ok mi autorispondo da solo

da questo sito (anche è disponibile in molti altri) ho trovato le funzioni imagebmp e imagecraetefrombmp come se fossero direttamente disponibili dalle gd

http://www.hotscripts.com/Detailed/41077.html

per correttezza allego le funzioni

Codice PHP:
<?
/*
*------------------------------------------------------------
*                   BMP Image functions
*------------------------------------------------------------
*                      By JPEXS
*/




/*
*------------------------------------------------------------
*                    ImageBMP
*------------------------------------------------------------
*            - Creates new BMP file
*
*         Parameters:  $img - Target image
*                      $file - Target file to store
*                            - if not specified, bmp is returned
*
*           Returns: if $file specified - true if OK
                     if $file not specified - image data
*/
function imagebmp($img,$file="",$RLE=0)
{


$ColorCount=imagecolorstotal($img);

$Transparent=imagecolortransparent($img);
$IsTransparent=$Transparent!=-1;


if(
$IsTransparent$ColorCount--;

if(
$ColorCount==0) {$ColorCount=0$BitCount=24;};
if((
$ColorCount>0)and($ColorCount<=2)) {$ColorCount=2$BitCount=1;};
if((
$ColorCount>2)and($ColorCount<=16)) { $ColorCount=16$BitCount=4;};
if((
$ColorCount>16)and($ColorCount<=256)) { $ColorCount=0$BitCount=8;};


                
$Width=imagesx($img);
                
$Height=imagesy($img);

                
$Zbytek=(4-($Width/(8/$BitCount))%4)%4;

                if(
$BitCount<24$palsize=pow(2,$BitCount)*4;

                
$size=(floor($Width/(8/$BitCount))+$Zbytek)*$Height+54;
                
$size+=$palsize;
                
$offset=54+$palsize;

                
// Bitmap File Header
                
$ret 'BM';                        // header (2b)
                
$ret .= int_to_dword($size);        // size of file (4b)
                
$ret .= int_to_dword(0);        // reserved (4b)
                
$ret .= int_to_dword($offset);        // byte location in the file which is first byte of IMAGE (4b)
                // Bitmap Info Header
                
$ret .= int_to_dword(40);        // Size of BITMAPINFOHEADER (4b)
                
$ret .= int_to_dword($Width);        // width of bitmap (4b)
                
$ret .= int_to_dword($Height);        // height of bitmap (4b)
                
$ret .= int_to_word(1);        // biPlanes = 1 (2b)
                
$ret .= int_to_word($BitCount);        // biBitCount = {1 (mono) or 4 (16 clr ) or 8 (256 clr) or 24 (16 Mil)} (2b)
                
$ret .= int_to_dword($RLE);        // RLE COMPRESSION (4b)
                
$ret .= int_to_dword(0);        // width x height (4b)
                
$ret .= int_to_dword(0);        // biXPelsPerMeter (4b)
                
$ret .= int_to_dword(0);        // biYPelsPerMeter (4b)
                
$ret .= int_to_dword(0);        // Number of palettes used (4b)
                
$ret .= int_to_dword(0);        // Number of important colour (4b)
                // image data

                
$CC=$ColorCount;
                
$sl1=strlen($ret);
                if(
$CC==0$CC=256;
                if(
$BitCount<24)
                   {
                    
$ColorTotal=imagecolorstotal($img);
                     if(
$IsTransparent$ColorTotal--;

                     for(
$p=0;$p<$ColorTotal;$p++)
                     {
                      
$color=imagecolorsforindex($img,$p);
                       
$ret.=inttobyte($color["blue"]);
                       
$ret.=inttobyte($color["green"]);
                       
$ret.=inttobyte($color["red"]);
                       
$ret.=inttobyte(0); //RESERVED
                     
};

                    
$CT=$ColorTotal;
                  for(
$p=$ColorTotal;$p<$CC;$p++)
                       {
                      
$ret.=inttobyte(0);
                      
$ret.=inttobyte(0);
                      
$ret.=inttobyte(0);
                      
$ret.=inttobyte(0); //RESERVED
                     
};
                   };


if(
$BitCount<=8)
{

 for(
$y=$Height-1;$y>=0;$y--)
 {
  
$bWrite="";
  for(
$x=0;$x<$Width;$x++)
   {
   
$color=imagecolorat($img,$x,$y);
   
$bWrite.=decbinx($color,$BitCount);
   if(
strlen($bWrite)==8)
    {
     
$retd.=inttobyte(bindec($bWrite));
     
$bWrite="";
    };
   };

  if((
strlen($bWrite)<8)and(strlen($bWrite)!=0))
    {
     
$sl=strlen($bWrite);
     for(
$t=0;$t<8-$sl;$t++)
      
$sl.="0";
     
$retd.=inttobyte(bindec($bWrite));
    };
 for(
$z=0;$z<$Zbytek;$z++)
   
$retd.=inttobyte(0);
 };
};

if((
$RLE==1)and($BitCount==8))
{
 for(
$t=0;$t<strlen($retd);$t+=4)
  {
   if(
$t!=0)
   if((
$t)%$Width==0)
    
