如何在PHP中使用GD设置透明背景？

Question

We have a script that generates MP3 waveform images, and it currently uses a specified foreground and background color. What I'd like is to have the background completely transparent.

I've messed with the below script a bit but I have no idea what I'm doing. Any ideas on how to get it to where it only generates the foreground color and not the background?

 /**
       * Image generation
       */
      // how much detail we want. Larger number means less detail
      // (basically, how many bytes/frames to skip processing)
      // the lower the number means longer processing time
      define("DETAIL", 5);

      // get user vars from form
      $width = isset($_POST["width"]) ? (int) $_POST["width"] : 775;
      $height = isset($_POST["height"]) ? (int) $_POST["height"] : 122;
      $background = isset($_POST["background"]) ? $_POST["background"] : $background_color;
      $foreground = isset($_POST["foreground"]) ? $_POST["foreground"] : $foreground_color;

      // create original image width based on amount of detail
      $img = imagecreatetruecolor(sizeof($data) / DETAIL, $height);

      // fill background of image
      list($r, $g, $b) = html2rgb($background);
      imagefilledrectangle($img, 0, 0, sizeof($data) / DETAIL, $height, imagecolorallocate($img, $r, $g, $b));

      // generate background color
      list($r, $g, $b) = html2rgb($foreground);

      // loop through frames/bytes of wav data as genearted above
      for($d = 0; $d < sizeof($data); $d += DETAIL) {
        // relative value based on height of image being generated
        // data values can range between 0 and 255
        $v = (int) ($data[$d] / 255 * $height);
        // draw the line on the image using the $v value and centering it vertically on the canvas
        imageline($img, $d / DETAIL, 0 + ($height - $v), $d / DETAIL, $height - ($height - $v), imagecolorallocate($img, $r, $g, $b));
      }

      $outPngName= $outputName.".png";

Answer 1

This is what you are looking for.

    // fill background with transparent color / white
    $transColor = imagecolortransparent($img, imagecolorallocatealpha($img, 255, 255, 255, 127));
    imagefill($img, 0, 0, $transColor);