TIFF Technical Note 1: TIFF Trees   
An Aldus TIFF Technical Note:  1 June 1993

---Motivation---
TIFF has always supported what amounts to a singly linked list of IFDUs in a single
TIFF file, via the Rnext IFD pointer,S though most applications currently ignore any
IFD beyond the first one.

Probably the best use for a linked list of IFDUs is when you want to store multiple
different but related images in the same file. The classic example is multiple pages of
a fax transmission.

But suppose we want to define an image mask for an image. (This is defined within the
TIFF spec, but currently rarely used.) That would mean two IFDUs in the file, one for
the main image and one for the image mask. Perhaps the image is really large, so that
we would like to include a lower resolution version (or RsubimageS) of the image; if
so, we must include a lower resolution version of the image mask, too. Now we have four
IFDUs in our TIFF file. The order of the files in the linked list is nowhere defined in
the TIFF spec, so the TIFF reader has to be careful that it doesnUt try to use the
wrong subimage for the wrong purpose. The situation is still workable, but is starting
to get complicated.
LetUs complicate the scenario further by saying that we want to include subimages and
masks for each of the pages in a multiple-page fax transmission. Now things are really
messy. Which subimage goes with which main image? We could make a list of rules, but
they would be quite arbitrary and difficult to use.

---Solution---
If only we had the concept of a tree within a TIFF file, we would have a natural way to
associate a main or RparentS image with a subordinate or RchildS image such as a
reduced resolution version or an image mask. 

One way to create a tree structure within a TIFF file is to define a new SubIFDs tag,
of type LONG. Each LONG value points to a RchildS IFD structure. 

This method is safe because old TIFF readers will not recognize the new tag, and will
simply ignore it. It is versatile because there is no built-in restriction concerning
what you store in a child IFDQa feature that may come in handy for future private and
public enhancements.

So when should you use the NextIFD pointer at the end of an IFD and when should you use
the SubIFDs tag? Use the NextIFD pointer if your application requires that multiple
visually unrelated images be stored in the same TIFF fileQa multi-page fax
transmission, for example. Use the SubIFDs tag for pointing to images that modify or
add information to or otherwise RhelpS the Parent imageQtransparency masks and
subsampled versions, for example.

No particular order or precedence is defined for child images. For example, if we have
two child images, such as a transparency mask and a subsampled version, they can appear
in either order.

New Tag

SubIFDs
Tag	= 330 (14A)
Type	= LONG
N	= number of child IFDs

Each LONG value is an offset (from the beginning of the TIFF file, as always) to a
child IFD. Child images provide extra information for the parent imageQsuch as a
transparency mask or subsampled version of the parent image.


---Examples---
LetUs revisit our first example. We have a main image, a mask for the main image, a low
res subimage, and a mask for the low res subimage. The IFDs in our file would look like
this:

              Main Image
                       |
                       QQQQQQ
                       |                       |
     LowRes Child              Mask Child
               |
 LowRes Mask Child

The SubIFDs tag is used in both the main image and the LowRes Child image. In the main
image, the SubIFDs tag has two values, which point to the beginning of the IFD
structures for a LowRes Child and Mask Child images. In the LowRes Child image, the
SubIFDs tag has only one value, the location of the IFD for a LowRes mask image.
If there is more than 1 child image for a given parent image, the NextIFD value of
Child #1 must point to Child #2, and so on. The last ChildUs NextIFD value must be
zero.

This completes the TIFF Tree tech note.