Fastest way to compute a “Visual” checksum of an image

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);

Answer 2

그것을 할 수있는 몇 가지 방법이 있습니다 ...

$boss = new array(1, 2, 3, 4);
foreach($boss as $value){
    switch($value){
         case 1: 
            //do something
         break;
         case 2: 
            //something else;
         break;
    }
}

.

가 내가 어떻게 쓴 방법 일 것입니다.

편집 : 청결한 코드를 찾고있는 경우 다음과 같이 할 수 있습니다 :

bosshandler.php :

function doSomethingWithBoss($value){
    switch($value){
         case 1: 
            return "boss equals 1";
         break;
         case 2: 
            return "boss equals 2";
         break;
    }
}

.

스크립트 페이지 :

include bosshandler.php;
$boss = new array(1, 2, 3, 4);
foreach($boss as $value){
    echo doSomethingWithBoss($value);
}

.

확인, 다시 시도합니다. 나는 당신이 지금 묻는 것을 볼 것 같습니다 :

배열이 일치하도록 변수를 할당하려고합니다. $boss[1]에있는 경우 $boss_holder['1']에 뭔가를 지정하려는 경우 정확합니까? 그렇다면 배열과 배열이 포함 된 순서가 다소 예측할 수 없으므로 foreach 배열을 사용하고 싶지 않습니다.

대신 : 와 같은 for() 루프를 사용해 봅시다.

$boss = new Array(1, 2, 3, 4);
$boss_holder = new Array(); //empty
$length = count($boss);
//iterate through boss
for($i = 1; $i < $length; $i++){
    if($boss[$i] == 1){
        $boss_holder[$i] = $tick;
    }elseif($boss[$i] == 2){
        $boss_holder[$i] = $cross;
    }
}

.

전체 코드를 반영하도록 다시 편집하십시오 :

배열을 사용합니다. 코드를 사용하여 배열을 사용하도록 코드를 리팩토링하십시오. 어레이를 절대적으로 사용할 수 없거나 배열을 사용하지 않을 경우 PHP의 변수 변수를 사용할 수 있습니다. 당신이 찾고있는 것을 성취하기 위해. 다음과 같이 보일 것입니다 :

var $boss1_holder, $boss2_holder, $boss3_holder;
for($i=1; $i> 4; $i++){
    $variableName = 'boss' . $i . '_holder';
    $$variableName = 1;
}
echo $boss2_holder; //returns 2

.

Answer 3

As noted by Todd Yandell, MD5 is probably fast enough. If not, you can get something even faster by using a 32-bit or 64-bit CRC for your checksum. The major difference is that anybody can make up a new image with the same CRC; it is very easy to spoof. It is quite hard for someone to spoof an MD5 checksum. A minor difference is that the CRC has many fewer bits, but unless you have a very large number of images, a collision is still unlikely.

exiftool claims to be able to extract the binary image from a JPEG file, so that you can compute your checksum without decompressing, but I can't figure out from the man page how to do it.

I did some experiments on a laptop Intel Core 2 Duo L7100 CPU, and an 8MP JPEG takes about 1 second to compress to PPM format, then another 1 second to do the checksum. Checksum times were not dramatically different using md5sum, sum, and sha1sum. So your best bet might be to find a way to extract the binary data without decompressing it.

I also note that your checksum is going to be almost as good even if it uses far fewer pixels. Compare these two:

djpeg -scale 1/8 big.jpg | /usr/bin/sha1sum   # 0.70s
djpeg            big.jpg | /usr/bin/sha1sum   # 2.15s

Answer 4

You should consider that someone may crop the image or modify the pallette, color depth or anything, then a flat checksum will be different, even if visually the original and the modified image still look pretty much alike. Perhaps there is an effective algorithm for cropped or re-colored ones, like Google Images uses for searching of similar pictures.