Convert color PDF to black and white (monochrome) PDF in PHP [for Twilio Fax]

Question

I am trying to convert a color pdf to black and white pdf. I am going to use the pdf to send it in fax and I am using Twilio and it is explicitly converting the color pdf to monochrome pdf however, I want to do it in my server-side to be able to preview the outcome.

As I have Imagick, and found some topics mainly on Imagick, I wanted to give it a try but I couldn't find the necessary class in Imagick. I found some for grayscale but fax is explicitly black & white (monochrome), so no identical to the fax case.

Closest I could find was:

->transformImageColorSpace(\Imagick::COLORSPACE_SRGB) and

->setImageColorSpace(Imagick::COLORSPACE_GRAY)

But these are grayscale, not monochrome.

Also, on Imagick formums, I found these commands but I don't want to execute with shell_exec (as I've read there are couple of down-sides)

// I am pretty sure this one should work, but couldn't find how to do it in php:
convert -density 288 in.pdf -resize 25% out.png  

// thus this one:
convert -density 600 in.pdf -threshold 15% -type bilevel -compress fax out.pdf

// Also found this one:
convert -density 288 image.pdf -resize 25% -threshold 50% -type bilevel image.tiff

How to achieve what I am trying to achieve in php either using above commands or any other php compatible way? How does Twilio do it?

Update:

Expected output (how Twilio does it):

Using below answer:

$img = new Imagick('path/to/document.pdf');
$img->quantizeImage(2,                        // Number of colors
                Imagick::COLORSPACE_GRAY, // Colorspace
                50,                        // Depth tree
                TRUE,                     // Dither
                FALSE);                   // Error correction
$img->writeImage('path/to/output.png')

Update 2:

Using $img->quantizeImage(2, Imagick::COLORSPACE_GRAY, 1, TRUE, FALSE);

emcconville emcconville · Accepted Answer · 2018-01-23T16:00:42

Use Imagick::quantizeImage to monochrome the PDF

$img = new Imagick('path/to/document.pdf');
$img->quantizeImage(2,                        // Number of colors
                    Imagick::COLORSPACE_GRAY, // Colorspace
                    1,                        // Depth tree
                    TRUE,                     // Dither
                    FALSE);                   // Error correction
$img->writeImage('path/to/output.png')

For example...

$img = new Imagick();
$img->newPseudoImage(300, 300, 'radial-gradient:');
$img->quantizeImage(2, Imagick::COLORSPACE_GRAY, 1, TRUE, FALSE);
$img->writeImage('output.png');

**

Or increase color count to allow gray values between black & white. See the usage docs Color Quantization and Dithering for great examples.

$img = new Imagick();
$img->newPseudoImage(300, 300, 'radial-gradient:');
$img->quantizeImage(255, Imagick::COLORSPACE_GRAY, 1, TRUE, FALSE);
$img->writeImage('output.png');

Convert color PDF to black and white (monochrome) PDF in PHP [for Twilio Fax]

1 Answers