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  • imagecolorallocatealpha()

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

    为一幅图像分配颜色+ alpha

    说明

    imagecolorallocatealpha(resource $image,int $red,int $green,int $blue,int $alpha): int

    imagecolorallocatealpha()的行为和imagecolorallocate()相同,但多了一个额外的透明度参数$alpha,其值从01270表示完全不透明,127表示完全透明。

    如果分配失败则返回FALSE

    使用imagecolorallocatealpha()的例子

    <?php
    $size = 300;
    $image=imagecreatetruecolor($size, $size);
    // 用白色背景加黑色边框画个方框
    $back = imagecolorallocate($image, 255, 255, 255);
    $border = imagecolorallocate($image, 0, 0, 0);
    imagefilledrectangle($image, 0, 0, $size - 1, $size - 1, $back);
    imagerectangle($image, 0, 0, $size - 1, $size - 1, $border);
    $yellow_x = 100;
    $yellow_y = 75;
    $red_x    = 120;
    $red_y    = 165;
    $blue_x   = 187;
    $blue_y   = 125;
    $radius   = 150;
    // 用 alpha 值分配一些颜色
    $yellow = imagecolorallocatealpha($image, 255, 255, 0, 75);
    $red    = imagecolorallocatealpha($image, 255, 0, 0, 75);
    $blue   = imagecolorallocatealpha($image, 0, 0, 255, 75);
    // 画三个交迭的圆
    imagefilledellipse($image, $yellow_x, $yellow_y, $radius, $radius, $yellow);
    imagefilledellipse($image, $red_x, $red_y, $radius, $radius, $red);
    imagefilledellipse($image, $blue_x, $blue_y, $radius, $radius, $blue);
    // 不要忘记输出正确的 header!
    header('Content-type: image/png');
    // 最后输出结果
    imagepng($image);
    imagedestroy($image);
    ?>
    

    Note:此函数需要 GD 2.0.1 或更高版本(推荐 2.0.28 及更高版本)。

    参见imagecolorallocate()和imagecolordeallocate()。

    参数

    $image

    由图象创建函数(例如imagecreatetruecolor())返回的图象资源。

    $red

    红色成分的值。

    $green

    绿色成分的值。

    $blue

    蓝色成分的值。

    $alpha

    A value between0and127.0indicates completely opaque while127indicates completely transparent.

    The colors parameters are integers between 0 and 255 or hexadecimals between 0x00 and 0xFF.

    返回值

    A color identifier or FALSE if the allocation failed.

    Warning

    此函数可能返回布尔值FALSE,但也可能返回等同于FALSE的非布尔值。请阅读布尔类型章节以获取更多信息。应使用===运算符来测试此函数的返回值。

    更新日志

    版本说明
    Prior to 5.1.3 Returns -1 if the allocation failed.

    范例

    Example of using imagecolorallocatealpha()

    <?php
    $size = 300;
    $image=imagecreatetruecolor($size, $size);
    // something to get a white background with black border
    $back = imagecolorallocate($image, 255, 255, 255);
    $border = imagecolorallocate($image, 0, 0, 0);
    imagefilledrectangle($image, 0, 0, $size - 1, $size - 1, $back);
    imagerectangle($image, 0, 0, $size - 1, $size - 1, $border);
    $yellow_x = 100;
    $yellow_y = 75;
    $red_x    = 120;
    $red_y    = 165;
    $blue_x   = 187;
    $blue_y   = 125;
    $radius   = 150;
    // allocate colors with alpha values
    $yellow = imagecolorallocatealpha($image, 255, 255, 0, 75);
    $red    = imagecolorallocatealpha($image, 255, 0, 0, 75);
    $blue   = imagecolorallocatealpha($image, 0, 0, 255, 75);
    // drawing 3 overlapped circle
    imagefilledellipse($image, $yellow_x, $yellow_y, $radius, $radius, $yellow);
    imagefilledellipse($image, $red_x, $red_y, $radius, $radius, $red);
    imagefilledellipse($image, $blue_x, $blue_y, $radius, $radius, $blue);
    // don't forget to output a correct header!
    header('Content-Type: image/png');
    // and finally, output the result
    imagepng($image);
    imagedestroy($image);
    ?>
    

    以上例程的输出类似于:

    注释

    Note:此函数需要 GD 2.0.1 或更高版本(推荐 2.0.28 及更高版本)。

    参见

    If you only wish to extract the alpha value for a color, you can simply extract it like so:
    <?php
      $color = imagecolorat($im, 50, 50);
      $alpha = $color >> 24;
    ?>
    It actually shifts off the first 24 bits (where 8x3 are used for each color), and returns the remaining 7 allocated bits (commonly used for alpha)
    If you need to calculate the integer representation of a color with an alpha channel, without initialising an image and using the imagecolorallocatealpha function. Then this function might be of some help:
    <?php
    function alphaColor($hexColor,$alpha)
    {
        return bindec(decbin($alpha).decbin(hexdec($hexColor));
    }
    echo alphaColor("FFFFFF",127);
    ?>
    
    When working with transparency, avoid imagecreate() and use imagecreatetruecolor() instead. Transparency effects may not work as expected within a palette-based image.