En un TIFF crea un Sub IFD con una miniatura (libtiff)

Question

Sé que thumbnail.c incluye algún código que crea una miniatura y lo coloca en un IDF secundario, pero hay muchas cosas sucediendo en ese código (generando la miniatura, aplicando una curva de contraste, etc.) y estoy teniendo dificultades para reproducir simplemente escribiendo una miniatura. Google tampoco ha sido de ayuda.

Mi pregunta es, después de haber abierto un archivo de salida y tener un TIFF *, tengo mis datos en miniatura listos para usar (así como mis datos de imagen principal), ¿cómo los agrego de tal manera que la miniatura está en un sub IFD de la imagen principal IFD?

Answer 1

Así que después de escarbar a través del código fuente libtiff por un tiempo, me encontré con esto en 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); 
... 

(que incluía la información de derechos de autor, ya que no estaba seguro si tuviera que cuando publicar el código de la biblioteca aquí)

por lo tanto, para responder a mi propia pregunta (cómo escribir una miniatura en un sub-IFD de la imagen principal 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); 

espero que esto ayuda a alguien y no tienen que gastar tanto esfuerzo como yo para descubrir cómo hacerlo. Realmente es una pena que libtiff esté tan poco documentado, especialmente si se considera cuán ampliamente se usa.