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brought to you by the letters R, G and B...

Posted Apr 16, 2008 21:49 UTC (Wed) by roelofs (subscriber, #2599)
In reply to: What If I Don't Actually Like My Users? by jzbiciak
Parent article: What If I Don't Actually Like My Users?

Erm... how often do you have 2 billion elements in a single array?

    unsigned char *pix = (unsigned char *)malloc(32768*32768*3*sizeof(unsigned char));

Not even a tiny bit far-fetched.

Greg

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brought to you by the letters R, G and B...

Posted Apr 16, 2008 21:55 UTC (Wed) by jzbiciak (✭ supporter ✭, #5246) [Link]

Hmm... GIMP at least tiles that and uses a higher order tile structure.  Are there really
programs that actually allocate that in a single 3GB malloc()?

brought to you by the letters R, G and B...

Posted Apr 16, 2008 22:19 UTC (Wed) by roelofs (subscriber, #2599) [Link]

XV used to try. It included multiple image-format decoders that checked the height and/or width separately but either failed to check the product of the two or else did so but subsequently failed to check the product of the result with the byte-depth. Or they did so incorrectly--i.e., failing only for negative products, not realizing that h*w*d can easily wrap back into the positive range. You really want to use this pattern or its equivalent:

  // int, long, or other signed type:  w, h, npixels, bufsize
  npixels = w * h;
  bufsize = 3 * npixels;
  if (w <= 0 || h <= 0 || npixels/w != h || bufsize/3 != npixels) {
    FAIL();
  }
  buf = malloc(bufsize*sizeof(whatever_buf_is_made_of));

But realistically, you're right--you absolutely don't want to use a program that tries to allocate the whole thing simultaneously, regardless of whether it's in one piece or many. And you may want to avoid certain image formats for the same reason--tiled TIFF, for example, is well suited to very large images, but many (most?) other image formats are not. PNG, for all its simplicity (well, relative to TIFF, anyway), basically requires you to decode at least two full rows simultaneously, and rows can be up to 16 GB each (2^31 - 1 pixels wide * 8 bytes deep for 64-bit RGBA). Of course, 2G x 2G x 64-bit images are still in the realm of fantasy, AFAIK...

Greg