当Firefox说“因为包含错误而无法显示图像”时，如何找出错误是什么？

I have a PHP file that makes an image. I have PHP configured to report errors on the screen. When I run the file in php file in Firefox 23, it returns the Image cannot be displayed because it contains errors.

How can I find out what particular error is at fault in the image?

Extra info Here is the code

<?php

$tfont[] = 'cylburn-webfont.ttf'; 

$cryptinstall="./cryptographp.fct.php";
include $cryptinstall; 

error_reporting(E_ALL ^ E_NOTICE);
srand((double)microtime()*1000000); 

if ($_GET[$_GET['sn']]=="") unset ($_GET['sn']); 
session_start();

include('getcaptcha.php'); 
$question = getcaptcha();
$questionLen = strlen($question);
$cryptwidth  = 25 * $questionLen;

$cryptinstall2="./cryptographp.cfg.php";
include $cryptinstall; 

$cryptwidth=900;
$cryptheight=200;

// CrÈation du cryptogramme temporaire
$imgtmp = imagecreatetruecolor($cryptwidth,$cryptheight);
$blank  = imagecolorallocate($imgtmp,255,255,255);
$black   = imagecolorallocate($imgtmp,0,0,0);
imagefill($imgtmp,0,0,$blank);

$word ='';
$x = 10; 
$pair = rand(0,1);
$charnb = rand($charnbmin,$charnbmax);


for ($i=0;$i<= $questionLen;$i++) {            
     $tword[$i]['font'] =  $tfont[array_rand($tfont,1)];
     $tword[$i]['angle'] = (rand(1,2)==1)?rand(0,$charanglemax):rand(360-$charanglemax,360);

     $tword[$i]['element'] = str_split($question)[$i-1];

     $tword[$i]['size'] = rand($charsizemin,$charsizemax);
     $tword[$i]['y'] = ($charup?($cryptheight/2)+rand(0,($cryptheight/5)):($cryptheight/1.5));
     $word .=$tword[$i]['element'];

     $lafont="fonts/".$tword[$i]['font'];
     imagettftext($imgtmp,$tword[$i]['size'],$tword[$i]['angle'],$x,$tword[$i]['y'],$black,$lafont,$tword[$i]['element']);

     $x +=$charspace;
     } 

// Calculate horizontal racadrage temporary cryptogram
$xbegin=0;
$x=0;
while (($x<$cryptwidth)and(!$xbegin)) {
     $y=0;
     while (($y<$cryptheight)and(!$xbegin)) {
           if (imagecolorat($imgtmp,$x,$y) != $blank) $xbegin = $x;
           $y++;
           }
     $x++;
     } 

$xend=0;
$x=$cryptwidth-1;
while (($x>0)and(!$xend)) {
     $y=0;
     while (($y<$cryptheight)and(!$xend)) {
           if (imagecolorat($imgtmp,$x,$y) != $blank) $xend = $x;
           $y++;
           }
     $x--;
     } 

$xvariation = round(($cryptwidth/2)-(($xend-$xbegin)/2));
imagedestroy ($imgtmp);

// Create the final cryptogram
// Create the background
$img = imagecreatetruecolor($cryptwidth,$cryptheight); 

if ($bgimg and is_dir($bgimg)) {
                    $dh  = opendir($bgimg);
                    while (false !== ($filename = readdir($dh))) 
                          if(eregi(".[gif|jpg|png]$", $filename))  $files[] = $filename;
                    closedir($dh);
                    $bgimg = $bgimg.'/'.$files[array_rand($files,1)];
                    }
if ($bgimg) {
            list($getwidth, $getheight, $gettype, $getattr) = getimagesize($bgimg);
                $imgread = imagecreatefrompng($bgimg); break;
                imagecopyresized ($img, $imgread, 0,0,0,0,$cryptwidth,$cryptheight,$getwidth,$getheight);
                imagedestroy ($imgread);
            }
            else {
                 $bg = imagecolorallocate($img,$bgR,$bgG,$bgB);
                 imagefill($img,0,0,$bg);
                 if ($bgclear) imagecolortransparent($img,$bg);
                 }


function ecriture()
{
// CrÈation de l'Ècriture
global  $img, $ink, $charR, $charG, $charB, $charclear, $xvariation, $charnb, $charcolorrnd, $charcolorrndlevel, $tword, $charspace;
$ink = imagecolorallocatealpha($img,$charR,$charG,$charB,$charclear);

global $question;
global $questionLen;

