Thresholding an image producing strange values at output.

[ocpaul20]

I am trying to produce a thresholded image with only an rgb value of zero or 255.

 

As you can see, although I only output 0 and 255,

I am finding that if I output as jpg file, I get other values in there as well.

If I output as gif, I also get other values in there too.

 

I am sure I am doing something stupid as there must be a way to do this.

 

My test file is this attached jpg greyscale file.

 

Is there a way to process this test file so that only 0 or 255 appears in the output image please? Thanks.

 

Here is my program

<?php
$url = 'greyscale_image.jpg';
$outfile = 'greyscaleout';
$src = imagecreatefromjpeg($url);
$w = imagesx($src);$h = imagesy($src);
$loband = 80;$hiband = 255;

$im_r = imagecreatetruecolor($w,$h);
imagefill($im_r,0,0,imagecolorallocate($im_r,255,0,0));

$im_j = imagecreatetruecolor($w,$h);
imagefill($im_j,0,0,imagecolorallocate($im_j,255,0,0));

for ($x=0;$x<$w;$x++) {
	for ($y=0;$y<$h;$y++) {
			$idx = imagecolorat($src,$x,$y);
			$rgb = imagecolorsforindex($src,$idx); // this is safest (for gifs) rather than some calculation - apparently
			$r =$rgb['red'];
			$g = $rgb['green'];
			$b = $rgb['blue'];

			$gryout = $r;
			if ($r <= $loband) {
				$gryout = 0; // everything less than= loband is black
			}
			if ($r > $loband) {
				$gryout = 255; // everything else is white
			}
			if ( ($gryout == 0) || ($gryout == 255) ) {
					// only zero or 255 allowed
					$idx = imagecolorallocate($im_r,$gryout,$gryout,$gryout);
					imagesetpixel($im_r,$x,$y,$idx); //
					imagesetpixel($im_j,$x,$y,$idx); //

					$idx2 = imagecolorat($im_r,$x,$y);//
					if ($idx != $idx2) {
						die($idx2." at x,y ".$x.",".$y);
					}
					$rgb = imagecolorsforindex($im_r,$idx); // this is safest (for gifs) rather than some calculation - apparently
					$r =$rgb['red'];
					$g = $rgb['green'];
					$b = $rgb['blue'];
					if ($r != 0) {
						if ($r != 255) {
							die($r." at x,y ".$x.",".$y);
						}
					}
			} else {
				die('<br>Grey going out to image is '.$gryout);
			}
	}
}
imagegif($im_r,$outfile.'.gif');imagedestroy($im_r);
imagejpeg($im_j,$outfile.'.jpg');imagedestroy($im_j);
// ==========================
// done the writing, now check it
// ==========================
// $outfile = 'greyscale_image'; // test using original
$src = imagecreatefromjpeg($outfile.'.jpg');
$w = imagesx($src);$h = imagesy($src);
$errcount = 0;
$x = 0;
while($x<$w) {
	$y = 0;
	while($y<$h) {
			$idx = imagecolorat($src,$x,$y);//
			$rgb = imagecolorsforindex($src,$idx); // this is safest (for gifs) rather than some calculation
			$r =$rgb['red'];
			$g = $rgb['green'];
			$b = $rgb['blue'];

			if ($r != 0) {
				if ($r != 255) {
					echo sprintf('<br>r(%d,%d) r=%d g=%d b=%d',$x,$y,$r,$g,$b);$errcount++;
				}
			}
			if ($g != 0) {
				if ($g != 255) {
					echo sprintf('<br>g(%d,%d) r=%d g=%d b=%d',$x,$y,$r,$g,$b);$errcount++;
				}
			}
			if ($b != 0) {
				if ($b != 255) {
					echo sprintf('<br>b(%d,%d) r=%d g=%d b=%d',$x,$y,$r,$g,$b);$errcount++;
				}
			}
			if ($errcount > 10) {break 2;}

			$y++;
	}
	$x++;
}
if ($errcount == 0) {die( ' Checked OK' );}
die( ' Checked  NOT OK' );
// =================
?>

 

 

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