Question

I’m looking to create an ID system for cataloging images. I can’t use md5() since that will change if I alter the EXIF tags of the image.

I am currently using the SHA1 checksum computed by imagemagick. It works perfectly, but it’s really, really slow on larger images (~15 seconds on a quad-core xeon for a 21 megapixel JPG).

Are there any other “visual” methods of uniquely identifying an image that would be faster?

Answer 1

You could try running MD5 on the actual bitmap data instead of the JPEG file. I tested on my machine (also a quad core Xeon) and the following runs in about 900ms on a 23 megapixel image.

uint32_t width  = MagickGetImageWidth(imageWand);
uint32_t height = MagickGetImageHeight(imageWand);

uint8_t *imageData = malloc(width * height * 3);

MagickExportImagePixels(imageWand,
   0, 0, width, height, "RGB", CharPixel, imageData);

unsigned char *imageDigest = MD5(imageData, width * height * 3, NULL);

free(imageData);