I have an approximately 7 gigabyte .ppm image, which I wish to open with GIMP. My computer (Windows 8) has 16GB of RAM, so I figured that GIMP should be able to open it fairly easily, as I have set the tile cache size to 12GB. However, upon opening the file, it will load to 100% fine, but then suddenly spike the RAM usage from about 8GB to maximum. GIMP will then show as not responding, and I have to force quit the application. Why would GIMP suddenly require gigabytes of extra memory after the image has been loaded, and is there a workaround/way to allow my image to load? (If there is a totally logical reason why it requires more memory, fair enough, I just don't have enough to load it, but I don't see why it does...)
I am not a GIMP developer, but I would guess it has to do with the conversion of formats. At least some of the original file has to be in memory at the same time as the new XCF-format image that GIMP is building in RAM. If GIMP is loading the entire PPM and then converting it all at once, RAM would spike.
You've probably already figured out a solution, but if not, I recommend using ImageMagick to convert the PPM to a more space-efficient format (e.g., PNG) before loading it into GIMP. ImageMagick can also break up large files into pieces you can load individually.