How to create grid based on luma numbers - php

Question

I have 555 photos in a directory. They get scanned and given a number from 1 through 100 based on the brightness of the photo.
My goal is to have these images in a 37x15 rectangular grid where the centermost horizontal row of images are the brightest and then the outward images are the darkest.
So the ending result would be the images being renamed img[1].jpg img[2].jpg img[3].jpg from the top left to the bottom right, once I combine these later, they would be a gradient of the bottom and top being the darkest images, and the center most horizontal images being the brightest in color.
I already have a function that determines the brightness between 0-100, and a loop for the rows that determine the values row, but stuck after that.
function getAvgLuminance($filename, $num_samples=30) {
global $rows;
// needs a mimetype check
$img = imagecreatefromjpeg($filename);
$width = imagesx($img);
$height = imagesy($img);
$x_step = intval($width/$num_samples);
$y_step = intval($height/$num_samples);
$total_lum = 0;
$sample_no = 1;
for ($x=0; $x<$width; $x+=$x_step) {
for ($y=0; $y<$height; $y+=$y_step) {
$rgb = imagecolorat($img, $x, $y);
$r = ($rgb >> 16) & 0xFF;
$g = ($rgb >> 8) & 0xFF;
$b = $rgb & 0xFF;
// choose a simple luminance formula from here
// http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
$lum = ($r+$r+$b+$g+$g+$g)/6;
$total_lum += $lum;
$sample_no++;
}
}
// work out the average
$avg_lum = $total_lum / $sample_no;
return ($avg_lum / 255) * $rows;
}
$rows=36;
$col=14;
$imgs_resource_dir = 'imgs';
$allFiles = scandir($imgs_resource_dir);
$imlist=array_diff($allFiles, array('.', '..'));
$rowCenter = $rows/2;
while($rows > $ri++){
$FromTop = ( $rowCenter - $ri );
$FromBottom = $rowCenter- ( $rows-$ri) ;
$RowPoint = $FromTop > 0 ? $FromTop : $FromBottom;
}// end row loop
//start image loop
foreach($imlist as $imgfile ){
$pathtoImage = $imgs_resource_dir.'/'.$imgfile ;
$lumss= getAvgLuminance($pathtoImage);
echo 'str '.$lumss.'<br>';
}//end foreach loop