在图像中用透明替换多个颜色

Question

i have a rain radar image here with big grey and white parts as backgroound. i need to remove the background ( grey / white ) and make in transparent. but it does not work. i already tried

   // replace white
   $rgb = imagecolorexact($im, 255, 255, 255);
   imagecolortransparent($im, $rgb);
   // replace grey
   $rgb = imagecolorexact($im, 189, 189, 189);
   imagecolortransparent($im, $rgb);

But this is not working. Its only one part transparent ( either the white one or the grey one ). I cannot remove both colors at the same time.

I really have no idea how images work .. so please if you know a way to achieve what i want please let me know.

Thanks

Answer 1

First, make all the grey pixels white. Then make all the white pixels transparent. That's it. Read it again :-)

Refer to PHP GD documentation to check actual parameters and details.

// Load up the original image
$src=imagecreatefrompng('weather.png');

// Ensure image is palettised
if(imageistruecolor($src)){
   imagetruecolortopalette($src);
}

// Find nearest colours to white and grey 189
$whiteindex=imagecolorclosest($src,255,255,255);
$greyindex =imagecolorclosest($src,189,189,189);

// Make all greys white and all nearly whites white, and both transparent
imagecolorset($src,$greyindex,255,255,255,127);
imagecolorset($src,$whiteindex,255,255,255,127);

// Write result 
imagepng($src,"result.png");

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Note that the code you started with, and the code above is using the GD library that comes ready-installed with most PHP interpreters. You could, instead, use the IMagick library (which is the PHP binding of ImageMagick) which is much more comprehensive. Your code would become something like:

// Move to a format which supports transparency
$imagick->setimageformat('png');

// Set $color to white first
$imagick->transparentPaintImage($color, $alpha, 10 * \Imagick::getQuantum(),false);

// Set $color to grey first
$imagick->transparentPaintImage($color, $alpha, 10 * \Imagick::getQuantum(),false);