how can i change the dpi of an image with the imagick extension

Question

I need to change all uploaded files to 72 dpi. I'm using the php imagick extension.

heres what i've tried (the image i'm using is 300dpi):

$image = new Imagick();
$image->setResolution(72,72) ;
$image->readImage($img);
$image->resampleImage  (72,72,imagick::FILTER_UNDEFINED,1);
$image->writeImage($target)

this doesn't seem to anything. the image is uploading, but stays at 300dpi

Answer 1

MatTheCat's answer is spot on. You might also try setImageUnits() to ensure it's working with inches and not centimeters.

$image->setImageUnits(imagick::RESOLUTION_PIXELSPERINCH);
$image->setImageResolution(72,72);

Resampling isn't necessary just to change dpi.

Note that changing the dpi alone will not affect file size and only applies to resampling and printing.