How to get PNG Alpha Channel Transparency

[Guud Spelar]

This post applies to the FPDF driver only, not pdflib or ghostscript.

Currently with FPDF, if you have a PNG with embedded alpha-channel transparency, the alpha channel is ignored. I found a nice add-on class for FPDF that enables alpha-channel transparency for PNGs here: http://staff.dasdeck.de/valentin/fpdf/fpdf_alpha/. I took it apart and put it in the html2ps code.

Basically, if it runs into a PNG with an embedded alpha layer, it will make two flat PNGs and use one to mask the other. It's pretty groovy. I learned a lot by playing with this code. Both HTML2PS and FPDF are very impressive and well-made. Thanks to Derren and Konstantin and also to Valentin Schmidt, the writer of the FPDF extension I copied from.

( lines that are new have a ### after them, unless the entire function is new.)

File: pdf.fpdf.php
Class: FPDF

All the changes to this file are to the FPDF class (line 1036)

Add a var for keeping track of temp files to the FPDF class. (Around line 1092)

var $_forms;
var $_form_radios;
var $_pages;
var $tmpFiles = array(); ###

add garbage cleanup to the Close() function:

function Close() {
	//Terminate document
	if ($this->state == FPDF_STATE_COMPLETED) {
		return;
	};
	
	if ($this->page==0) {
		$this->AddPage();
	};
	
	//Close page
	$this->_endpage();
	//Close document
	$this->_enddoc();
	
	foreach($this->tmpFiles as $tmp) @unlink($tmp);	 ### 
}

Replace Image() function:

function Image($file,$x,$y,$w=0,$h=0,$typeBogus='',$link='', $isMask=false, $maskImg=0, $type='', $alpha=true)
{
	//Put an image on the page
	if(!isset($this->images[$file]))
	{
		//First use of image, get info
		if($type=='')
		{
			$pos=strrpos($file,'.');
			if(!$pos)
				$this->Error('Image file has no extension and no type was specified: '.$file);
			$type=substr($file,$pos+1);
		}
		$type=strtolower($type);
		$mqr=get_magic_quotes_runtime();
		set_magic_quotes_runtime(0);
		if($type=='jpg' || $type=='jpeg'){
			$info=$this->_parsejpg($file);
		}
		elseif($type=='png'){		
		
			$info=$this->_parsepng($file);		
							
			if ($alpha){
				$this->ImagePngWithAlpha($file,$x,$y,$w,$h,$link);
				return;
			}
		}
		else
		{
			//Allow for additional formats
			$mtd='_parse'.$type;
			if(!method_exists($this,$mtd))
				$this->Error('Unsupported image type: '.$type);
			$info=$this->$mtd($file);
		}
		set_magic_quotes_runtime($mqr);
		
		if ($isMask){
		  $info['cs']="DeviceGray"; // try to force grayscale (instead of indexed)
		}
		
		$info['i']=count($this->images)+1;
		
		if ($maskImg>0){
			$info['masked'] = $maskImg;
		}
		
		$this->images[$file]=$info;
	}
	else
	{
		$info=$this->images[$file];
	}
	
	//Automatic width and height calculation if needed
	if($w==0 && $h==0)
	{
		//Put image at 72 dpi
		$w=$info['w']/$this->k;
		$h=$info['h']/$this->k;
	}
	if($w==0)
		$w=$h*$info['w']/$info['h'];
	if($h==0)
		$h=$w*$info['h']/$info['w'];
		
	if ($isMask){
		$x = $this->fwPt + 10; // embed hidden, ouside the canvas  
	}

	$info = $this->images[$file];
				
	$this->_out(sprintf('q %.2f 0 0 %.2f %.2f %.2f cm /I%d Do Q',
					  $w*$this->k,
					  $h*$this->k,
					  $x*$this->k,
					  ($this->h-($y+$h))*$this->k,
					  $info['i']));
					  
	return $info['i'];
}

Add these two new functions after Image(): ImagePngWithAlpha() and _gamma()

function ImagePngWithAlpha($file,$x,$y,$w=0,$h=0,$link='')
{	
	$tmp_alpha = tempnam(WRITER_TEMPDIR, 'mska');
	$this->tmpFiles[] = $tmp_alpha;
	$tmp_plain = tempnam(WRITER_TEMPDIR, 'mskp');
	$this->tmpFiles[] = $tmp_plain;
	
	list($wpx, $hpx) = getimagesize($file);
	
