在TIFF中创建带缩略图的子IFD（libtiff）

Question

I know thumbnail.c includes some code that creates a thumbnail and places it in a sub IDF, but there is a lot going on in that code (generating the thumbnail, applying a contrast curve, etc.) and I am having difficulty reproducing just writing a thumbnail. Google has not been any help either.

My question is, after I have opened an output file and have a TIFF*, I have my thumbnail data all ready to go (as well as my main image data), how do I add them in such a way that the thumbnail is in a sub IFD of the main image IFD?

Answer 1

So after digging around through the libtiff source code for a while, I stumbled across this in tif_dirwrite.c:

 /*
 * Copyright (c) 1988-1997 Sam Leffler
 * Copyright (c) 1991-1997 Silicon Graphics, Inc.
 *
 * Permission to use, copy, modify, distribute, and sell this software and
 * its documentation for any purpose is hereby granted without fee, provided
 * that (i) the above copyright notices and this permission notice appear in
 * all copies of the software and related documentation, and (ii) the names of
 * Sam Leffler and Silicon Graphics may not be used in any advertising or
 * publicity relating to the software without the specific, prior written
 * permission of Sam Leffler and Silicon Graphics.
 */

...
if (!n)
    return(0);
/*
 * Total hack: if this directory includes a SubIFD
 * tag then force the next <n> directories to be
 * written as ``sub directories'' of this one.  This
 * is used to write things like thumbnails and
 * image masks that one wants to keep out of the
 * normal directory linkage access mechanism.
 */
tif->tif_flags|=TIFF_INSUBIFD;
tif->tif_nsubifd=tif->tif_dir.td_nsubifd;
if (tif->tif_dir.td_nsubifd==1)
    tif->tif_subifdoff=0;
else
    tif->tif_subifdoff=m;
return(1);
...

(I included the copyright info because I wasn't sure if I had to when posting code from the library here)

So, to answer my own question (how to write a thumbnail in a sub-IFD of the main image IFD):

//...
//For the sake of this demo we will assume that I have opened a 
//TIFF (TIFF* created_TIFF) in write mode and have included the correct header
//files

//set all of your TIFF fields for the main image
//...

//Define the number of sub-IFDs you are going to write
//(assuming here that we are only writing one thumbnail for the image):
int number_of_sub_IFDs = 1;
toff_t sub_IFDs_offsets[1] = { 0UL };

//set the TIFFTAG_SUBIFD field:
if(!TIFFSetField(created_TIFF, TIFFTAG_SUBIFD, number_of_sub_IFDs, 
    sub_IFDs_offsets))
{
    //there was an error setting the field
}

//Write your main image raster data to the TIFF (using whatever means you need,
//such as TIFFWriteRawStrip, TIFFWriteEncodedStrip, TIFFWriteEncodedTile, etc.)
//...

//Write your main IFD like so:
TIFFWriteDirectory(created_TIFF);

//Now here is the trick: like the comment in the libtiff source states, the 
//next n directories written will be sub-IFDs of the main IFD (where n is 
//number_of_sub_IFDs specified when you set the TIFFTAG_SUBIFD field)

//Set up your sub-IFD
if(!TIFFSetField(created_TIFF, TIFFTAG_SUBFILETYPE, FILETYPE_REDUCEDIMAGE))
{
    //there was an error setting the field
}

//set the rest of the required tags here, as well as any extras you would like
//(remember, these refer to the thumbnail, not the main image)
//...

//Write this sub-IFD:
TIFFWriteDirectory(created_TIFF);

//Assuming you are only writing one sub-IFD and are done with the file, you 
//can close it now. If you specified more than one sub-IFD, you need repeat 
//the above code (starting where we set TIFFTAG_SUBFILETYPE) for each of your
//sub-IFDs
TIFFClose(created_TIFF);

I hope that this helps somebody and that they don't have to expend as much effort as I did to figure out how to do this. It really is a shame that libtiff is so poorly documented, especially considering how widely it is used.

Answer 2

I think @KSletmoe has pointed out a key point that the SubIFD tag(330) should be added in IFD0 before you write any subIFD, but there is still a problem.

The definition of SubIFD tag is "Offset to child IFDs". So, if you don't set the offset correctly, tiff parser can not parse the whole tiff correctly.

There are two ways to handle this situation.

First, pre-calculate the size of each IFD/SubIFD, then fill-in to SubIFD tag when you set, instead of setting 0x0.

Or you can write every IFD as normal then go back to IFD0 and add SubIFD tag with final offset that calculated by libtiff. Like following:

//Write down every thing you need in a tiff
// ... IFD0
TIFFWriteDirectory(tif);
// ... IFD1
TIFFWriteDirectory(tif);
// ... IFD2
TIFFWriteDirectory(tif);

//set current Dir as IFD0 in the end
TIFFSetDirectory(tif, 0);  

//get next dir offset
sub_offset[1] = TIFFGetNextDirOff(tif, 2);
sub_offset[0] = TIFFGetNextDirOff(tif, 1);
//only for clean next dir offset in IFD0
no_use_offset = TIFFGetNextDirOff(tif, 0);                                                    

TIFFSetField(tif, TIFFTAG_SUBIFD, 2, sub_offset);    

You may need to add flowing function in tif_dir.c

uint64 TIFFGetNextDirOff(TIFF* tif, uint16 dirn)                                                 
{                                                                                                
    uint64 nextdir;                                                                              
    uint16 n;                                                                                    
    if (!(tif->tif_flags&TIFF_BIGTIFF))                                                          
        nextdir = tif->tif_header.classic.tiff_diroff;                                           
    else                                                                                         
        nextdir = tif->tif_header.big.tiff_diroff;                                               
    for (n = dirn; n > 0 && nextdir != 0; n--)                                                   
        if (!TIFFAdvanceDirectory(tif, &nextdir, NULL))                                          
            return (0);

    /*!!Watchout!! Here will reset the next dir offset to 0*/
    tif->tif_nextdiroff = 0;                                                                                                                                     
    return nextdir;                                                                              
}

Thanks for @KSletmoe. Hope this can help someone, and looking forward some much elegant way to do so.