PHP imitates GD to generate BMP pictures

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']) . "";
		}

		// 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("", $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 = "