller volumes. Note that the /3GB switch can cause problems with some drivers as it reduces the amount of address space available to the kernel.</p>
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