image upload script to make thumbnails and getting unwanted posterized images

I have an image upload script, and the thumbnails that its producing are posterized and of poor quality.

Im not sure which specific function in the script alters this so ill post the whole thing.

if (isset($_POST['submitted'])) { 

$idir = "images/";   // Path To Images Directory
$tdir = "images/";   // Path To Thumbnails Directory
$twidth = "300";   // Maximum Width For Thumbnail Images
$theight = "100";   // Maximum Height For Thumbnail Images 


if($_FILES["imagefile"]["size"] >= 2120000) {
  echo "Too Big";
  die();
} else {
    $imageData = @getimagesize($_FILES["imagefile"]["tmp_name"]);

    if($imageData === FALSE || !($imageData[2] == IMAGETYPE_GIF || $imageData[2] == IMAGETYPE_JPEG || $imageData[2] == IMAGETYPE_PNG)) {
      echo "Image Must Be GIF, JPG, or PNG";
      die();
    }
}

$url = $_FILES['imagefile']['name'];   // Set $url To Equal The Filename For Later Use
  if ($_FILES['imagefile']['type'] == "image/jpg" || $_FILES['imagefile']['type'] == "image/jpeg" || $_FILES['imagefile']['type'] == "image/pjpeg" || $_FILES['imagefile']['type'] == "image/png" || $_FILES['imagefile']['type'] == "image/gif") {
    $file_ext = strrchr($_FILES['imagefile']['name'], '.');   // Get The File Extention In The Format Of , For Instance, .jpg, .gif or .php

            $saltdate = date( 'U' );
            $saltuser = $_SERVER[REMOTE_ADDR];
            $saltname = md5($saltdate.$saltuser);

        $copy = copy($_FILES['imagefile']['tmp_name'], "$idir" . "$saltname" . "$file_ext");   // Move Image From Temporary Location To Permanent Location
    if ($copy) {   // If The Script Was Able To Copy The Image To It's Permanent Location
      $cfunction = 'imagecreatefromjpeg';      
      if ($_FILES['imagefile']['type'] == "image/png") {
        $cfunction = 'imagecreatefrompng';
      } else if ($_FILES['imagefile']['type'] == "image/gif") {
        $cfunction = 'imagecreatefromgif';
      } 
开发者_如何学运维      $simg = $cfunction("$idir" . "$saltname" . "$file_ext");   // Make A New Temporary Image To Create The Thumbanil From
      $currwidth = imagesx($simg);   // Current Image Width
      $currheight = imagesy($simg);   // Current Image Height
      if ($currheight > $currwidth) {   // If Height Is Greater Than Width
         $zoom = $twidth / $currheight;   // Length Ratio For Width
         $newheight = $theight;   // Height Is Equal To Max Height
         $newwidth = $currwidth * $zoom;   // Creates The New Width
      } else {    // Otherwise, Assume Width Is Greater Than Height (Will Produce Same Result If Width Is Equal To Height)
        $zoom = $twidth / $currwidth;   // Length Ratio For Height
        $newwidth = $twidth;   // Width Is Equal To Max Width
        $newheight = $currheight * $zoom;   // Creates The New Height
      }
      $dimg = imagecreate($newwidth, $newheight);   // Make New Image For Thumbnail
      imagetruecolortopalette($simg, false, 256);   // Create New Color Pallete
      $palsize = ImageColorsTotal($simg);
      for ($i = 0; $i < $palsize; $i++) {   // Counting Colors In The Image
       $colors = ImageColorsForIndex($simg, $i);   // Number Of Colors Used
       ImageColorAllocate($dimg, $colors['red'], $colors['green'], $colors['blue']);   // Tell The Server What Colors This Image Will Use
      }
      imagecopyresized($dimg, $simg, 0, 0, 0, 0, $newwidth, $newheight, $currwidth, $currheight);   // Copy Resized Image To The New Image (So We Can Save It)

                imagejpeg($dimg, "$tdir" . "thumb_" . $saltname . "$file_ext");   // Saving The Image

                        $full = "$saltname" . "$file_ext";
                        $thumb = "thumb_" . "$saltname" . "$file_ext";

      imagedestroy($simg);   // Destroying The Temporary Image
      imagedestroy($dimg);   // Destroying The Other Temporary Image
    } else {

    }
  } else {
  }


echo "<img src=images/$thumb>";
} 

Any idea what i can change to improve the quality of the pictures to keep them from being posterized?

Didn't really read your code, bust scanned it for the typical gotcha:

Don't use imagecopyresized().

Instead, use imagecopyresampled()

imagetruecolortopalette($simg, false, 256); converts a truecolor image to a palette image with at most 256 colours. Since you're saving to JPEG afterwards, this line seems rather pointless. Along with the for loop following it. Well, this entire block, really:

      imagetruecolortopalette($simg, false, 256);   // Create New Color Pallete
      $palsize = ImageColorsTotal($simg);
      for ($i = 0; $i < $palsize; $i++) {   // Counting Colors In The Image
       $colors = ImageColorsForIndex($simg, $i);   // Number Of Colors Used
       ImageColorAllocate($dimg, $colors['red'], $colors['green'], $colors['blue']);   // Tell The Server What Colors This Image Will Use
      }