$ret.=chr(0).chr(0);

   if((
$t+5)%$Width==0)
   {
     
$ret.=chr(0).chr(5).substr($retd,$t,5).chr(0);
     
$t+=1;
   }
   if((
$t+6)%$Width==0)
    {
     
$ret.=chr(0).chr(6).substr($retd,$t,6);
     
$t+=2;
    }
    else
    {
     
$ret.=chr(0).chr(4).substr($retd,$t,4);
    };
  };
  
$ret.=chr(0).chr(1);
}
else
{
$ret.=$retd;
};


                if(
$BitCount==24)
                {
                for(
$z=0;$z<$Zbytek;$z++)
                 
$Dopl.=chr(0);

                for(
$y=$Height-1;$y>=0;$y--)
                 {
                 for(
$x=0;$x<$Width;$x++)
                        {
                           
$color=imagecolorsforindex($img,ImageColorAt($img,$x,$y));
                           
$ret.=chr($color["blue"]).chr($color["green"]).chr($color["red"]);
                        }
                 
$ret.=$Dopl;
                 };

                 };

  if(
$file!="")
   {
    
$r=($f=fopen($file,"w"));
    
$r=$r and fwrite($f,$ret);
    
$r=$r and fclose($f);
    return 
$r;
   }
  else
  {
   echo 
$ret;
  };
};


/*
*------------------------------------------------------------
*                    ImageCreateFromBmp
*------------------------------------------------------------
*            - Reads image from a BMP file
*
*         Parameters:  $file - Target file to load
*
*            Returns: Image ID
*/

function imagecreatefrombmp($file)
{
global  
$CurrentBit$echoMode;

$f=fopen($file,"r");
$Header=fread($f,2);

if(
$Header=="BM")
{
 
$Size=freaddword($f);
 
$Reserved1=freadword($f);
 
$Reserved2=freadword($f);
 
$FirstByteOfImage=freaddword($f);

 
$SizeBITMAPINFOHEADER=freaddword($f);
 
$Width=freaddword($f);
 
$Height=freaddword($f);
 
$biPlanes=freadword($f);
 
$biBitCount=freadword($f);
 
$RLECompression=freaddword($f);
 
$WidthxHeight=freaddword($f);
 
$biXPelsPerMeter=freaddword($f);
 
$biYPelsPerMeter=freaddword($f);
 
$NumberOfPalettesUsed=freaddword($f);
 
$NumberOfImportantColors=freaddword($f);

if(
$biBitCount<24)
 {
  
$img=imagecreate($Width,$Height);
  
$Colors=pow(2,$biBitCount);
  for(
$p=0;$p<$Colors;$p++)
   {
    
$B=freadbyte($f);
    
$G=freadbyte($f);
    
$R=freadbyte($f);
    
$Reserved=freadbyte($f);
    
$Palette[]=imagecolorallocate($img,$R,$G,$B);
   };




if(
$RLECompression==0)
{
   
$Zbytek=(4-ceil(($Width/(8/$biBitCount)))%4)%4;

for(
$y=$Height-1;$y>=0;$y--)
    {
     
$CurrentBit=0;
     for(
$x=0;$x<$Width;$x++)
      {
         
$C=freadbits($f,$biBitCount);
       
imagesetpixel($img,$x,$y,$Palette[$C]);
      };
    if(
$CurrentBit!=0) {freadbyte($f);};
    for(
$g=0;$g<$Zbytek;$g++)
     
freadbyte($f);
     };

 };
};


if(
$RLECompression==1//$BI_RLE8
{
$y=$Height;

$pocetb=0;

while(
true)
{
$y--;
$prefix=freadbyte($f);
$suffix=freadbyte($f);
$pocetb+=2;

$echoit=false;

if(
$echoit)echo "Prefix: $prefix Suffix: $suffix
"
;
if((
$prefix==0)and($suffix==1)) break;
if(
feof($f)) break;

while(!((
$prefix==0)and($suffix==0)))
{
 if(
$prefix==0)
  {
   
$pocet=$suffix;
   
$Data.=fread($f,$pocet);
   
$pocetb+=$pocet;
   if(
$pocetb%2==1) {freadbyte($f); $pocetb++;};
  };
 if(
$prefix>0)
  {
   
$pocet=$prefix;
   for(
$r=0;$r<$pocet;$r++)
    
$Data.=chr($suffix);
  };
 
$prefix=freadbyte($f);
 
$suffix=freadbyte($f);
 
$pocetb+=2;
 if(
$echoit) echo "Prefix: $prefix Suffix: $suffix
"
;
};

for(
$x=0;$x<strlen($Data);$x++)
 {
  
imagesetpixel($img,$x,$y,$Palette[ord($Data[$x])]);
 };
$Data="";

};

};


if(
$RLECompression==2//$BI_RLE4
{
$y=$Height;
$pocetb=0;

/*while(!feof($f))
 echo freadbyte($f)."_".freadbyte($f)."
";*/
while(true)
{
//break;
$y--;
$prefix=freadbyte($f);
$suffix=freadbyte($f);
$pocetb+=2;

