How to clean background captcha image using imagick - php

Question

I try to clean background captcha image using php imagick method and i used convert method as an alternative on console but i got same result. But it's not really worked.
Then i will use tesseract ocr for getting captcha code. There is a code for tesseract:
tesseract -psm 7 captcha.png output
Captcha image:
captcha
There is my code:
//Example Tutorial::backgroundMasking
function backgroundMasking()
{
//Load the image
$imagick = new \Imagick(realpath('captchaa.png'));
$backgroundColor = "rgb(162, 234, 160)";
$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);
$max = $imagick->getQuantumRange();
$max = $max["quantumRangeLong"];
$canvas->thresholdimage(0.77 * $max);
$canvas->negateImage(false, \Imagick::CHANNEL_ALL);
//Output the final image
$canvas->setImageFormat('png');
header("Content-Type: image/png");
echo $canvas->getImageBlob();
}
Result captcha image: enter image description here
what can i do for getting better result?