imagesavealpha

(PHP 4 >= 4.3.2, PHP 5, PHP 7, PHP 8)

imagesavealpha — Whether to retain full alpha channel information when saving images

Description

imagesavealpha(GdImage $image, bool $enable): bool

imagesavealpha() sets the flag which determines whether to retain full alpha channel information (as opposed to single-color transparency) when saving images. This is only supported for image formats which support full alpha channel information, i.e. PNG, WebP and AVIF.

Note: imagesavealpha() is only meaningful for PNG images, since the full alpha channel is always saved for WebP and AVIF. It is not recommended to rely on this behavior, as it may change in the future. Thus, imagesavealpha() should be called deliberately also for WebP and AVIF images.

Alphablending has to be disabled (imagealphablending($im, false)) to retain the alpha-channel in the first place.

Parameters

image: A GdImage object, returned by one of the image creation functions, such as imagecreatetruecolor().
enable: Whether to save the alpha channel or not. Defaults to false.

Return Values

Returns true on success or false on failure.

Changelog

Version	Description
8.0.0	`image` expects a GdImage instance now; previously, a valid `gd` resource was expected.

Examples

Example #1 Basic imagesavealpha() Usage

<?php
// Load a png image with alpha channel
$png = imagecreatefrompng('./alphachannel_example.png');

// Turn off alpha blending
imagealphablending($png, false);

// Do desired operations

// Set alpha flag
imagesavealpha($png, true);

// Output image to browser
header('Content-Type: image/png');

imagepng($png);
imagedestroy($png);
?>

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User Contributed Notes 2 notes

down

ray hatfield ¶

13 years ago

After much trial and error and gnashing of teeth I finally figured out how to composite a png with an 8-bit alpha onto a jpg. This was not obvious to me so I thought I'd share. Hope it helps.



I'm using this to create a framed thumbnail image:



<?php

// load the frame image (png with 8-bit transparency)

$frame = imagecreatefrompng('path/to/frame.png');



// load the thumbnail image

$thumb = imagecreatefromjpeg('path/to/thumbnail.jpg');



// get the dimensions of the frame, which we'll also be using for the

// composited final image.

$width = imagesx( $frame );

$height = imagesy( $frame );



// create the destination/output image.

$img=imagecreatetruecolor( $width, $height );



// enable alpha blending on the destination image.

imagealphablending($img, true);



// Allocate a transparent color and fill the new image with it.

// Without this the image will have a black background instead of being transparent.

$transparent = imagecolorallocatealpha( $img, 0, 0, 0, 127 );

imagefill( $img, 0, 0, $transparent );



// copy the thumbnail into the output image.

imagecopyresampled($img,$thumb,32,30,0,0, 130, 100, imagesx( $thumb ), imagesy( $thumb ) );



// copy the frame into the output image (layered on top of the thumbnail)

imagecopyresampled($img,$frame,0,0,0,0, $width,$height,$width,$height);



imagealphablending($img, false);



// save the alpha

imagesavealpha($img,true);



// emit the image

header('Content-type: image/png');

imagepng( $img );



// dispose

imagedestroy($img);



// done.

exit;

?>