Create horizontal(1000x16) and vertical(16x1000) images of rules (measured in pixels) to be used in, for example, online editors, using PHP and ImageMagick. (inspiration: http://www.ibm.com/developerworks/br/linux/library/l-pixelruler/index.html)

createRulesImages.php

<?php
$rule1 = "img/rule1.jpg";
$ruleV = "img/ruleV.jpg";
$ruleH = "img/ruleH.jpg";
$width = 1000;
$height = 1000;

// horizontal rule
$str = "line 0,0 $width,0 line 0,15 $width,15";
$text = "";

for ($i = 0; $i <= $width; $i += 5){
  $str .= "line $i,12 $i,15 ";
}

for ($i = 0; $i <= $width; $i += 10){
	$str .= "line $i,10 $i,15 ";
}

for ($i = 0; $i <= $width; $i += 50){
	$str .= "line $i,0 $i,15 ";
}

for ($i = 0; $i <= $width; $i += 50){
	$text .= "-draw \"text ".($i+2).",9 '$i' \" ";
}
	
$str01 = "convert -size ".$width."x16 xc:white $rule1";
$str02 = "convert -fill black -draw \"$str\" -font arial -pointsize 10 $text $rule1 $ruleH";
exec ($str01);
exec ($str02);

// vertical rule
$str = "line 0,0 0,$height line 15,0 15,$height";
$text = "";

for ($i = 0; $i <= $height; $i += 5){
	$str .= "line 12,$i 15,$i ";
}

for ($i = 0; $i <= $height; $i += 10){
	$str .= "line 10,$i 15,$i ";
}

for ($i = 0; $i <= $height; $i += 50){
	$str .= "line 0,$i 15,$i ";
}

for ($i = 0; $i <= $height; $i += 50){
	$c = strval($i);
	$a = str_split($c);
	for ($j = 0; $j < count($a); $j++) {
		$text .= "-draw \"text 3,".($i+9 + $j*8)." '$a[$j]' \" ";;
	}
}
	
$str01 = "convert -size 16x".$height." xc:white $rule1";
$str02 = "convert -fill black -draw \"$str\" -font arial -pointsize 10 $text $rule1 $ruleV";
exec ($str01);
exec ($str02);
?>