Example

function backgroundMasking()
{
    //Load the image
    $imagick = new \Imagick(realpath('images/chair.jpeg'));

    $backgroundColor = "rgb(255, 255, 255)";
    $fuzzFactor = 0.1;

    // Create a copy of the image, and paint all the pixels that
    // are the background color to be transparent
    $outlineImagick = clone $imagick;
    $outlineImagick->transparentPaintImage(
        $backgroundColor, 0, $fuzzFactor * \Imagick::getQuantum(), false
    );
    
    // Copy the input image
    $mask = clone $imagick;
    // Deactivate the alpha channel if the image has one, as later in the process
    // we want the mask alpha to be copied from the colour channel to the src
    // alpha channel. If the mask image has an alpha channel, it would be copied
    // from that instead of from the colour channel.
    $mask->setImageAlphaChannel(\Imagick::ALPHACHANNEL_DEACTIVATE);
    //Convert to gray scale to make life simpler
    $mask->transformImageColorSpace(\Imagick::COLORSPACE_GRAY);

    // DstOut does a "cookie-cutter" it leaves the shape remaining after the
    // outlineImagick image, is cut out of the mask.
    $mask->compositeImage(
        $outlineImagick,
        \Imagick::COMPOSITE_DSTOUT,
        0, 0
    );
    
    // The mask is now black where the objects are in the image and white
    // where the background is.
    // Negate the image, to have white where the objects are and black for
    // the background
    $mask->negateImage(false);

    $fillPixelHoles = false;
    
    if ($fillPixelHoles == true) {
        // If your image has pixel sized holes in it, you will want to fill them
        // in. This will however also make any acute corners in the image not be
        // transparent.
        
        // Fill holes - any black pixel that is surrounded by white will become
        // white
        $mask->blurimage(2, 1);
        $mask->whiteThresholdImage("rgb(10, 10, 10)");

        // Thinning - because the previous step made the outline thicker, we
        // attempt to make it thinner by an equivalent amount.
        $mask->blurimage(2, 1);
        $mask->blackThresholdImage("rgb(255, 255, 255)");
    }

    //Soften the edge of the mask to prevent jaggies on the outline.
    $mask->blurimage(2, 2);

    // We want the mask to go from full opaque to fully transparent quite quickly to
    // avoid having too many semi-transparent pixels. sigmoidalContrastImage does this
    // for us. Values to use were determined empirically.
    $contrast = 15;
    $midpoint = 0.7 * \Imagick::getQuantum();
    $mask->sigmoidalContrastImage(true, $contrast, $midpoint);

    // Copy the mask into the opacity channel of the original image.
    // You are probably done here if you just want the background removed.
    $imagick->compositeimage(
        $mask,
        \Imagick::COMPOSITE_COPYOPACITY,
        0, 0
    );

    // To show that the background has been removed (which is difficult to see
    // against a plain white webpage) we paste the image over a checkboard
    // so that the edges can be seen.
    
    // Create the check canvas
    $canvas = new \Imagick();
    $canvas->newPseudoImage(
        $imagick->getImageWidth(),
        $imagick->getImageHeight(),
        "pattern:checkerboard"
    );

    // Copy the image with the background removed over it.
    $canvas->compositeimage($imagick, \Imagick::COMPOSITE_OVER, 0, 0);
    
    //Output the final image
    $canvas->setImageFormat('png');
    header("Content-Type: image/png");
    echo $canvas->getImageBlob();
}