$x = $xvariation;
for ($i=1;$i<$questionLen;$i++) {       

    if ($charcolorrnd){   // Choisit des couleurs au hasard
       $ok = false;
       do {
          $rndcolor = 0;
          switch ($charcolorrndlevel) {
                 case 1  : if ($rndcolor<200) 
                 $ok=true; break; // tres sombre
                 default : $ok=true;               
                 }
          } while (!$ok);

      $rndink = imagecolorallocatealpha($img,$rndR,$rndG,$rndB,$charclear);

    }  

    $lafont="fonts/".$tword[$i]['font'];
    imagettftext($img,$tword[$i]['size'],$tword[$i]['angle'],$x,$tword[$i]['y'],$charcolorrnd?$rndink:$ink,$lafont,$tword[$i]['element']);

    $x +=$charspace;
    } 
}


function noisecolor()
// Fonction permettant de dÈterminer la couleur du bruit et la forme du pinceau
 {
 global $img, $noisecolorchar, $ink, $bg, $brushsize;
 switch ($noisecolorchar) {
         case 1  : $noisecol=$ink; break;
         case 2  : $noisecol=$bg; break;
         case 3  : 
         default : $noisecol=imagecolorallocate ($img,0,0,0); break;               
         }
 if ($brushsize and $brushsize>1 and function_exists('imagesetbrush')) {
    $brush = imagecreatetruecolor($brushsize,$brushsize);
    imagefill($brush,0,0,$noisecol);
    imagesetbrush($img,$brush);
    $noisecol=IMG_COLOR_BRUSHED;
    }
 return $noisecol;    
}


function bruit()
//Adding noise: points, lines and circles random
{
global $noisepxmin, $noisepxmax, $noiselinemin, $noiselinemax, $nbcirclemin, $nbcirclemax,$img, $cryptwidth, $cryptheight;
$nbpx = rand($noisepxmin,$noisepxmax);
$nbline = rand($noiselinemin,$noiselinemax);
for ($i=1;$i<$nbpx;$i++) imagesetpixel ($img,rand(0,$cryptwidth-1),rand(0,$cryptheight-1),noisecolor());
for ($i=1;$i<=$nbline;$i++) imageline($img,rand(0,$cryptwidth-1),rand(0,$cryptheight-1),rand(0,$cryptwidth-1),rand(0,$cryptheight-1),noisecolor());
} 


if ($noiseup) {
   ecriture();
   bruit();
   } else {
          bruit();
          ecriture();
          }


// Create the frame
if ($bgframe) {
   $framecol = imagecolorallocate($img,($bgR*3+$charR)/4,($bgG*3+$charG)/4,($bgB*3+$charB)/4);
   imagerectangle($img,0,0,$cryptwidth-1,$cryptheight-1,$framecol);
   }


// Transformations supplÈmentaires: Grayscale et Brouillage
// VÈrifie si la fonction existe dans la version PHP installÈe
if ($cryptgrayscal) imagefilter ( $img,IMG_FILTER_GRAYSCALE);
if ($cryptgaussianblur) imagefilter ( $img,IMG_FILTER_GAUSSIAN_BLUR);

    /* if i comment out these lines, then the file runs fine, but of course does not return an image
    header("Content-type: image/png"); //this is the only header(...) that is included in the file. I am requesting the file directly with the URL.
    imagepng($img);

    imagedestroy ($img);
    unset ($word,$tword);
    unset ($_SESSION['cryptreload']); */

    ?>

If I comment out the header("content-type: image/png"), it gets rid of the error but returns nonsense to the browser that looks like this:

(All this said, I am less interested in fixing this particular image file than in understanding how to go about debugging errors/unexpected behavior in image-creation files. I want to learn how to fix problematic image-generating files on my own.)

Comment out the header() so it just sends back data and not image/png.

Read your PHP error log and you will find out the exact error. Most probably PHP is throwing warning that's the reason you are seeing this error on your browser.

Perhaps your php script messed up your images. Logical errors can't be reported, you have to find it by yourself. To turn on regular error reporting check out this link : How do I enable error reporting in PHP?

To enable error logging Where does PHP store the error log? (php5, apache, fastcgi, cpanel)

That's a trick question, isn't it?

Your script contains the line

echo('here!'); //prints here to the screen

This will prepend the ASCII string "here" to the PNG data stream, thus ruining the image.

Hint: If you download the result of your script (using e.g. wget, or simply "Save page" in Firefox), and open the file in an editor, you'll quickly see the problem.

In an ASP.Net CORE application you may be tempted to put picture files in a folder called Images under the solution, but you need to put them in the wwwroot/images folder: