用GD PHP水印PNG图像时的局部黑色背景

Question

I have pieced together a PHP class to perform various image related functions using GD functions of PHP.

It works great for all image types. Rotate, flip, resize, crop and to a lesser extent, watermark.

All but the latter work perfectly. For example after a few changes, rotated PNG images retained their transparency whereas before they were losing that and the background turning black. Common problem, it appears. But all working now.

Where I'm still getting stuck is watermarking a PNG image with another PNG image. It appears to work fine with JPG and other images. This is the code (simplified):

public function writeWatermarkSimple()
{
    $watermarkFile = 'watermark.png';
    $watermarkImage = imagecreatefrompng($watermarkFile);

    imagealphablending($watermarkImage, false);
    imagesavealpha($watermarkImage, true);

    $imageFile = 'image.png';
    $baseImage = imagecreatefrompng($imageFile);

    imagealphablending($baseImage, false);
    imagesavealpha($baseImage, true);

    $marginH = imagesx($baseImage) - imagesx($watermarkImage);
    $marginV = imagesy($baseImage) - imagesy($watermarkImage);

    $cut = imagecreatetruecolor(imagesx($watermarkImage), imagesy($watermarkImage));
    imagecopy($cut, $baseImage, 0, 0, $marginH, $marginV, imagesx($watermarkImage), imagesy($watermarkImage));
    imagecopy($cut, $watermarkImage, 0, 0, 0, 0, imagesx($watermarkImage), imagesy($watermarkImage));

    imagecopymerge($baseImage, $cut, $marginH, $marginV, 0, 0, imagesx($watermarkImage), imagesy($watermarkImage), 80);

    if (!imagepng($baseImage, 'watermarked_image.png'))
    {
        return false;
    }

    return true;
}

This has been pieced together with various guides and advice people have given based on a similar issue. Again, working perfectly with JPG images and PNG watermarks, but not PNG & PNG.

Some example images:

http://i.imgur.com/hHRWinj.png - This is the watermark I'm using. http://i.imgur.com/6sy8Ncs.png - This is the image I'm applying the watermark to. http://i.imgur.com/ghovYLm.png - This is the end result.

The bit I find interesting is that any part of the watermark that is overlaid on a non-transparent portion of the image is working fine. Just the rest of it has the black background.

This leads me to believe I'm close, and I hope that the expertise of you fine people may lead me to the solution.

Thanks ever so for reading.

Answer 1

So, I'm not giving up on finding the correct answer to do this using GD. However, I was overjoyed to find that what needed up to 30 lines of code with GD can be achieved using much less with ImageMagick:

    $image = new Imagick();
    $image->readimage($this->_image);

    $watermark = new Imagick();
    $watermark->readimage($this->_watermark->_getImage());
    $watermark->evaluateImage(Imagick::EVALUATE_DIVIDE, 2, Imagick::CHANNEL_ALPHA);

    $image->compositeImage($watermark, imagick::COMPOSITE_OVER, $marginH, $marginV);

So this is before (with GD): http://i.imgur.com/AlS0TcO.png

And after (with ImageMagick and the code above): http://i.imgur.com/zBxlC3R.png

If anyone has an answer that is purely GD then I'd be immensely grateful.

Answer 2

Ran into some similar issues recently and while this may not exactly solve your problem, these were some useful discoveries that I made.

In my case, I have an original .jpg image and a watermark .png image. The watermark image has a fully transparent background. I wanted to specify the opacity in my script and have it change the watermark opacity before placing it on top of the origina image. Most posts out there regarding PHP watermarking assume that the original watermark .png file already has the solid watermark portion set to the correct opacity rather than changing it via the script.

1. gd didn't like a 24 bit .png and caused some goofy issues. Switching to 8 bit resolved that with gd . On the other hand, imagick works very well with a 24 bit .png and the final result seems to be better.
2. For me, using gd worked just fine if I was opening the original watermark .png and using imagecopymerge() to set the watermark transparency. If however I tried to scale the original watermark .png (which has transparent background) first, then I would get similar results as you with black or white background portion of where watermark image is. See How do I resize pngs with transparency in PHP? for a partial solution by filling the new wm image with transparent rectangle first. For me this still produced an opaque white background on the final result no matter what I tried.
3. I switched to imagick and was using setImageOpacity() to change the transparency of my watermark .png before applying it on top of my original image and I was still getting the same effect with a black background. Finally read in the PHP doc for setImageOpacity() that if the original .png has any transparent pixels and you try to lower the opacity, those pixels become opaque (black) with the new transparency applied. Instead, need to use the evaluateImage() function. This will instead evaluate each pixel's alpha channel only and divide by the specifid number.
4. I assume the black / white background issue with gd is likely due to similar ways that it treats alpha channels when scaling / combining as compared to imagick and if you want to do it all in gd you just need to find some similar way to evaluate and manipulate the alpha channel per-pixel because the "easy" ways seem to take an already transparent background and make it opaque.

So, the solution:

Assuming you want to apply your watermark at an opacity of 45% and you're using imagick , then instead of this:

$watermark->setImageOpacity(.45);

do this

$watermark->evaluateImage(Imagick::EVALUATE_DIVIDE, (1/.45), Imagick::CHANNEL_ALPHA);

You need to divide 1 by your opacity to get the demoninator by which the function will divide the alpha channel value for each pixel. In this case, 1/.45 = 2.2222 , so then the function will divide the alpha channel of each pixel by 2.2222 . This means a solid pixel (alpha of 1 ) would result in 1/2.2222 or .45 alpha or transparency when finished. Any pixels that were already transparent (alpha 0 ) would stay transparent because 0 divided by anything is always what? Zero!

After you change the watermark transparency then you can use compositeImage() to merge the watermark onto the original image.