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17 years ago
[Editor's note: fixed according to the note of roberto at ilpiola.it]



I could not find any information on changing the DPI information on a JPG file using the GD lib. Since changing this does not resize or scale the actual image, it is only a header-setting.

The following snipplet will save your $image to $file and set the DPI to 150.



<?php



  imagejpeg($image, $file, 75);



  // Change DPI

  $dpi_x   = 150;

  $dpi_y   = 150;

  

  // Read the file

  $size    = filesize($file);

  $image   = file_get_contents($file);



  // Update DPI information in the JPG header

  $image[13] = chr(1);

  $image[14] = chr(floor($dpi_x/256));

  $image[15] = chr(      $dpi_x%256);

  $image[16] = chr(floor($dpi_y/256));

  $image[17] = chr(      $dpi_y%256);



  // Write the new JPG

  $f = fopen($file, 'w');

  fwrite($f, $msg, $size);

  fclose($f);



?>



P.s. not fully tested (yet) but it works for my images ...
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