imagecolorat()

(PHP 4, PHP 5, PHP 7)

取得某像素的颜色索引值

说明

返回$image所指定的图形中指定位置像素的颜色索引值。

如果 PHP 编译时加上了 GD 库 2.0 或更高的版本并且图像是真彩色图像，则本函数以整数返回该点的 RGB 值。用移位加掩码来取得红，绿，蓝各自成分的值：

取得各自的 RGB 值

参见imagecolorset()和imagecolorsforindex()。

I made a function that calculates the average color of a given image resource and returns it in "#rrggbb" format (hex):
function average($img) {
  $w = imagesx($img);
  $h = imagesy($img);
  $r = $g = $b = 0;
  for($y = 0; $y > 16;
      $g += $rgb >> 8 & 255;
      $b += $rgb & 255;
    }
  }
  $pxls = $w * $h;
  $r = dechex(round($r / $pxls));
  $g = dechex(round($g / $pxls));
  $b = dechex(round($b / $pxls));
  if(strlen($r) 
As creamdog noted before, the alpha channel IS available from this function! (The manual should probably be updated to include this!)
$rgba = imagecolorat($im,$x,$y);
$alpha = ($rgba & 0x7F000000) >> 24;
$alpha will then contain the TRANSPARENCY (NOT OPACITY) level. So 127, the max, would be completely transparent, and 0 would be completely opaque.
Using this information, it is possible to write a dithering png-to-gif function like the completely working simple one below: 
trimImage ( resource $image , int $colour , int $tolerance )
trimImage() will return top-most, right-most, bottom-most and left-most positions of wanted pixels for an image (i.e. find minimum image area so you can then trim given colour from the outer edges of an image).
Parameters
image
An image resource, returned by one of the image creation functions, such as imagecreatetruecolor().
colour
colour to be 'trimmed'. Allowable range 0 (black) to 255 (white). if null (or outside 0 - 255) will use top/left corner colour as default.
tolerance
Acceptable range +- from colour. 0 (trim only exact colour) to 255 (trim all colours).
Return Values
Returns an array of outermost pixels which are outside the tolerance range from colour. 
array( int $top, int $right, int $bottom, int $left )
Example (and actual function) 
 
 
Look mom, no tables :)
I made some changes to the code from 'hazard AT krankteil DOTTILLYDO de' so the function would output a div that displays the image.
As for the size of the outputted file I can say the original png file was lots smaller, but maybe its a nice feature for small buttons or such.
The way you can use it is the same as the code from 'hazard AT krankteil DOTTILLYDO de'.
litle note: each div contains a bogus image. When this is not in IE will screw up the output. 
This improves upon to my previous function (which only really worked on a few greyscale colours). Colour should now be a hexadecimal colour value. Colour and tolerance are now optional parameters.
trimImage ( resource $image [, str $colour [, int $tolerance]] )
trimImage() will return top-most, right-most, bottom-most and left-most positions of wanted pixels for an image (i.e. find minimum image area so you can then trim given colour from the outer edges of an image).
Parameters
image
An image resource, returned by one of the image creation functions, such as imagecreatetruecolor().
colour
Hexadecimal value of colour to be 'trimmed'. If omitted the top/left corner colour will be used as default.
tolerance
Acceptable range +- from colour. Allowable range 0 to 255. If omitted 10 will be used as default.
Return Values
Returns an array of outermost pixels which are beyond the tolerance range from colour. 
array( int $top, int $right, int $bottom, int $left )
function trimImage($im,$c=null,$t=10) {
 // if hex colour ($c) exists attempt to convert to decimal
 if ($c) $rgb = @hexdec($c);
 // if hexdec failed to get a value between black (0) and white (16777215)
 // grab the colour from the top left corner (2 pixels in to avoid messy edges)
 if (!is_numeric($rgb) || $rgb  16777215) $rgb = imagecolorat($im, 2, 2); 
 // split $rgb into red, green and blue
 $r = ($rgb >> 16) & 0xFF;
 $g = ($rgb >> 8) & 0xFF;
 $b = $rgb & 0xFF;
 // make sure tolerance ($t) is a number between 0 - 255
 if (!is_numeric($t) || $t  255) $t = 255;
 $w = imagesx($im); $h = imagesy($im); // image width and height
 for($x = 0; $x > 16) & 0xFF;
   $grn = ($rgb >> 8) & 0xFF;
   $blu = $rgb & 0xFF;
   if (
    $red  $r+$t || // not trim red (nor within tolerance) 
    $grn  $g+$t || // not trim green (nor within tolerance) 
    $blu  $b+$t // not trim blue (nor within tolerance)
   ) {
    $y_axis[$y] = $y; $x_axis[$x] = $x; // wanted pixel coordinates stored
   }
  }
 }
 if (!$y_axis) $y_axis = $x_axis = array(0); // avoid errors if all pixels are trimmed
 // sort so first and last occurances are at start and end
 sort($y_axis); sort($x_axis); 
 $t = array_shift($y_axis); // first wanted pixel on Y axis (top)
 $r = array_pop($x_axis); // last wanted pixel on X axis (right)
 $b = array_pop($y_axis); // last wanted pixel on Y axis (bottom)
 $l = array_shift($x_axis); // first wanted pixel on X axis (left)
 return array($t,$r,$b,$l);
}
imagecolorat() works differently for png's with true color and for paletted png's - for true color it returns value of color, for paletted it returns index number and you have to use imagecolorsforindex() to get rgb color value.
In GD 2.x there is support for true color images complete with an alpha channel. GD 2.x has a 7-bit (0-127) alpha channel.
While most people are used to an 8-bit (0-255) alpha channel, it is actually quite handy that GD's is 7-bit (0-127). Each pixel is represented by a 32-bit signed integer, with the four 8-bit bytes arranged like this:
 High Byte  Low Byte
{Alpha Channel} {Red} {Green} {Blue}
For a signed integer, the leftmost bit, or the highest bit, is used to indicate whether the value is negative, thus leaving only 31 bits of actual information. PHP's default integer value is a signed long into which we can store a single GD palette entry. Whether that integer is positive or negative tells us whether antialiasing is enabled for that palette entry.
Here is a contribution for change tint.
function colorize($path_image, $red, $green, $blue)
{
   $im = imagecreatefrompng($path_image);
  $pixel = array();
    
  $n_im = imagecreatetruecolor(imagesx($im),imagesy($im));
  $fond = imagecolorallocatealpha($n_im, 255, 255, 255, 0);
  imagefill($n_im, 0, 0, $fond);
  
  for($y=0;$ypx; height: px; position: absolute; display: block;} 

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