在TIFF中，创建带有缩略图的Sub IFD(libtiff) [英] In a TIFF create a Sub IFD with thumbnail (libtiff)

问题描述

我知道thumbnail.c包含一些创建缩略图并将其放置在子IDF中的代码，但是该代码中有很多事情(生成缩略图，应用对比度曲线等)，我正在仅写缩略图很难复制. Google也没有任何帮助.

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.

我的问题是，在打开输出文件并具有TIFF *之后，我已经准备好要使用的缩略图数据(以及我的主图像数据)，如何以这样的方式添加它们:缩略图位于主图像IFD的子IFD中?

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?

推荐答案

因此，在研究libtiff源代码一段时间后，我在tif_dirwrite.c中偶然发现了这一点:

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);
...

(我包括了版权信息，因为不确定在从库中发布代码时是否必须这样做)

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

因此，要回答我自己的问题(如何在主图像IFD的子IFD中写入缩略图):

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);

我希望这对某人有帮助，并且他们不必像我那样花很多精力去弄清楚该怎么做.如此薄弱的文档记录真的很可惜，尤其是考虑到它的使用范围.

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.

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