How to check if an image has transparency using GD?

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How do I check if an image has transparent pixels with php's GD library?

It doesn't look like you can detect transparency at a glance.

The comments on the imagecolorat manual page suggest that the resulting integer when working with a true-color image can actually be shifted four times total, with the fourth being the alpha channel (the other three being red, green and blue). Therefore, given any pixel location at $x and $y, you can detect alpha using:

$rgba = imagecolorat($im,$x,$y);
$alpha = ($rgba & 0x7F000000) >> 24;
$red = ($rgba & 0xFF0000) >> 16;
$green = ($rgba & 0x00FF00) >> 8;
$blue = ($rgba & 0x0000FF);

An $alpha of 127 is apparently completely transparent, while zero is completely opaque.

Unfortunately you might need to process every single pixel in the image just to find one that is transparent, and then this only works with true-color images. Otherwise imagecolorat returns a color index, which you must then look up using imagecolorsforindex, which actually returns an array with an alpha value.

How to check if an image has transparency using GD?, How do I check if an image has transparent pixels with php's GD library? It doesn'​t look like you can detect transparency at a glance. The comments on the  On that basis, the following code will load a GIF and check that no palette entry contains transparency - rather than checking every single pixel in a very slow double loop over height and width of an image: This is a neat idea for 8-bit PNG images, but GIF doesn't support alpha channel transparency.

How to check if an image has transparency using GD?, How do I check if an image has transparent pixels with php's GD library? It doesn'​t look like you can detect transparency at a glance. The comments on the  #3.1: Transparency: Transparency of PNG images is retained using imagealphablending and imagesavealpha GD functions. The first one is here just for completeness. It enables blending alpha values between several layers. It becomes handy when we merge two images with transparency. But, here we are just flipping and rotating the images, so it was not needed.

It can be done!

I've combined all answers and comments into one function which should be fast & reliable:

function hasAlpha($imgdata) {
    $w = imagesx($imgdata);
    $h = imagesy($imgdata);

    if($w>50 || $h>50){ //resize the image to save processing if larger than 50px:
        $thumb = imagecreatetruecolor(10, 10);
        imagealphablending($thumb, FALSE);
        imagecopyresized( $thumb, $imgdata, 0, 0, 0, 0, 10, 10, $w, $h );
        $imgdata = $thumb;
        $w = imagesx($imgdata);
        $h = imagesy($imgdata);
    }
    //run through pixels until transparent pixel is found:
    for($i = 0; $i<$w; $i++) {
        for($j = 0; $j < $h; $j++) {
            $rgba = imagecolorat($imgdata, $i, $j);
            if(($rgba & 0x7F000000) >> 24) return true;
        }
    }
    return false;
}

//SAMPLE USE:
hasAlpha( imagecreatefrompng("myfile.png") );   //returns true if img has transparency

How to check if an image has transparency using GD?, How do I check if an image has transparent pixels with php's GD library? Answer 1. It doesn't look like you can detect transparency at a glance. It says if the image is opaque. If true opaque, if false, then the image has some transparency. It is relatively new v6.6.8-9, so you may need an upgrade. convert image -format "%[opaque]" info: The alternate is to turn on and extract the alpha channel. Then look at its statistics. If the mean is less than 1, then it has some transparency in it.

I know this is an old thread, but in my opinion it needs improvement since walking through a huge png by checking all pixels only to find out it is not transparent is a waste of time. So after some googleing I found Jon Fox's Blog and I improved his code with the help of the W3C PNG Specification further to be reliable, fast and have a minimum on memory imprint:

function IsTransparentPng($File){
    //32-bit pngs
    //4 checks for greyscale + alpha and RGB + alpha
    if ((ord(file_get_contents($File, false, null, 25, 1)) & 4)>0){
        return true;
    }
    //8 bit pngs
    $fd=fopen($File, 'r');
    $continue=true;
    $plte=false;
    $trns=false;
    $idat=false;
    while($continue===true){
        $continue=false;
        $line=fread($fd, 1024);
        if ($plte===false){
            $plte=(stripos($line, 'PLTE')!==false);
        }
        if ($trns===false){
            $trns=(stripos($line, 'tRNS')!==false);
        }
        if ($idat===false){
            $idat=(stripos($line, 'IDAT')!==false);
        }
        if ($idat===false and !($plte===true and $trns===true)){
            $continue=true;
        }
    }
    fclose($fd);
    return ($plte===true and $trns===true);
}

How to check if a GIF has transparency using GD?, I saw this thread and the solutions perfectly works but for PNG only. Is there a solution for checking if a GIF image has transparency in PHP-GD? Answer 1. I use exiftool to check two images, but it doesn't say whether the image is transparent. How can I check that then? Thanks. Examples are an PBM image and an PPM image $ exiftool test-001-001.pbm ExifTool Version Number : 8.60 File Name : test-001-001.pbm Directory : .

