Signature generation

[boan-jfm]

Hi

Does php-vips support signature generation like imagemagick?

SignatureImage() computes a message digest from an image pixel stream with an implementation of the NIST SHA-256 Message Digest algorithm. This signature uniquely identifies the image and is convenient for determining if an image has been modified or whether two images are identical.
-- https://imagemagick.org/api/signature.php

If not, are there alternatives to generate a likewise signature based on pixeldata only?

[jcupitt]

Hi again, no there's nothing like that. I think I'd just shell out to md5sum on the source file.

[boan-jfm]

I must admit, I am not familiar with Ruby, and I do not understand how you can divide an object by 255 (image / 255). PHP-VIPS seems to have a different signature for the conv method; it uses an image object rather than an array:

Image conv(Image $mask, array $options = []) Convolution operation.
-- https://github.com/libvips/php-vips/blob/v2.3.0/src/ImageAutodoc.php#L153

(libvips: https://www.libvips.org/API/current/libvips-convolution.html#vips-conv)

I have tried to convert it to PHP:

        $image = Image::thumbnail($test_image_path, 9, ['height' => 8, 'size' => 'force']);
        $image = $image->colourspace('b-w');
        $image = $image->cast('int')->conv($image, ['precision' => 'integer']);
        $image = $image->crop(0, 0, 8, 8);
        $array = $image->cast('uchar')->writeToArray();
        $binaryString = implode('', $array);
        $hash = bindec($binaryString);

Without the 'conv' call, it executes with a huge number as a result

[jcupitt]

In php it'd be:

#!/usr/bin/env php
<?php

require __DIR__ . '/vendor/autoload.php';
use Jcupitt\Vips;

function dhash($filename)
{
    $image = Vips\Image::thumbnail($filename, 9, [
        'height' => 8,
        'size' => 'force'
    ]);
    $image = $image->colourspace('b-w');
    $image = $image->cast('int')->conv([[-1,  1]], ['precision' => 'integer']);
    $image = $image->crop(1, 0, 8, 8)->more(0)->divide(255)->cast('uchar');
    $array = $image->writeToArray();
    $binaryString = implode('', $array);
    $hash = bindec($binaryString);

    return $hash;
}

for ($i = 1; $i < count($argv); $i++)
{
    $hash = dhash($argv[$i]);
    echo "${argv[$i]}: $hash\n";
}

Though it looks like php doesn't have 64-bit ints, unfortunately.

[jcupitt]

I do not understand how you can divide an object by 255 (image / 255)

That returns a new image with every pixel divided by 255. php doesn't have operator overloads, so you have to write ->divide(255) instead.

PHP-VIPS seems to have a different signature for the conv method; it uses an image object rather than an array:

You can pass an array to conv in PHP as well. It'll get cast into an image for you.

[boan-jfm]

Thank you, it seems to work.

Just curious.

1. In Ruby, you provide the matrix [1, -1], whereas in PHP, you provide [[-1, 1]] as an argument. In PHP, it's a nested array, and the numbers appear to be switched?

2. Why is it cropped from the left by 1 and not 0, with the top being 0 and not 1 when calling the crop method?

The size of an int is platform-dependent, although a maximum value of about two billion is the usual value (that's 32 bits signed). 64-bit platforms usually have a maximum value of about 9E18. PHP does not support unsigned ints. int size can be determined using the constant PHP_INT_SIZE, maximum value using the constant PHP_INT_MAX, and minimum value using the constant PHP_INT_MIN.
https://www.php.net/manual/en/language.types.integer.php

[jcupitt]

1. It's computing taking the left-right pixel difference, so it doesn't really matter which is 1 and which is -1. It's supposed to cast a 1D array into a 2D array for you, but php-vips doesn't seem to do this (argh!), so I made a 2D array.
2. It's computing left-right difference, so it shrinks to 9 pixels across, then drops a column. Again, it doesn't really matter which column you drop. Probably dropping 0 is best.