在TIFF中创建带缩略图的子IFD(libtiff)

Question

我知道thumbnail.c包含一些创建缩略图并将其放在子IDF中的代码,但是代码中有很多(生成缩略图,应用对比曲线等),我很难再现写一个缩略图.谷歌也没有任何帮助.

我的问题是,在我打开一个输出文件并且有一个TIFF*之后,我的缩略图数据都已准备就绪(以及我的主图像数据),如何以缩略图所在的方式添加它们主图像IFD的子IFD？

Answer 1

因此,在深入了解libtiff源代码一段时间之后,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);
...

(我包含版权信息,因为我不确定在这里从库中发布代码时是否需要)

那么,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);

我希望这对某些人有所帮助,并且他们不必花费尽可能多的努力来弄清楚如何做到这一点.令人遗憾的是,诽谤的记录很差,特别是考虑到它的使用范围.