Why is the quality on generated PDFs so low with this code? - php

Question

I have the following code. It's used to combine various image attachments (and pdfs) into one PDF. For some reason, when I take even a single PDF and put it through the code, the end result comes out looking very bad compared to the original PDF. In addition, I can select text in the source PDF, but in the generated one I cannot.
Any help would be greatly appreciated.
// PDF object
$pdf = new Imagick();
$max_resolution = array('x' => 100, 'y' => 100);
foreach($attachment_ids as $attachment_id) {
$attachment = DAO_Attachment::get($attachment_id);
$file = Storage_Attachments::get($attachment);
// Temporarily store our attachment
$im = new Imagick();
$im->readImageBlob($file);
// We need to reset the iterator otherwise only one page will be rotated
$im->resetIterator();
// Get the resolution
$resolution = $im->getImageResolution();
if($resolution['x'] > $max_resolution['x']) {
$max_resolution['x'] = $resolution['x'];
}
if($resolution['y'] > $max_resolution['y']) {
$max_resolution['y'] = $resolution['y'];
}
$num_pages = $im->getNumberImages();
$rotation = array_shift($rotations);
$degrees = $rotation > 0 ? 360 - $rotation : 0;
$pages = array();
if($degrees > 0) {
// Rotate each page
for($i = 1; $i <= $num_pages; $i++) {
$im->nextImage();
$im->rotateImage(new ImagickPixel(), $degrees);
}
}
// We need to reset the iterator again so all of our pages will be added to the pdf
$im->resetIterator();
// If the image format isn't a pdf, convert it to a png
if($im->getImageFormat !== 'pdf') {
$im->setImageFormat('png');
// Opacity
if(method_exists($im, 'setImageOpacity'))
$im->setImageOpacity(1.0);
}
$im->setImageCompression(imagick::COMPRESSION_LOSSLESSJPEG);
$im->setImageCompressionQuality(100);
$im->stripImage();
// Add the rotated attachment to the PDF
$pdf->addImage($im);
// Free
$im->destroy();
}
// Create a composite
$pdf->setImageFormat('pdf');
// Compress output
$pdf->setImageCompression(imagick::COMPRESSION_LOSSLESSJPEG);
$pdf->setImageCompressionQuality(100);
$pdf->stripImage();
// Set resolution
$pdf->setImageResolution($max_resolution['x'], $max_resolution['y']);

Answer 1

This may be obvious to you already but a low quality image will not result in a high quality pdf. I don't know how good Imagick's pdf generation capabilities are, but it seems from your code you are converting images? You could compare by doing the same thing with TcPDF, though if the image is low quality I doubt you will get better results.
Also, if you have access to higher DPI resolution images than the usual web-optimised format, I recommend you use those to build your PDF instead. The quality will be a lot better.

Answer 2

ImageMagick uses GhostScript to convert PDFs to various raster image formats. GhostScript is quite good at this, but you're hand-cuffing it by scaling the page down to a max of 100x100.
An 8.5x11 (inches) page at 72 dpi, is 612x792 pixels.
Perhaps you meant to restrict DPI rather than resolution? The output still won't scale all that well (vector formats vs pixel formats), but I suspect it would be a big improvement.

Answer 3

It turns out the answer to this is to set the DPI using setResolution(). We do this before using readImageBlob() to get read the file containing our image, as it will change the DPI of the image based on the current resolution (so setting it afterwards won't work).
You could also use some math and use resampleImage() to do it after the fact, but setResolution() seems to be working perfectly for us.