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，但该代码中有很多内容（生成缩略图，应用对比度曲线等）并且我正在使用难以复制只是写缩略图。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 *后，我的缩略图数据都已准备就绪（以及我的主要图像数据），如何以这样的方式添加它们thumbnail在主图像IFD的子IFD中？

Answer 1

所以在深入了解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);
...

（我包含了版权信息，因为我不确定在这里从库中发布代码时是否必须这样做）

那么，回答我自己的问题（如何在主图像IFD的子IFD中写缩略图）：

//...
//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，并且他们不必像我一样花费太多努力来弄清楚如何做到这一点。令人遗憾的是，诽谤的记录非常糟糕，特别是考虑到它的使用范围有多广泛。

Answer 2

我认为@KSletmoe指出了一个关键点，即在编写任何subIFD之前应在IFD0中添加SubIFD标签（330），但是仍然存在问题。

SubIFD标签的定义是“对子IFD的偏移量”。因此，如果您未正确设置偏移量，则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，然后在设置时填充SubIFD标签，而不是设置为0x0。

或者您可以正常编写每个IFD，然后返回IFD0并添加具有libtiff计算的最终偏移量的SubIFD标签。如下所示：

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType. APPLICATION_JSON);
headers.set("x-api-key", "XXXXXXX");
headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
HttpEntity<StreamsForCodecs> requestEntity = new HttpEntity<>(streams, headers);
ResponseEntity<StreamsForCodecsResponse> response = new RestTemplate().exchange(url, HttpMethod.POST, requestEntity, StreamsForCodecsResponse.class);

您可能需要在tif_dir.c中添加流动函数

//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);

感谢@KSletmoe。希望这可以对某人有所帮助，并期待一些优雅的方法。

在TIFF中创建带缩略图的子IFD（libtiff）

2 个答案: