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February 1998

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1998.02.18 10:25 "tiff lib question / bug (?)", by Raymond Walsh


Heres my setup:
Version 3.4
OS: Win NT ( 4.0 servicepak 3 in case you're interested )
Compiler: VC++ 5.0
Target: Win32 ( debug )

I created the *.lib by using the *.mak file I found in the \contrib\WinNT
I know thats not a supported version, but the Windows specific aspect doesnt
seem to be my trouble... Please read on :)

i'm dabbling with this wonderful tiff library.
I have TIFFs created in Photoshop 4.0 Macintosh, saved as PC & Mac tiffs
with and without LZW ( all 4 cases )

I open the tiff,
tif = TIFFOpen( (*path), "ru" ); //read, un-memory mapped ( the
memory-mapped version crashes my machine but thats another story )

...get the height & width...

....get my memory buffer....

....and read the bits....
TIFFReadRGBAImage( tif, w, h, raster, 0 );

...and close the file

TIFFReadRGBAImage has the alpha channel as all 0xff.
Every darn pixel!
I know for a fact that there are other alpha values in there.... so i step
into code. :)

I follow it down into:
pickTileContigCase(TIFFRGBAImage* img)
switch (img->photometric)
switch (img->bitspersample)
case 8:
//ok so I have a 8 bit per sample, RGB, photometric TIFF...
if (!img->Map)    //map == 0, and my ->alpha == 0, so i get the "ignore
alpha" "put" routine... but wait darn it, the Alpha shouldnt be 0
     if (img->alpha == EXTRASAMPLE_ASSOCALPHA)
     put = putRGBAAcontig8bittile;
     else if (img->alpha == EXTRASAMPLE_UNASSALPHA)
     put = putRGBUAcontig8bittile;
     put = putRGBcontig8bittile;    <--------------------------- im getting
this one, but I want one of the other 2...

//so if I add a line ( or two ) to:
"int TIFFRGBAImageBegin(TIFFRGBAImage* img, TIFF* tif, int stop, char
emsg[1024])"...I can make the image appear the way I want it to.

in "TIFFRGBAImageBegin" ( tif_getimage.c )
theres one "switch" right away. then the following code:

    img->alpha = 0;
    TIFFGetFieldDefaulted(tif, TIFFTAG_EXTRASAMPLES,&extrasamples,
    if (extrasamples == 1)
{    //i've added a bracket here cuz of the line(s) i'm adding
   switch (sampleinfo[0])
  case EXTRASAMPLE_ASSOCALPHA: /* data is pre-multiplied */
  case EXTRASAMPLE_UNASSALPHA: /* data is not pre-multiplied */
   img->alpha = sampleinfo[0];
.........................AFTER THAT CODE I'M
 //if the alpha is still empty, but there are 4 channels, then one of those
channels is the alpha, damnit
 //so we're gunna make it one of the 2 types of alphachannels
 //photoshop4.0 Macintosh seems to make sampleinfo[0] == 0
  if( img->alpha == 0 )
   if( img->samplesperpixel == 4 )
   //this is arbitrary, take your pick...

Am I messing up your awesome library?
or have I found some obscure alphachannel related quirk?
or does photoshop generate substandard TIFF images?
Or am I an idiot who should just learn to live with it? :)

Did you get all that? If you want or need more information I can probably
explain that in more detail.

thanks for your attention