PHP, manipulating very big image files without loading them into RAM

Question

I've been searching about this for a while but I didn't find what I wanted, so here is my problem:

using PHP, I want to create a very big image file,lets say 20000 gigapixels, then I want to add a small image to specific location on this big image. My computer doesn't have enough RAM to load up the entire image and manipulate pixels that way, so I think I need to access the image data on hard disk and manipulate them in some way, so anyone knows how to do this? thanks for helping me out :)

Answer 1

Just keep a simple file, not image, and store pixel data in it in any custom format. PHP has a fseek function, which allows you to jump to any location in the file, so you can calculate needed location & perform read/write on it. If you have image with size W x H, and if each pixel takes 3 bytes, then the address of pixel (X, Y) in the file will be (W * Y + X) * 3.

Answer 2

What would you do with an image that size? You couldn't serve it a browser, and even if you did manage to load it into the server, it would take up all the server resources, so you wouldn't be using the server for anything else in the meanwhile.

The short answer is that handling an image of that kind of scale as a single file in RAM is out of the question unless you've got an extremely powerful machine dedicated to it, and nothing else. At 20k x 20k pixels, even a simple monchrome image is going to take 400mb. Scale that up to any useful colour depth, and you're talking about gigabytes of RAM just to hold the graphic, and that's before we even start thinking about actually doing stuff with it.

I guess the solution is to look at what other people do, given the same problem.

Real applications that use images of that scale (eg mapping apps or panorama photos like this one) store their image as a series of much smaller blocks. Each block is a smaller image in its own right. They'd also usually have separate sets of blocks for each zoom level too. Handling a single massive image file is implausible for any realistic server environment, but smaller chunks make it easy to handle for both browser and server. The server just sends the blocks to the user that are in the current view; when the user scrolls or zooms, they get sent more blocks.

Your question mentions adding a smaller image to a specific location on the big one. Again, looking at how others do this, google maps and others handle this kind of thing using a layering system. The layers are built up and sent to the browser separately.

I know that doesn't directly answer the question, but I hope it gives you some options to think about.

Answer 3

ImageMagick supports operations on very large files. I don't see support in the PHP/ImageMagick API but you could call out (exec) to the command line program and use one of it's disk caching or streaming options.

There is some documentation for dealing with large files here: www.imagemagick.org.