PDF Thumbnail always generated as grayscale using Imagick

Question

I am stuck and could really use some help on this one. I am using PHP and Imagick to generate a thumbnail which is working great. However, I noticed that CMYK PDFs are generated always as grayscale. So I tested this by taking the CMYK PDF and manually converting it to RGB with Adobe Acrobat Pro. Then I re-ran it through the following code and it produced a color image. I know about

$image->transformImageColorSpace(1);
or
$image->setImageColorSpace(1);

However this doesn't work. What is the correct way for converting a pdf to a color PNG image? I have looked at the following links with no luck:

http://php.net/manual/en/imagick.setimagecolorspace.php

Convert PDF to JPEG with PHP and ImageMagick

Any help on this one would be great.

Here is the code:

$filePath = fileSaveUserUpload("path/to/file", ""); //path changed here...
    $_SESSION['FILEPATH'] = $filePath;

    //-------------first makes a thumbnail of first page in image/pdf
    $extension_pos = strrpos($filePath, '.');                        // find position (number) of the last dot, so where the extension starts

    $image = new Imagick();
    $image->readImage($filePath."[0]");                              //reads an image at a path(first page only in this case)
    $image->transformImageColorSpace(1);                             //convert to RGB
    $image->setbackgroundcolor('white');                             //replace transparency with this color
    $image->setCompression(Imagick::COMPRESSION_LOSSLESSJPEG);
    $image->setCompressionQuality(150);
    $image->setImageAlphaChannel(Imagick::ALPHACHANNEL_REMOVE);      //remove transparency
    $image->mergeImageLayers(Imagick::LAYERMETHOD_FLATTEN);          //make everything that was transparent white
    $image->thumbnailImage(0,250);                                   //max height 300 but try and preserve aspect ratio (wdithxheight)
    $thumbnail = substr($filePath, 0, $extension_pos) . '_thumb.png';// . substr($filePath, $extension_pos);

    $image->writeImage($thumbnail);
    $image->clear();
    $image->destroy();

UPDATE:

I am using the following imagick version:

ImageMagick 6.9.1-2 Q16 x86 2015-04-14

3.3.0RC2

GhostScript Version: 9.18

Here is the original PDF (changed it to a picture here):

Here is the thumbnail that it produced:

This ONLY happens with CMYK PDFs. If I take this same PDF and convert it to RGB through adobe acrobat it comes out color. I tested this and it still holds true.

Answer 1

greeting from 2019. was having this problem still on gs 9.26 (9.27 doesnt work at all)

set the colourspace BEFORE loading the file, transform it AFTER.

// setup imagick for colour
$Img = new Imagick();
$Img->SetResolution(200,200);
$Img->SetColorspace(Imagick::COLORSPACE_SRGB);

// now read pdf first page.
$Img->ReadImage("{$File}[0]");
$Img->TransformImageColorSpace(Imagick::COLORSPACE_SRGB);

// the rest of your stuff.

Answer 2

You may try changing:

$image->transformImageColorSpace(1);
to
$image->transformImageColorSpace(Imagick::COLORSPACE_RGB);.

I'm not sure what you are trying to do with the 1, but according to PHP.net, there are predefined colorspaces that can be added to transformImageColorSpace(); to have the image output with the correct colorspace.

Answer 3

The problem you are seeing is likely to be an old version of GhostScript that is not doing the conversion correctly. Below is the image produced when calling your code on a system that has GhostScript version 8.70 installed:

btw you almost certainly want to be using SRGB colorspace, not plain old RGB. SRGB is the correct one to use for displaying images on computer screens.

$image->transformImageColorSpace(\Imagick::COLORSPACE_SRGB);

Here is a command line to test ghostscript by itself doing the conversion:

./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 tree-blackandwhite.pdf

It should convert the image to a PNG. You'd probably need to replace ./gs-916-linux_x86_64 with just gs.