How to read a portion of an image using RMagick?

I'm trying to load a large image and crop it into tiles really fast and with low memory consumption. An example is the way gdal2tiles s开发者_如何学Pythoncript http://www.maptiler.org/ does it, but gdal2tiles doesn't output the tiles the way I want them to be. Therefore I decided to develop my own version using ruby and RMagick(Imagemagick).(gdal2tiles barely uses any memory).

What I notice when using RMgick is that I need to load the whole image to memory first which kinda kills my system. Is there a way to read a portion of a image file manipulate it and then read the rest ?? Or maybe I should do it in some other way or use other libraries.

First thing to do is call descructive methods only. Don't call img.crop but img.crop!, that will save some memory.

But you might be interested in one of the alternatives. The problem with memory you describe, is almost unavoidable with RMagick. There are quite a lot of alternatives for RMagick for this very reason.

Because you need simple tasks only, I think the following libraries are usefull for you, because they are extremely optimised, but their featureset is limited.

• MiniMagick
• Chunky PNG
• Oily PNG