$echoit=false;

if(
$echoit)echo "Prefix: $prefix Suffix: $suffix
"
;
if((
$prefix==0)and($suffix==1)) break;
if(
feof($f)) break;

while(!((
$prefix==0)and($suffix==0)))
{
 if(
$prefix==0)
  {
   
$pocet=$suffix;

   
$CurrentBit=0;
   for(
$h=0;$h<$pocet;$h++)
    
$Data.=chr(freadbits($f,4));
   if(
$CurrentBit!=0freadbits($f,4);
   
$pocetb+=ceil(($pocet/2));
   if(
$pocetb%2==1) {freadbyte($f); $pocetb++;};
  };
 if(
$prefix>0)
  {
   
$pocet=$prefix;
   
$i=0;
   for(
$r=0;$r<$pocet;$r++)
    {
    if(
$i%2==0)
     {
      
$Data.=chr($suffix%16);
     }
     else
     {
      
$Data.=chr(floor($suffix/16));
     };
    
$i++;
    };
  };
 
$prefix=freadbyte($f);
 
$suffix=freadbyte($f);
 
$pocetb+=2;
 if(
$echoit) echo "Prefix: $prefix Suffix: $suffix
"
;
};

for(
$x=0;$x<strlen($Data);$x++)
 {
  
imagesetpixel($img,$x,$y,$Palette[ord($Data[$x])]);
 };
$Data="";

};

};


 if(
$biBitCount==24)
{
 
$img=imagecreatetruecolor($Width,$Height);
 
$Zbytek=$Width%4;

   for(
$y=$Height-1;$y>=0;$y--)
    {
     for(
$x=0;$x<$Width;$x++)
      {
       
$B=freadbyte($f);
       
$G=freadbyte($f);
       
$R=freadbyte($f);
       
$color=imagecolorexact($img,$R,$G,$B);
       if(
$color==-1$color=imagecolorallocate($img,$R,$G,$B);
       
imagesetpixel($img,$x,$y,$color);
      }
    for(
$z=0;$z<$Zbytek;$z++)
     
freadbyte($f);
   };
};
return 
$img;

};


fclose($f);


};





/*
* Helping functions:
*-------------------------
*
* freadbyte($file) - reads 1 byte from $file
* freadword($file) - reads 2 bytes (1 word) from $file
* freaddword($file) - reads 4 bytes (1 dword) from $file
* freadlngint($file) - same as freaddword($file)
* decbin8($d) - returns binary string of d zero filled to 8
* RetBits($byte,$start,$len) - returns bits $start->$start+$len from $byte
* freadbits($file,$count) - reads next $count bits from $file
* RGBToHex($R,$G,$B) - convert $R, $G, $B to hex
* int_to_dword($n) - returns 4 byte representation of $n
* int_to_word($n) - returns 2 byte representation of $n
*/

function freadbyte($f)
{
 return 
ord(fread($f,1));
};

function 
freadword($f)
{
 
$b1=freadbyte($f);
 
$b2=freadbyte($f);
 return 
$b2*256+$b1;
};


function 
freadlngint($f)
{
return 
freaddword($f);
};

function 
freaddword($f)
{
 
$b1=freadword($f);
 
$b2=freadword($f);
 return 
$b2*65536+$b1;
};



function 
RetBits($byte,$start,$len)
{
$bin=decbin8($byte);
$r=bindec(substr($bin,$start,$len));
return 
$r;

};



$CurrentBit=0;
function 
freadbits($f,$count)
{
 global 
$CurrentBit,$SMode;
 
$Byte=freadbyte($f);
 
$LastCBit=$CurrentBit;
 
$CurrentBit+=$count;
 if(
$CurrentBit==8)
  {
   
$CurrentBit=0;
  }
 else
  {
   
fseek($f,ftell($f)-1);
  };
 return 
RetBits($Byte,$LastCBit,$count);
};



function 
RGBToHex($Red,$Green,$Blue)
  {
   
$hRed=dechex($Red);if(strlen($hRed)==1$hRed="0$hRed";
   
$hGreen=dechex($Green);if(strlen($hGreen)==1$hGreen="0$hGreen";
   
$hBlue=dechex($Blue);if(strlen($hBlue)==1$hBlue="0$hBlue";
   return(
$hRed.$hGreen.$hBlue);
  };

        function 
int_to_dword($n)
        {
                return 
chr($n 255).chr(($n >> 8) & 255).chr(($n >> 16) & 255).chr(($n >> 24) & 255);
        }
        function 
int_to_word($n)
        {
                return 
chr($n 255).chr(($n >> 8) & 255);
        }


function 
decbin8($d)
{
return 
decbinx($d,8);
};

function 
decbinx($d,$n)
{
$bin=decbin($d);
$sbin=strlen($bin);
for(
$j=0;$j<$n-$sbin;$j++)
 
$bin="0$bin";
return 
$bin;
};

function 
inttobyte($n)
{
return 
chr($n);
};

?>