imagewbmp()

(PHP 4 >= 4.0.1, PHP 5, PHP 7)

以 WBMP 格式将图像输出到浏览器或文件

说明

imagewbmp(resource $image[,string $filename[,int $foreground]]): bool

imagewbmp()从$image图像创建一个名为$filename的WBMP文件。$image参数是imagecreatetruecolor()的返回值。

$filename参数是可选项，如果省略，则直接将原图像流输出。通过用header()发送image/vnd.wap.wbmp的 Content-type，可以创建直接输出 WBMP 图像的 PHP 脚本。

Note:

WBMP 支持仅能用于 PHP 编译时加入了 GD-1.8 或更高版本时。

用可选的$foreground参数可以设定前景色，用imagecolorallocate()函数返回的颜色标识符。默认前景色是黑色。

参见image2wbmp()，imagepng()，imagegif()，imagejpeg()和imagetypes()。

As has been commented before, GD doesnt do a very good translation to 2-colours, especially for photos. The following routine converts to two colours, I believe using error diffusion (the algorithm's nicked off news). It's slow, but just about adequate for small images and low load. I suspect it can be made much more efficient :-)
function ImageColorFloydSteinberg($dst_img, $src_img) {
  ImageColorAllocate($dst_img, 0,0,0);
  ImageColorAllocate($dst_img, 255,255,255);
  $grey_img = ImageCreate(ImageSX($src_img), ImageSY($src_img));
  for ($a = 0; $a <= 255; $a++) ImageColorAllocate($grey_img, $a,$a,$a);
  for($x = 0; $x <= ImageSX($src_img); $x++) {
    for($y = 0; $y <= ImageSY($src_img); $y++) {
      $color = ImageColorsForIndex($src_img, ImageColorAt($src_img, $x, $y));
      $greyscale = .299 * $color["red"] + .587 * $color["green"] + .114 * $color["blue"];
      ImageSetPixel($grey_img, $x, $y, ImageColorClosest($grey_img, $greyscale, $greyscale, $greyscale));
    }
  }
  for($x = 0; $x <= ImageSX($src_img); $x++) {
    for($y = 0; $y <= ImageSY($src_img); $y++) {
      $color = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x, $y));
      if ($color["red"] > 128) {
        ImageSetPixel($dst_img, $x, $y, ImageColorClosest($dst_img,255,255,255));
        $err = $color["red"] - 255;
      } else {
        ImageSetPixel($dst_img, $x, $y, ImageColorClosest($dst_img,0,0,0));
        $err = $color["red"];
      }
      if ($x != ImageSx($src_img)) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x+1, $y));
        $newgrey = $color2["red"] + $err * 7 / 16;
        ImageSetPixel($grey_img, $x+1, $y, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      if ($x != 0) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x-1, $y));
        $newgrey = $color2["red"] + $err * 3 / 16;
        ImageSetPixel($grey_img, $x-1, $y, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      if ($y != ImageSy($src_img)) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x, $y+1));
        $newgrey = $color2["red"] + $err * 5 / 16;
        ImageSetPixel($grey_img, $x, $y+1, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      if ($x != ImageSx($src_img) && $y != ImageSy($src_img)) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x+1, $y+1));
        $newgrey = $color2["red"] + $err / 16;
        ImageSetPixel($grey_img, $x+1, $y+1, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      
    }
  }
  imagedestroy($grey_img);
}
To output your WBMP, use
ImageWBMP($final_img, "", ImageColorClosest(255,255,255));

In the previous note by -> lukeross at sys3175 dot co dot uk <- there is a coding error in the for() loops. After correcting the error, I found that this function did a great job of converting color images to 2 colors.
The following is the corrected function code:
function ImageColorFloydSteinberg($dst_img, $src_img) {
  ImageColorAllocate($dst_img, 0,0,0);
  ImageColorAllocate($dst_img, 255,255,255);
  $isx = ImageSX($src_img);
  $isy = ImageSY($src_img);
  $grey_img = ImageCreate($isx, $isy);
  for ($a = 0; $a <= 255; $a++) ImageColorAllocate($grey_img, $a,$a,$a);
  $isx -= 1; // To correct pixel count in source image width starting from 0
  $isy -= 1; // Correcting pixel count in source image height starting from 0
  for($x = 0; $x <= $isx; $x++) {
    for($y = 0; $y <= $isy; $y++) {
      $color = ImageColorsForIndex($src_img, ImageColorAt($src_img, $x, $y));
      $greyscale = .299 * $color["red"] + .587 * $color["green"] + .114 * $color["blue"];
      ImageSetPixel($grey_img, $x, $y, ImageColorClosest($grey_img, $greyscale, $greyscale, $greyscale));
    }
  }
  for($x = 0; $x <= $isx; $x++) {
    for($y = 0; $y <= $isy; $y++) {
      $color = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x, $y));
      if ($color["red"] > 128) {
        ImageSetPixel($dst_img, $x, $y, ImageColorClosest($dst_img,255,255,255));
        $err = $color["red"] - 255;
      } else {
        ImageSetPixel($dst_img, $x, $y, ImageColorClosest($dst_img,0,0,0));
        $err = $color["red"];
      }
      if ($x != $isx) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x+1, $y));
        $newgrey = $color2["red"] + $err * 7 / 16;
        ImageSetPixel($grey_img, $x+1, $y, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      if ($x != 0) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x-1, $y));
        $newgrey = $color2["red"] + $err * 3 / 16;
        ImageSetPixel($grey_img, $x-1, $y, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      if ($y != $isy) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x, $y+1));
        $newgrey = $color2["red"] + $err * 5 / 16;
        ImageSetPixel($grey_img, $x, $y+1, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      if ($x != $isx && $y != $isy) {
        $color2 = ImageColorsForIndex($grey_img, ImageColorAt($grey_img, $x+1, $y+1));
        $newgrey = $color2["red"] + $err / 16;
        ImageSetPixel($grey_img, $x+1, $y+1, ImageColorClosest($grey_img,$newgrey, $newgrey, $newgrey));
      }
      
    }
  }
  imagedestroy($grey_img);
}