How do I find out what the error is when Firefox says "the Image cannot be displayed because it contains errors"?

Question

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 the 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?

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;
        }
    }


    // Fonction permettant de dÈterminer la couleur du bruit et la forme du pinceau
    function noisecolor()
    {
        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;
    }


    //Adding noise: points, lines and circles random
    function bruit()
    {
        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 it 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 it 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.)

Answer 1

First make sure that no part of your code before the header() command outputs anything.

If you have any includes before your header, make sure you don't finish your PHP code with ?> because sometimes this generates a byte.

Finally try wrapping your includes inside of ob_start(); and ob_end_clean(); as this will make sure that no random bytes are cast to the output due to any other reasons (I had this happen once).

Answer 2

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

Answer 3

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? (PHP 5, Apache, FastCGI, and cPanel)