imagecolorsforindex

说明

array imagecolorsforindex ( resource image, int index )

本函数返回一个具有 red，green，blue 和 alpha 的键名的关联数组，包含了指定颜色索引的相应的值。

例子 1. imagecolorsforindex() 例子
<?php // 打开一幅图像 $im = imagecreatefrompng('nexen.png'); // 取得一点的颜色 $start_x = 40; $start_y = 50; $color_index = imagecolorat($im, $start_x, $start_y); // 使其可读 $color_tran = imagecolorsforindex($im, $color_index); // 显示该颜色的值 echo '<pre>'; print_r($color_tran); echo '</pre>'; ?>
本例将输出：
Array ( [red] => 226 [green] => 222 [blue] => 252 [alpha] => 0 )

参见 imagecolorat() 和 imagecolorexact()。

add a note User Contributed Notes

adspeed.com
24-Aug-2005 07:05


To correct m4551 at abasoft dot it example:



ImageTrueColorToPalette($im,1,$t);



might give less colors than $t, so the for loop should call "$i<ImageColorsTotal($im)" instead of "$i<$t" just to be sure, or you'll get the warning: Color index [0-9] out of range

strozek(a)deas()harvard()edu
26-Jun-2004 04:32


Regarding m4551's method of conversion -- the actual CCIR-approved RGB-to-grayscale conversion is as follows:



grayscale component = 0.2125*R + 0.7154*G + 0.0721*B



(cf. CCIR Recommendation 709 for modern monitors)

m4551 at abasoft dot it
24-Apr-2004 12:23


here's a function to greyscale an image even from a truecolor source (jpeg or png).



slightly poor quality, but very fast...



function imagegreyscale(&$img, $dither=1) {    

    if (!($t = imagecolorstotal($img))) {

        $t = 256;

        imagetruecolortopalette($img, $dither, $t);    

    }

    for ($c = 0; $c < $t; $c++) {    

        $col = imagecolorsforindex($img, $c);

        $min = min($col['red'],$col['green'],$col['blue']);

        $max = max($col['red'],$col['green'],$col['blue']);

        $i = ($max+$min)/2;

        imagecolorset($img, $c, $i, $i, $i);

    }

}

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