	$img = imagecreatefrompng($file);
	$alpha_img = imagecreate( $wpx, $hpx );

	// generate gray scale pallete
	for($c=0;$c<256;$c++){
		imagecolorallocate($alpha_img, $c, $c, $c);
	}
					
	// extract alpha channel
	$xpx=0;
	while ($xpx<$wpx){
		$ypx = 0;
		while ($ypx<$hpx){
			$color_index = imagecolorat($img, $xpx, $ypx);
			$alpha = ($color_index & 0x7F000000) >> 24;
			imagesetpixel($alpha_img, $xpx, $ypx, $this->_gamma( (127-$alpha)*255/127)  );
			$ypx++;
		}
		$xpx++;
	}

	imagepng($alpha_img, $tmp_alpha);
	imagedestroy($alpha_img);
	
	// extract image without alpha channel
	$plain_img = imagecreatetruecolor ( $wpx, $hpx );
	imagecopy ($plain_img, $img, 0, 0, 0, 0, $wpx, $hpx );
	imagepng($plain_img, $tmp_plain);
	imagedestroy($plain_img);
	
	//first embed mask image (w, h, x, will be ignored)
	$maskImg = $this->Image($tmp_alpha, 0,0,0,0, 'PNG', '', true, NULL, 'PNG', false); 
	
	//embed image, masked with previously embedded mask
	$this->Image($tmp_plain,$x,$y,$w,$h,'PNG',$link, false, $maskImg, 'PNG', false);
}

// GD seems to use a different gamma, this method is used to correct it again
function _gamma($v){
	return pow ($v/255, 2.2) * 255;
}

Add a new "mask" flag for masked images in _putimages():

function _putimages()
{
	$filter=($this->compress) ? '/Filter /FlateDecode ' : '';
	reset($this->images);
	while(list($file,$info)=each($this->images))
	{
		$this->_newobj();
		$this->images[$file]['n']=$this->n;
		$this->_out('<</Type /XObject');
		$this->_out('/Subtype /Image');
		$this->_out('/Width '.$info['w']);
		$this->_out('/Height '.$info['h']);
		
		if (isset($info["masked"])) $this->_out('/SMask '.($this->n-1).' 0 R'); ###
		
		if($info['cs']=='Indexed')
			$this->_out('/ColorSpace [/Indexed /DeviceRGB '.(strlen($info['pal'])/3-1).' '.($this->n+1).' 0 R]');
		else
		{
			$this->_out('/ColorSpace /'.$info['cs']);
			if($info['cs']=='DeviceCMYK')
				$this->_out('/Decode [1 0 1 0 1 0 1 0]');
		}
		$this->_out('/BitsPerComponent '.$info['bpc']);
		if(isset($info['f']))
			$this->_out('/Filter /'.$info['f']);
		if(isset($info['parms']))
			$this->_out($info['parms']);
		if(isset($info['trns']) && is_array($info['trns']))
		{
			$trns='';
			for($i=0;$i<count($info['trns']);$i++)
				$trns.=$info['trns'][$i].' '.$info['trns'][$i].' ';
			$this->_out('/Mask ['.$trns.']');
		}
		$this->_out('/Length '.strlen($info['data']).'>>');
		$this->_putstream($info['data']);
		unset($this->images[$file]['data']);
		$this->_out('endobj');
		//Palette
		if($info['cs']=='Indexed')
		{
			$this->_newobj();
			$pal=($this->compress) ? gzcompress($info['pal']) : $info['pal'];
			$this->_out('<<'.$filter.'/Length '.strlen($pal).'>>');
			$this->_putstream($pal);
			$this->_out('endobj');
		}
	}
}

Continued in next post >>

[Guud Spelar]