Pretty strait forward function, it will check if there is any transparent pixel in the image, if it is, it will return true.

$im = imagecreatefrompng('./transparent.png');
if(check_transparent($im)) {
    echo 'DA';
}
else {
    echo 'NU';
}

function check_transparent($im) {

    $width = imagesx($im); // Get the width of the image
    $height = imagesy($im); // Get the height of the image

    // We run the image pixel by pixel and as soon as we find a transparent pixel we stop and return true.
    for($i = 0; $i < $width; $i++) {
        for($j = 0; $j < $height; $j++) {
            $rgba = imagecolorat($im, $i, $j);
            if(($rgba & 0x7F000000) >> 24) {
                return true;
            }
        }
    }

    // If we dont find any pixel the function will return false.
    return false;
}

🤱 🤚 How to check if image has transparency with GD?, It doesn't look like you can immediately detect transparency. The comments on the imagecolorat page suggest that the final integer when working with an image​  This is a PHP Class useful if you need to add transparency to an image. Have a look at the source code and the explanations below: class.image.transparency.php

How to check if an image has transparency using Imagick in PHP , The raster file formats that support transparency are GIF, PNG, BMP, TIFF, and JPEG 2000, through either a transparent color or an alpha channel  To check if an image has transparency or not, you can use the getImageAlphaChannel method of Imagick. This method returns an integer identified as the constant of the colorspace of the image ( check the colorspace constants of image for more info ).

imagecolortransparent - Manual, If color is not specified, and the image has no transparent color, the returned Transparency is copied only with imagecopymerge() and true color images, not if(!empty($transparent_color)) /* simple check to find wether transparent color was set or not */ This function has a very strange behaviour with GD version > 2​. First use the mouse to Right-Click in the Content Browser and then from the Create Basic Asset section of the pop-up Menu, select Material. Name the Material Transparency_Material and then open it up by using the Left Mouse Button and Double-Clicking on the Material in the Content Browser.

imagesavealpha - Manual, Alphablending has to be disabled ( imagealphablending($im, false) ) to retain the alpha-channel in the first place. Load a png image with alpha channel When using the current GD methodologies, you are reading content from an image and manipulating it. By then writing that content to a brand new file, you are losing the EXIF data. For purposes when you want to retain EXIF data, it is recommended that you compile in and use the PECL Imagick extension. It has great resizing methods built right in and the EXIF data is retained.

Comments
  • What kind of image? GIF, PNG-8 or PNG-24?
  • is there any other way without GD?
  • Probably, but this specific question is all about GD. Considering your other (dupe) question, you're already using GD, so this isn't a problem.
  • What I meant is if there is an other way without GD so you don't need to check every single pixel?
  • If you have the Imagick extension, the getImageAlphaChannel method looks like it could do this. The Imagick extension is non-standard, and I'm not sure if this can be done with plain old vanilla called-through-exec Image Magick.
  • This will not work. This value indicates a particular PNG supports transparency. If the value indicates it does, you still need to check if it actually contains at least a single transparent pixel.
  • @Jongware You're absolutely right. However, I think it's a great way to avoid the overhead of checking every pixel in every image. Depending on your requirements, you can get away with guessing if the image has transparency with this function or you could use it to determine if you need to further process it.
  • a solution could be to first check with that is_alpha_png function, and only make deep search for transparent pixel if it's true
  • Not quite solid enough. I had a transparent PNG where the 25th byte was 3. I will be using @Anubis answer instead as it will be checking for PLTE and tRNS as well as per libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.2
  • file_get_contents() will copy the whole file into memory. Why not just only read byte 25?
  • what if the image is 1px by 10000px.. your function will unnecessarily resize what is still a pretty small image. i suggest you divide the width by the height and check the ratio instead.. or simply check the actual filesize.. also, if there is say a single transparent pixel you might squish it out by compressing the image to 10x10.. you may want to consider making that a bit bigger, and possibly retaining the aspect ratio. other than that, well done! +1
  • Specific mention of PNG palettes with transparency: libpng.org/pub/png/book/chapter08.html#png.ch08.div.5.2
  • Could it happen that PLTE or one of the other keywords would be spanning two chunks of 1024 bytes? Then your script would fail... Otherwise this seems to be made the accepted answer.
  • I'm not sure the answer to @TheStoryCoder's question but I've been using this script and it works quite well. The one downside is that this function will return true if the image has a transparency layer. Even if no transparent pixels are visible (or rather seen (seen through ?)). For example if you add alpha transparency to the background of a PNG and then cover that background with a solid black layer. - Which in turn. If you're using GD and trying to keep transparency, this will add some odd transparent pixels/sections to images that fit the prior described properties.
  • Correction, I was just saving the image without correctly setting the transparencies. So... Works pretty well. (Couldn't edit my original comment.)