imagesavealpha

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

imagesavealpha — 保存图像时是否保留完整的 alpha 通道信息

说明

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

imagesavealpha() 设置标记，确定在保存图像时是否保存完整的 alpha 通道信息（与单一透明色相反）。这仅支持支持完整 alpha 通道信息的图像格式，即 PNG、WebP 和 AVIF。

注意: 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.

必须禁用 alpha 混合（imagealphablending($im, false)），以首先保留 alpha 通道。

参数

image: 由图象创建函数(例如imagecreatetruecolor())返回的 GdImage 对象。
enable: 是否保存透明（alpha）通道。默认 false。

返回值

成功时返回 true，或者在失败时返回 false。

更新日志

版本	说明
8.0.0	`image` 现在需要 GdImage 实例；之前需要有效的 `gd` resource。

示例

示例 #1 基础 imagesavealpha() 用法

<?php
// 载入带 alpha 通道的 png 图像
$png = imagecreatefrompng('./alphachannel_example.png');

// 关闭 alpha 混合
imagealphablending($png, false);

// Do desired operations

//并设置 alpha 标志
imagesavealpha($png, true);

// 输出图像到浏览器
header('Content-Type: image/png');

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

参见

imagealphablending() - 设定图像的混色模式

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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;

?>

down

-1

phil at unabacus dot net ¶

16 years ago


The comment left by "doggz at mindless dot com" will cause a duplication in layering of the transparent image - AlphaImageLoader loads the image as if it were a floating layer on top of the <img> element - so your image will double up.. so don't go thinking something very strange is happening with your PHP it's the silly browser ;)

The easiest (although not the best) way to get around this is to use the CSS background property instead of an image src - because as of yet you can't change an image's src dynamically using currently supported CSS:

<div style="width:200px; height:200px; background: url(my-trans-image.php); *background:url(); *filter:progid:
DXImageTransform.Microsoft.AlphaImageLoader(src='my-trans-image.php', sizingMethod='scale');"></div>

The above (although not pretty) keeps the image loaded as a background for any good browser as they should ignore the starred (*) CSS items and should support Alpha PNGs natively. IE will listen to the starred items and blank out the background whilst applying it's AlphaLoader on top. Obviously you need to know the width and height of your image but you can get this using getimagesize() or just by hardcoding.

Downsides to know:

1. Unless the user has 'backgrounds enabled when printing' your image wont show up when the webpage is printed.

2. You can't stretch or shrink a background image - if you change the div's dimensions from that of the image you will stretch it in IE (due to the 'scale' property - which you can change for sake of standardness to 'crop') but you will crop it in any other browser.

3. Most browsers treat images and backgrounds differently, in load priority and in the way the user can interact with them.

Other Options:

Other methods resort to using JavaScript or Browser Detection on the Server Side.

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