imagick creates wrong colorspace / colors

I have the following code on my site where i have to create images out of uploaded pdf files, my problem is that i cannot get the colors right (and yes there can be both text and images in the pdf files):

        $img = new imagick();
        $img->setResolution(72, 72);
        $img->readImage($dir_f.$file);
        $img->set ImageCompression(imagick::COMPRESSION_NO);
        $img->setImageCompressionQuality(100);

        $num_pages = $img->getNumberImages();
        for($i = 0;$i < $num_pages; $i++) {
            $fina l_name     = str_replace(" ", "+", basename(str_replace(".".$file_ext,"", $file_name)));
            $final_name     = preg_replace("/[^a-zA-Z0-9-+]/", "", $final_name);

            $save_to        = $pdf_dir."/".str_replace(".".$file_ext,"", $final_name).'-'.$i.'.png';

            $img->setIteratorIndex($i);
            $img->setImageCompression(imagick::COMPRESSION_NO);
            $img->setImageCompressionQuality(100);
            $img->setimageformat("png");
            $img->writeImage($save_to);

            $file_image = str_replace(dirname(__FILE__)."/../../", "/", $save_to);
            $file_images[] = array($file_image, $index[$ii]);
        }
        $img->destroy();

I have gotten different results depending on wich type of colorspace i have been using but non of them are correctly made colors.

when i dont set a colorspace it gets to bright. when i use colorspace srgb or cmyk the image gets to bright, when i use colorspace COMPRESSION_NO it gets to bright when i use colorspace rgb it gets to dark

Hopefully someone is able to help me out here.

Solution

This looks like it's a bug in Ghostscript, which is the delegate program ImageMagick normally uses to convert PDFs to bitmap format.

Using Ghostscript version 8.70 I see the same output you are seeing, i.e. the image is lighter than it is when viewing the PDF directly.

After downloading Ghostscript version 9 and running the command below to do the conversion, I see the same colours as in the PDF.

You're probably going to need to either upgrade to Ghostscript 9 or figure out what is special about the PDF that makes Ghostscript not convert it correctly. I think the PDF might either have a non-standard color profile or weird colorspace. But as converting it with GS9 works fine, that is probably the easiest way to fix the problem.

./ghostscript-9.16-linux-x86_64/gs-916-linux_x86_64 -q -dQUIET -dSAFER -dBATCH \
  -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 \
  -dAlignToPixels=0 -dGridFitTT=1 -sDEVICE=pngalpha \
  -dFirstPage=1 -dLastPage=3 \
  -dTextAlphaBits=4 -dGraphicsAlphaBits=4 -r72 \
  -sOutputFile=gs-%d.png 153311-flyers-sample-papers-volume-2.pdf