PHP仿GD生成BMP图片

function imagebmp(&$im, $filename = '', $bit = 8, $compression = 0)
{
    if (!in_array($bit, array(1, 4, 8, 16, 24, 32)))
    {
        $bit = 8;
    }
    else if ($bit == 32) // todo:32 bit
    {
        $bit = 24;
    }
    $bits = pow(2, $bit);

    // 调整调色板
    imagetruecolortopalette($im, true, $bits);
    $width  = imagesx($im);
    $height = imagesy($im);
    $colors_num = imagecolorstotal($im);

    if ($bit <= 8)
    {
        // 颜色索引
        $rgb_quad = '';
        for ($i = 0; $i < $colors_num; $i ++)
        {
            $colors = imagecolorsforindex($im, $i);
            $rgb_quad .= chr($colors['blue']) . chr($colors['green']) . chr($colors['red']) . "\\0";
        }

        // 位图数据
        $bmp_data = '';

        // 非压缩
        if ($compression == 0 || $bit < 8)
        {
            if (!in_array($bit, array(1, 4, 8)))
            {
            $bit = 8;
            }
            $compression = 0;

            // 每行字节数必须为4的倍数，补齐。
            $extra = '';
            $padding = 4 - ceil($width / (8 / $bit)) % 4;
            if ($padding % 4 != 0)
            {
            $extra = str_repeat("\\0", $padding);
            }

            for ($j = $height - 1; $j >= 0; $j --)
            {
                $i = 0;
                while ($i < $width)
                {
                    $bin = 0;
                    $limit = $width - $i < 8 / $bit ? (8 / $bit - $width + $i) * $bit : 0;

                    for ($k = 8 - $bit; $k >= $limit; $k -= $bit)
                    {
                        $index = imagecolorat($im, $i, $j);
                        $bin |= $index << $k;
                        $i ++;
                    }

                    $bmp_data .= chr($bin);
                }

                $bmp_data .= $extra;
            }
        }
        // RLE8 压缩
        else if ($compression == 1 && $bit == 8)
        {
            for ($j = $height - 1; $j >= 0; $j --)
            {
                $last_index = "\\0";
                $same_num   = 0;
                for ($i = 0; $i <= $width; $i ++)
                {
                    $index = imagecolorat($im, $i, $j);
                    if ($index !== $last_index || $same_num > 255)
                    {
                        if ($same_num != 0)
                        {
                            $bmp_data .= chr($same_num) . chr($last_index);
                        }

                        $last_index = $index;
                        $same_num = 1;
                    }
                    else
                    {
                    $same_num ++;
                    }
                }

                $bmp_data .= "\\0\\0";
            }

            $bmp_data .= "\\0\\1";
        }
        $size_quad = strlen($rgb_quad);
        $size_data = strlen($bmp_data);
    }
    else
    {
    // 每行字节数必须为4的倍数，补齐。
        $extra = '';
        $padding = 4 - ($width * ($bit / 8)) % 4;
        if ($padding % 4 != 0)
        {
            $extra = str_repeat("\\0", $padding);
        }
        // 位图数据
        $bmp_data = '';
        for ($j = $height - 1; $j >= 0; $j --)
        {
            for ($i = 0; $i < $width; $i ++)
            {
                $index  = imagecolorat($im, $i, $j);
                $colors = imagecolorsforindex($im, $index);
                if ($bit == 16)
                {
                    $bin = 0 << $bit;

                    $bin |= ($colors['red'] >> 3) << 10;
                    $bin |= ($colors['green'] >> 3) << 5;
                    $bin |= $colors['blue'] >> 3;

                    $bmp_data .= pack("v", $bin);
                }
                else
                {
                    $bmp_data .= pack("c*", $colors['blue'], $colors['green'], $colors['red']);
                }

                // todo: 32bit;
            }
            $bmp_data .= $extra;
        }
        $size_quad = 0;
        $size_data = strlen($bmp_data);
        $colors_num = 0;
    }

    // 位图文件头
    $file_header = "BM" . pack("V3", 54 + $size_quad + $size_data, 0, 54 + $size_quad);

    // 位图信息头
    $info_header = pack("V3v2V*", 0x28, $width, $height, 1, $bit, $compression, $size_data, 0, 0, $colors_num, 0);

    // 写入文件
    if ($filename != '')
    {
        $fp = fopen($filename, "wb");
        fwrite($fp, $file_header);
        fwrite($fp, $info_header);
        fwrite($fp, $rgb_quad);
        fwrite($fp, $bmp_data);
        fclose($fp);        
        return true;
    }

    // 浏览器输出
    header("Content-Type: image/bmp");
    echo $file_header . $info_header;
    echo $rgb_quad;
    echo $bmp_data;
    return true;
}
//该片段来自于http://outofmemory.cn