imagefilter的半灰度（）

I know PHP's GD library can apply grayscale filter to an image, for example:

$img = imagecreatefrompng('test.png');
$img = imagefilter($img, IMG_FILTER_GRAYSCALE);
imagepng($img, 'test_updated.png');

Is there any method that can apply half of the grayscale effects (which is similar to CSS3's filter: grayscale(50%);) ?

I read from this answer, the grayscale filter is actually a reduction of R, G & B channel. Can I customize my own grayscale filter in PHP?

Reference: imagefilter()

Is there any method that can apply half of the grayscale effects (which is similar to CSS3's filter: grayscale(50%);) ?

Found a script something similar what you are looking for..

<?php

function convertImageToGrayscale($source_file, $percentage)
{
    $outputImage = ImageCreateFromJpeg($source_file);
    $imgWidth    = imagesx($outputImage);
    $imgHeight   = imagesy($outputImage);

    $grayWidth  = round($percentage * $imgWidth);
    $grayStartX = $imgWidth-$grayWidth;

    for ($xPos=$grayStartX; $xPos<$imgWidth; $xPos++)
    {
        for ($yPos=0; $yPos<$imgHeight; $yPos++)
        {
            // Get the rgb value for current pixel
            $rgb = ImageColorAt($outputImage, $xPos, $yPos);

            // extract each value for r, g, b
            $rr = ($rgb >> 16) & 0xFF;
            $gg = ($rgb >> 8) & 0xFF;
            $bb = $rgb & 0xFF;

            // Get the gray Value from the RGB value
            $g = round(($rr + $gg + $bb) / 3);

            // Set the grayscale color identifier
            $val = imagecolorallocate($outputImage, $g, $g, $g);

            // Set the gray value for the pixel
            imagesetpixel ($outputImage, $xPos, $yPos, $val);
        }
    }
    return $outputImage;
}

$image = convertImageToGrayscale("otter.jpg", .25);

header('Content-type: image/jpeg');
imagejpeg($image);
?>

See if that works. I found that here