Development ≫ PHP | 2009-10-14 02:08:39

ImageCopyMergeAlpha

The GD-Lib is a very powerfull extension for PHP.

but in one case, it really fails.

Imagine, you want to merge an image with ৺alpha channels with another one.
No magic: u can use the ৺imagecopy() function. It will look like this:

imagecopy with an alpha channel watermark

Imagine, you want to merge an image without ৺alpha channels with another one,
AND add opacity.
No magic: u can use the imagecopymerge() function. (The ৺banner has white background)

imagecopymerge using alpha of 50

But now, imagine, you want to merge an image with ৺alpha channels with another one,
AND add opacity.

In this case, the GD-Lib fails:

imagecopymerge using alpha of 50 AND and watermark with alpha channels

Background:
Each pixel on an image is defined by 4 values: red, green, blue and ৺alpha.
If you use the ৺imagecopy function, the ৺alpha value will be preserved, which means:
what IS transparent will be transparent after merging.

If you use the imagecopymerge function you can set the new ৺alpha channel, BUT:
If you set up an ৺alpha channel of 100 (nearly transparent) EACH pixel will get this ৺alpha value.
This means, that if there are pixel with a value of 127 (totally transparent), they will be visible after merging - like in the third example above.

Solution:
i wrote an function called "imagecopymergealpha" which is able to solve this problem:

imagecopymergealpha

As u can see, transparent pixel are "staying" transparent, and opaque pixels are getting "more" transparent.

And this is, where the magic happens:

 
function imagecopymergealpha(&$dest_img, &$src_img, $dest_x, $dest_y, $src_x, $src_y, $src_width, $src_height, $alpha){
  $new_src = imagecreatetruecolor($src_width, $src_height);
  imagefill($new_src,0,0, imagecolorallocatealpha($new_src,255,255,255,127));
  for ($x=0; $x < $src_width; $x++){
    for ($y=0; $y < $src_height; $y++){
      $color = imagecolorsforindex($src_img,imagecolorat($src_img,$x,$y));
      $new_alpha = $color["alpha"];
      $new_alpha += $alpha;
      if ($new_alpha > 127) $new_alpha = 127;
      $newColor = imagecolorallocatealpha($src_img,$color["red"],$color["green"],$color["blue"], $new_alpha);
      imagesetpixel($new_src,$x,$y,$newColor);
    }
  }
  imagecopy($dest_img,$new_src,$dest_x,$dest_y,$src_x,$src_y,$src_width,$src_height);
  imagedestroy($new_src);
}
 

the last parameter - which is the "total" ৺alpha channel can get values from 0 - 127, where 0 means opaque and 127 totally invisible

Update 2009-11-09: Added imagedestroy command to free memory.

Related Tags:

৺ImageCopy
৺alpha

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