在TIFF中创建一个带有缩略图(libtiff）的子IFD

因此，在深入研究了 libtiff 源代码一段时间后，我在 tif_dirwrite.c 中偶然发现了这个：

 /*
 * 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);
...

（我包含了版权信息，因为我不确定在这里发布库中的代码时是否必须这样做）

所以，回答我自己的问题（如何在主图像IFD的子IFD中编写缩略图）：

//...
//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);

我希望这对某人有所帮助，并且他们不必像我那样花费那么多的努力来弄清楚如何做到这一点。libtiff的记录如此之少，这真的是一种耻辱，特别是考虑到它的使用范围有多广。

我认为@KSletmoe已经指出了一个关键点，即在编写任何子IFD之前应该在IFD0中添加SubIFD标签（330），但仍然存在问题。

SubIFD 标签的定义是"偏移到子 IFD"。因此，如果您没有正确设置偏移量，tiff 解析器将无法正确解析整个 tiff。

有两种方法可以处理这种情况。

首先，预先计算每个 IFD/SubIFD 的大小，然后在设置时填写 SubIFD 标签，而不是设置0x0。

或者您可以正常编写每个 IFD，然后返回 IFD0 并添加 SubIFD 标签，其中包含由 libtiff 计算的最终偏移量。像下面这样：

//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);    

您可能需要在 tif_dir.c 中添加流动函数

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;                                                                              
}

谢谢你的@KSletmoe。希望这可以帮助某人，并期待一些优雅的方式来做到这一点。