PDF文件是否可以定义0页，否则会导致0作为页面大小？

Question

I have a PHP script using Imagick, but there is the risk of a NAN error, should a PDF file provided by a user contain no pages or have a page with no height or no width. I am not sure if this is possible in a PDF structure. Also making a jpeg from a page number larger than the total pages will cause an error. Is it generally possible a valid PDF file wrapper is sent but without actual page content?

The core question: How can we count and measure pages for a proper error capture before entering the conversion from PDF to JPEG?

In the function below I assume it might be possible to have 0 height or 0 width. And use the code if($imH==0){$imH=1;} but having code based on an assumption doesn't feel right.

parts of the function were adopted from an article by umidjons: https://gist.github.com/umidjons/11037635

PHP code:

function genPdfThumbnail ( $src, $targ, $size=256, $page=1 ){

    if(file_exists($src) && !is_dir($src)): // source path must be available and cannot be a directory

        if(mime_content_type($src) != 'application/pdf'){return FALSE;} // source is not a pdf file returns a failure

        $sepa   =   '/'; // using '/' as path separation for nfs on linux.
        $targ   =   dirname($src).$sepa.$targ;
        $size   =   intval($size); // only use as integer, default is 256
        $page   =   intval($page); // only use as integer, default is 1     
        $page--; // default page 1, must be treated as 0 hereafter
        if ($page<0){$page=0;} // we cannot have negative values

        $img    =   new Imagick($src."[$page]");
        $imH    =   $img->getImageHeight();
        $imW    =   $img->getImageWidth();

        if ($imH==0) {$imH=1;} // if the pdf page has no height use 1 instead
        if ($imW==0) {$imW=1;} // if the pdf page has no width use 1 instead
        $sizR   =   round($size*(min($imW,$imH)/max($imW,$imH))); // relative pixels of the shorter side

        $img    ->  setImageColorspace(255); // prevent image colors from inverting
        $img    ->  setImageBackgroundColor('white'); // set background color before flatten
        $img    =   $img->flattenImages(); // prevent black zones on transparency in pdf
        $img    ->  setimageformat('jpeg');

        if ($imH == $imW){$img->thumbnailimage($size,$size);} // square page 
        if ($imH < $imW) {$img->thumbnailimage($size,$sizR);} // landscape page orientation
        if ($imH > $imW) {$img->thumbnailimage($sizR,$size);} // portrait page orientation      
        if(!is_dir(dirname($targ))){mkdir(dirname($targ),0777,true);} // if not there make target directory

        $img    ->  writeimage($targ);
        $img    ->  clear();
        $img    ->  destroy();

        if(file_exists( $targ )){ return $targ; } // return the path to the new file for further processing

    endif;
    return FALSE; // source file not available or Imagick didn't create jpeg file, returns a failure

}

call the function e.g. like:

$newthumb = genPdfThumbnail('/nfs/vsp/server/u/user/public_html/any.pdf','thumbs/any.p01.jpg',150,'01');

Answer 1

Sure, a PDF file is a container format that can contain pretty much anything, including (only) metadata with 0 pages. But even so, with this code it's quite possible to request a thumbnail for page 21 on a document that only contains 5 pages.

If that happens, the problem will occur on this line:

$img    =   new Imagick($src."[$page]");

This will throw an exception if the provided page does not exist. You can catch that exception and handle it however you want:

try {
    $img = new Imagick($src."[$page]");
} except (ImagickException $error) {
    return false;
}

If you want to read the number of pages beforehand, you can try to let Imagick parse the document first:

$pdf = new Imagick($src);
$pages = $pdf->getNumberImages();

The function name is a bit misleading, see this comment in the PHP manual:

"For PDFs this function indicates the number of pages on the PDF, NOT images that might be embedded within the PDF."

Here as well, if the PDF document is invalid in some way, this can throw an exception so you might want to catch that and handle it:

try {
    $pdf = new Imagick($src);
    $pages = $pdf->getNumberImages();
} except (ImagickException $error) {
    return false;
}

if ($pages < $page) {
    return false;
}