Black background when converting multi page PDF to JPG with Imagick php extension

Question

What is the best way to correct black background when converting multi page PDF to JPG with Imagick php extension?

Following is the code used on my application:

    $imagick = new Imagick($file);
    $imagick->setResolution(150,150);
    $imagick->setImageFormat("jpg");
    $imagick->setImageCompression(imagick::COMPRESSION_JPEG);
    $imagick->setImageCompressionQuality(70);
    foreach ($imagick as $c => $_page) {
        $_page->setImageBackgroundColor('white');
        $_page->adaptiveResizeImage($maxsize,$maxsize,true);
        $_page->writeImage("$file-$c.jpg");
    }

I'am aware that the flattenImage method can be used to remove black background, such as in:

    $imagick = $imagick->flattenImages();

But when the file has more the one pages, the flattenImages method puts all the pages on the same image, and therefore the result is a copy of the last page in all the JPGs generated.

I appreciate if anybody can help me.

Answer 1

Working code first - explanation to follow:

This code works, but is incredibly slow:

$file = "./YORK.pdf";

$maxsize = 500;

$imagick = new Imagick($file);
$imagick->setResolution(150,150);
$imagick->setImageFormat("jpg");
$imagick->setImageCompression(imagick::COMPRESSION_JPEG);
$imagick->setImageCompressionQuality(70);

foreach ($imagick as $c => $_page) {
    $_page->setImageBackgroundColor('white');
    $_page->adaptiveResizeImage($maxsize,$maxsize,true);
    $_page->setImageCompose(\Imagick::COMPOSITE_ATOP);
    $_page->flattenImages();
    $_page->writeImage("$file-$c-compose.jpg");
}

This code works and is fast:

foreach ($imagick as $c => $_page) {
    $_page->setImageBackgroundColor('white');
    $_page->adaptiveResizeImage($maxsize,$maxsize,true);
    $blankPage = new \Imagick();
    $blankPage->newPseudoImage($_page->getImageWidth(), $_page->getImageHeight(), "canvas:white");
    $blankPage->compositeImage($_page, \Imagick::COMPOSITE_ATOP, 0, 0);
    $blankPage->writeImage("$file-$c.jpg");
}

What I think is happening is that when it comes to write the image ImageMagick is doing:

• Convert the individual layers to JPG
• Merge them on top of each other.

For each of the layers that has transparency because JPG doesn't support transparency it is rendering the transparency as black and then merging it. The code above makes the compositing be done in the correct order.

An alternative way to fix the problem is to put the output as PNG. As it supports transparency, the individual layers with transparency are merged correctly, and then you could convert the final image to JPG if you really wanted to.

Using PNG as the intermediate format may also produce a slightly higher quality output, as it may skip a 'save to JPG and decode' step. I do recommend using PNG in your workflow wherever possible, and then converting to JPG only when you serve a file to an end-user if you really need that extra bit of compression.