GD and Image 函数
在线手册：中文 英文
PHP手册

imagecolorsforindex

(PHP 4, PHP 5)

imagecolorsforindex — 取得某索引的颜色

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。

Example #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()。

GD and Image 函数
在线手册：中文 英文
PHP手册
PHP手册 - N: 取得某索引的颜色

用户评论:

hofstadler dot andi at gmx dot at (07-Oct-2008 08:36)

I have optimized the rgb2hsl function from slepichev a bit, so that it is a bit shorter and hopefully a bit faster:

<?php
/**
 * Convert RGB colors array into HSL array
 *
 * @param array $ RGB colors set, each color component with range 0 to 255
 * @return array HSL set, each color component with range 0 to 1
 */
function rgb2hsl($rgb){
    $clrR = ($rgb[0]);
    $clrG = ($rgb[1]);
    $clrB = ($rgb[2]);
    
    $clrMin = min($clrR, $clrG, $clrB);
    $clrMax = max($clrR, $clrG, $clrB);
    $deltaMax = $clrMax - $clrMin;
    
    $L = ($clrMax + $clrMin) / 510;
    
    if (0 == $deltaMax){
        $H = 0;
        $S = 0;
    }
    else{
        if (0.5 > $L){
            $S = $deltaMax / ($clrMax + $clrMin);
        }
        else{
            $S = $deltaMax / (510 - $clrMax - $clrMin);
        }

        if ($clrMax == $clrR) {
            $H = ($clrG - $clrB) / (6.0 * $deltaMax);
        }
        else if ($clrMax == $clrG) {
            $H = 1/3 + ($clrB - $clrR) / (6.0 * $deltaMax);
        }
        else {
            $H = 2 / 3 + ($clrR - $clrG) / (6.0 * $deltaMax);
        }

        if (0 > $H) $H += 1;
        if (1 < $H) $H -= 1;
    }
    return array($H, $S,$L);
}
?>

matrebatre (07-Aug-2008 07:11)

Be aware that

<?php
$rgba     = imagecolorat($image, $x, $y);
$r = ($rgba >> 16) & 0xFF;
$g = ($rgba >> 8) & 0xFF;
$b = $rgba & 0xFF;
$a     = ($rgba & 0x7F000000) >> 24;
?>

will only work for truecolor images. With eg GIF images, this will have strange results. For GIF images, you should always use imagecolorsforindex().

derek at idreams dot co dot uk (30-Nov-2007 08:45)

The earlier microsoft sepia example seemed to have a factor in which made it pinky... here is a modified example which uses just the Microsoft sepia (as per the wiki sepia entry)

<?
function imagetosepia(&$img) {
  if (!($t = imagecolorstotal($img))) {
    $t = 256;
    imagetruecolortopalette($img, true, $t);
  }
  $total = imagecolorstotal( $img );
  for ( $i = 0; $i < $total; $i++ ) {
    $index = imagecolorsforindex( $img, $i );
    $red = ( $index["red"] * 0.393 + $index["green"] * 0.769 + $index["blue"] * 0.189 );
    $green = ( $index["red"] * 0.349 + $index["green"] * 0.686 + $index["blue"] * 0.168 );
    $blue = ( $index["red"] * 0.272 + $index["green"] * 0.534 + $index["blue"] * 0.131 );
    if ($red > 255) { $red = 255; }
    if ($green > 255) { $green = 255; }
    if ($blue > 255) { $blue = 255; }
    imagecolorset( $img, $i, $red, $green, $blue );
  }
}
?>

joe dot scylla at gmail dot com (28-Nov-2007 02:35)

While it's quite easy and intuitive to get the alpha transparency of a pixel with:

<?php
$rgba     = imagecolorsforindex($image, imagecolorat($image, $x, $y));
$alpha     = $rgba["alpha"];
?>

you should use the return value of the command imagecolorat to get the alpha transparency with the code below because it's much faster and will have a major impact if you process every pixel of an image:

<?php
$rgba     = imagecolorat($image, $x, $y);
$alpha     = ($rgba & 0x7F000000) >> 24;
?>

tim at leethost dot com (10-Sep-2007 06:01)

Here's a better grayscale, sepia, and general tinting function.  This function is better because:

1) Works with true color images (the other sepia code didn't).
2) Provides a more gooder grayscale conversion (yes, I said "more gooder").  The other grayscale code used imagetruecolortopalette, which just doesn't work well for grayscale conversion.
3) The other sepia code was really colorful, a little too much for my taste.  This function allows you to optionally set the tinting of the grayscale to anything you wish.
4) Single function for grayscale, sepia, and any other tinting you can dream up.