Replace the 'alpha' error message in _parsepng with a string return

// Extract info from a PNG file
function _parsepng($file) {
  $f = fopen($file,'rb');
  if (!$f) {
	$this->Error('Can\'t open image file: '.$file);
  };

  //Check signature
  if (fread($f,8)!=chr(137).'PNG'.chr(13).chr(10).chr(26).chr(10)) {
	$this->Error('Not a PNG file: '.$file);
  };

  //Read header chunk
  fread($f,4);
  if (fread($f,4)!='IHDR') {
	$this->Error('Incorrect PNG file: '.$file);
  };

  $w = $this->_freadint($f);
  $h = $this->_freadint($f);
  $bpc = ord(fread($f,1));

  if ($bpc>8) {
	$this->Error('16-bit depth not supported: '.$file);
  };

  $ct=ord(fread($f,1));
  if ($ct==0) {
	$colspace='DeviceGray';
  } elseif($ct==2) {
	$colspace='DeviceRGB';
  } elseif($ct==3) {
	$colspace='Indexed';
  } else {
	//$this->Error('Alpha channel not supported: '.$file); ###
	return 'alpha'; ###
  };

  if (ord(fread($f,1))!=0) {
	$this->Error('Unknown compression method: '.$file);
  };

  if (ord(fread($f,1))!=0) {
	$this->Error('Unknown filter method: '.$file);
  };

  if (ord(fread($f,1))!=0) {
	$this->Error('Interlacing not supported: '.$file);
  };

  fread($f,4);
  $parms='/DecodeParms <</Predictor 15 /Colors '.($ct==2 ? 3 : 1).' /BitsPerComponent '.$bpc.' /Columns '.$w.'>>';

  //Scan chunks looking for palette, transparency and image data
  $pal='';
  $trns='';
  $data='';
  do {
	$n=$this->_freadint($f);
	$type=fread($f,4);
	if ($type=='PLTE') {
	  //Read palette
	  $pal=fread($f,$n);
	  fread($f,4);
	} elseif($type=='tRNS') {
	  //Read transparency info
	  $t=fread($f,$n);
	  if ($ct==0) {
		$trns=array(ord(substr($t,1,1)));
	  } elseif($ct==2) {
		$trns=array(ord(substr($t,1,1)),ord(substr($t,3,1)),ord(substr($t,5,1)));
	  } else {
		$pos=strpos($t,chr(0));
		if ($pos!==false) {
		  $trns=array($pos);
		}
	  }
	  fread($f,4);
	} elseif ($type=='IDAT') {
	  //Read image data block
	  $data.=fread($f,$n);
	  fread($f,4);
	} elseif ($type=='IEND') {
	  break;
	} else {
	  fread($f,$n+4);
	};
  } while($n);

  if ($colspace=='Indexed' && empty($pal)) {
	$this->Error('Missing palette in '.$file);
  };
  fclose($f);
  return array('w'     => $w,
			   'h'     => $h,
			   'cs'    => $colspace,
			   'bpc'   => $bpc,
			   'f'     => 'FlateDecode',
			   'parms' => $parms,
			   'pal'   => $pal,
			   'trns'  => $trns,
			   'data'  => $data);
}

File: output.fpdf.class.php
Class: OutputDriverFPDF

Change imagepng() to copy(). I'm not sure if this is the best way to do this, but it worked for me. For some reason, I don't know why, when imagepng is used at this point, it causes the PNG to lose its alpha channel. So instead of using imagepng to create a new image in the temp directory from the cached file, it just copies the file from the cache folder to the temp folder.

function _mktempimage($image) {   
	$tempnam = tempnam(WRITER_TEMPDIR, WRITER_FILE_PREFIX);

	switch ($image->get_type()) {
		case 'image/png':
			$filename = $tempnam . '.png';
			copy($image->get_filename(),$filename);	  ##
			break;
		
		case 'image/jpeg':
			default:
			$filename = $tempnam . '.jpg';
			imagejpeg($image->get_handle(), $filename);
			break;
    }

    unlink($tempnam);
    return $filename;
}

[TTrek]

Will this be available in future release?

[kevin.mcisaac]

Guud thank you so much. This was tremendously helpful