Example PHP Code that tries to detect image sharpness and displays point of max contrast.

blur-eval.php

<?php

// Sample code to evaluate the sharpness of an image in PHP
// @author https://pastebin.com/u/ossifrage

// Note: this code has not been thoroughly tested, and does not
// perform any kind of input validation. Please don't run this
// on your website and then complain when someone uses it to 
// break your server.

// For testing purposes only, access the name of the input image
// from the query string (blur-eval.php?f=my_file.jpg)
$image = imagecreatefromjpeg($_GET['f']);

$w = imagesx($image);
$h = imagesy($image);

// It would take too long to examine the sharpness of every pixel.
// This variable specifies the size of the gaps between rows and
// columns of the pixels we actually check
$s = 10; // grid size

$start = microtime(true);

$maxdiff = 0;

// Check every $s-th row of the image and store the max difference
// between adjacent pixels
// Note: the formula 0.35*$r + 0.50*$g + 0.15*$b converts RGB values into
// perceptual brightness
for ($x=$s/2;$x<=$w-$s/2;$x+=$s) {
  $r = (imagecolorat($image, $x, $s/2) >> 16) & 0xff;
  $g = (imagecolorat($image, $x, $s/2) >> 8) & 0xff;
  $b = (imagecolorat($image, $x, $s/2)) & 0xff;
  $v0 = floor(0.35*$r + 0.50*$g + 0.15*$b);
  for ($y=$s/2+1;$y<=$h-$s/2;$y++) {
    $r = (imagecolorat($image, $x, $y) >> 16) & 0xff;
    $g = (imagecolorat($image, $x, $y) >> 8) & 0xff;
    $b = (imagecolorat($image, $x, $y)) & 0xff;
    $v = floor(0.35*$r + 0.50*$g + 0.15*$b);
    if ($maxdiff<abs($v-$v0)) {
      $maxdiff=abs($v-$v0);
      $xmax = $x;
      $ymax = $y;
    }
    $v0 = $v;
  }
}

// Do the same thing on the columns
for ($y=$s/2;$y<=$h-$s/2;$y+=$s) {
  $r = (imagecolorat($image, $s/2, $y) >> 16) & 0xff;
  $g = (imagecolorat($image, $s/2, $y) >> 8) & 0xff;
  $b = (imagecolorat($image, $s/2, $y)) & 0xff;
  $v0 = floor(0.35*$r + 0.50*$g + 0.15*$b);
  for ($x=$s/2+1;$x<=$w-$s/2;$x++) {
    $r = (imagecolorat($image, $x, $y) >> 16) & 0xff;
    $g = (imagecolorat($image, $x, $y) >> 8) & 0xff;
    $b = (imagecolorat($image, $x, $y)) & 0xff;
    $v = floor(0.35*$r + 0.50*$g + 0.15*$b);
    if ($maxdiff<abs($v-$v0)) {
      $maxdiff=abs($v-$v0);
      $xmax = $x;
      $ymax = $y;
    }
    $v0 = $v;
  }
}

// Now calculate the range of brightness in the 9x9 block of pixels
// centered on the location of maximum contrast found above
$maxv = 0;
$minv = 255;
for ($x=$xmax-4;$x<=$xmax+4;$x++) {
  for ($y=$ymax-4;$y<=$ymax+4;$y++) {
    $r = (imagecolorat($image, $x, $y) >> 16) & 0xff;
    $g = (imagecolorat($image, $x, $y) >> 8) & 0xff;
    $b = (imagecolorat($image, $x, $y)) & 0xff;
    $v = floor(0.35*$r + 0.50*$g + 0.15*$b);
    if ($v>$maxv) $maxv=$v;
    if ($v<$minv) $minv=$v;
  }
}

// Calculate a sharpness value based on this brightness range
$sharpness = ($maxdiff / (15 + $maxv - $minv)) * 27000 / 255;
    
$end =  microtime(true);
$howlong = $end - $start;

// Draw on the image to show where the maximum contrast was found, etc.
$bg = imagecolorallocatealpha($image,255,255,255,32);
$fg = imagecolorallocate($image,0,0,0);

imagerectangle($image,$xmax-4,$ymax-4,$xmax+4,$ymax+4,$bg);
imageline($image,$xmax-5,$ymax,0,$ymax,$bg);
imageline($image,$xmax+5,$ymax,$w-1,$ymax,$bg);
imageline($image,$xmax,$ymax-5,$xmax,0,$bg);
imageline($image,$xmax,$ymax+5,$xmax,$h-1,$bg);


$s = "Sharpness: ".number_format($sharpness,1)."%, " . number_format($howlong,3)." seconds";
imagealphablending($image,true);
imagefilledrectangle($image,4,$h-20,10+strlen($s)*7,$h-4,$bg);


imagestring($image, 2, 8, $h-18, $s,$fg);

header('Content-Type: image/jpeg');
imagejpeg($image);
imagedestroy($image);