Here's some examples:

imagegrayscaletint ($img);  // Grayscale, no tinting
imagegrayscaletint ($img,304,242,209);  // What I use for sepia
imagegrayscaletint ($img,0,0,255);  // A berry blue image

The RGB values for tinting are normally from 0 to 255.  But, you can use values larger than 255 to lighten and "burn" the image.  The sepia example above does this a little, the below example provides a better example of lightening the image and burning the light areas out a little:

imagegrayscaletint ($img,400,400,400);  // Lighten image
imagegrayscaletint ($img,127,127,127);  // Darken image

<?
function imagegrayscaletint (&$img, $tint_r = 255, $tint_g = 255, $tint_b = 255) {
  $width = imagesx($img); $height = imagesy($img);
  $dest = imagecreate ($width, $height);
  for ($i=0; $i<256; $i++) imagecolorallocate ($dest, $i, $i, $i);
  imagecopyresized ($dest, $img, 0, 0, 0, 0, $width, $height, $width, $height);
  for ($i = 0; $i < 256; $i++) imagecolorset ($dest, $i, min($i * abs($tint_r) / 255, 255), min($i * abs($tint_g) / 255, 255), min($i * abs($tint_b) / 255, 255));
  $img = imagecreate ($width, $height);
  imagecopy ($img, $dest, 0, 0, 0, 0, $width, $height);
  imagedestroy ($dest);
}
?>

slepichev at yahoo dot com (06-Sep-2007 11:50)

If you would like to change the intensity or lightness level of a specific color, you will need to convert the color format from RGB to HSL.
following function convert RGB array(red,green,blue) to HSL array(hue, saturation, lightness)
<?php
/**
 * Convert RGB colors array into HSL array
 *
 * @param array $ RGB colors set
 * @return array HSL set
 */
 function rgb2hsl($rgb){
   
     $clrR = ($rgb[0] / 255);
     $clrG = ($rgb[1] / 255);
     $clrB = ($rgb[2] / 255);
   
     $clrMin = min($clrR, $clrG, $clrB);
     $clrMax = max($clrR, $clrG, $clrB);
     $deltaMax = $clrMax - $clrMin;
   
     $L = ($clrMax + $clrMin) / 2;
   
     if (0 == $deltaMax){
         $H = 0;
         $S = 0;
         }
    else{
         if (0.5 > $L){
             $S = $deltaMax / ($clrMax + $clrMin);
             }
        else{
             $S = $deltaMax / (2 - $clrMax - $clrMin);
             }
         $deltaR = ((($clrMax - $clrR) / 6) + ($deltaMax / 2)) / $deltaMax;
         $deltaG = ((($clrMax - $clrG) / 6) + ($deltaMax / 2)) / $deltaMax;
         $deltaB = ((($clrMax - $clrB) / 6) + ($deltaMax / 2)) / $deltaMax;
         if ($clrR == $clrMax){
             $H = $deltaB - $deltaG;
             }
        else if ($clrG == $clrMax){
             $H = (1 / 3) + $deltaR - $deltaB;
             }
        else if ($clrB == $clrMax){
             $H = (2 / 3) + $deltaG - $deltaR;
             }
         if (0 > $H) $H += 1;
         if (1 < $H) $H -= 1;
         }
     return array($H, $S, $L);
     }
?>

admin at phpgfx dot com (26-Aug-2007 05:54)

this is a sepia filter using microsoft's definition

<?php

function imagesepia( $img ) {
    $total = imagecolorstotal( $img );
    for ( $i = 0; $i < $total; $i++ ) {
        $index = imagecolorsforindex( $img, $i );
        $red = ( $index["red"] * 0.393 + $index["green"] * 0.769 + $index["blue"] * 0.189 ) / 1.351;
        $green = ( $index["red"] * 0.349 + $index["green"] * 0.686 + $index["blue"] * 0.168 ) / 1.203;
        $blue = ( $index["red"] * 0.272 + $index["green"] * 0.534 + $index["blue"] * 0.131 ) / 2.140;
        imagecolorset( $img, $i, $red, $green, $blue );
    }
}

?>

adspeed.com (24-Aug-2005 12:05)

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 (25-Jun-2004 09:32)

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 (23-Apr-2004 05:23)

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);
    }
}