PHP Histogram Equalization (and RGB322)

<?php
function histeq( $im ){
    if ( is_string( $im ) )
        $im = imagecreatefromjpeg( $im );
    $_h = imagesy( $im );
    $_w = imagesx( $im );
    $_f = $_h * $_w / 255;
    for ( $y = 0; $y < $_h ; $y++ ) for ( $x = 0; $x < $_w; $x ++ ){
        $col = imagecolorat( $im, $x, $y );
        $originals[0][ ( $col >> 16 ) & 0xFF ]++;
        $originals[1][ ( $col >> 8 ) & 0xFF ]++;
        $originals[2][ $col & 0xFF ]++;
    }
    foreach ( array( 0, 1, 2 ) as $chn ){
        $ori = $originals[ $chn ];
        $i = 0;
        $sum = array();
        for ( $j = 0; $j < 255; $j++ ){
            $counter = $ori[$j];
            $map = array();
            while ( $counter ){
                while ( $sum[$i] >= $_f )
                    $i++;
                $count = min( $counter, $_f - $sum[$i] );
                $map[ min( $i,255 ) ] += $count / $ori[$j];
                $sum[$i] += $count;
                $counter -= $count;
            }
            $cum = 0;
            foreach( $map as $k => $v){
                $cum += $v;
                $matrix[$chn][$j][$k] = $cum;
            }
        }
    }
    for ( $y = 0; $y < $_h ; $y++ ) for ( $x = 0; $x < $_w; $x ++ ){
        $col = imagecolorat( $im, $x, $y );
        $kol[0] = ( $col >> 16 ) & 0xFF;
        $kol[1] = ( $col >> 8 ) & 0xFF;
        $kol[2] = $col & 0xFF;
        $rand = rand( 0, 1000000 ) / 1000000;
        foreach ( array( 0, 1, 2 ) as $chn ){
            foreach ( (array) $matrix[$chn][ $kol[$chn] ] as $k => $v ){
                if ( $rand <= $v ){
                    $kol[$chn] = $k;
                    break;
                }
            }
        }
        // FF:8bit, FE:7bit, FC:6bit, F8:5bit, F0:4bit, E0:3bit, C0:2bit, 80:1bit
        // ex FFFFFF:RGB888 E0E0C0:RGB332 // 0xF0FCE0;//0xE0F0C0;
        $col = ( ( $kol[0] << 16 ) | ( $kol[1] << 8 ) | $kol[2] ) & 0xE0E0C0;
        imagesetpixel( $im, $x, $y, $col );
    }

    ob_start();
    imagejpeg( $im );
    return ob_get_clean();
}

header( "Content-Type: image/jpg" );
echo histeq( __DIR__ . "/test.jpg" );