php-vips: writeToBuffer doesn't output edited/removed EXIF data

Here it is in PHP:

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

require __DIR__ . '/vendor/autoload.php';

use Jcupitt\Vips;

function sigmoid(bool $sharpen, float $midpoint, float $contrast,
    bool $ushort = false): Vips\Image
{   
    $lut = Vips\Image::identity(["ushort" => $ushort]);
    // Rescale so each element is in [0, 1]
    $range = $lut->max();
    $lut = $lut->divide($range);

    if ($sharpen) {
        $x = $lut->multiply(-1)->add($midpoint)->multiply($contrast)->
            exp()->add(1)->pow(-1);
        $min = $x->min();
        $max = $x->max();
        $result = $x->subtract($min)->divide($max - $min);
    } else { 
        $min = 1 / (1 + exp($contrast * $midpoint)); 
        $max = 1 / (1 + exp($contrast * ($midpoint - 1)));
        $x = $lut->multiply($max - $min)->add($min);
        $result = $x->multiply(-1)->add(1)->divide($x)->log()->
            divide($contrast)->multiply(-1)->add($midpoint);
    }

    // Rescale back to 0 - 255 or 0 - 65535
    $result = $result->multiply($range);
    $result = $result->cast($ushort ? 
        Vips\BandFormat::USHORT : Vips\BandFormat::UCHAR);

    return $result;
}

$midpoint = 0.5;
$contrast = 6;

$sig = sigmoid(TRUE, $midpoint, $contrast);
$sig->writeToFile("x1.v");

$sig = sigmoid(FALSE, $midpoint, $contrast);
$sig->writeToFile("x2.v");

Run nip2 x*.v to see the two curves.

That’s a very direct translation. You can make it a bit quicker: there are several sequences like:

$x->multiply(-1)->add($midpoint)->multiply($contrast)->...

That’s a simple linear transform of $x, so you can swap those three calls for a single ->linear(). Of course this is just making a LUT and won’t be run for every pixel, so perhaps speed doesn’t matter.

OK, I’ve updated everything with the fixed equation, it works much better now, though you can no longer decrease contrast.

I’ll have a look at the IM sources and see how they do contrast reduction.