- compresses the reference element pyramids in a pyramid dictionary
cxPyramidDictionary * cxPyrDictCompress( cxPyramidDictionary *src, long layer)
integer function cxPyrDictCompress(src, layer)
Input pyramid dictionary data structure.
Input layer to which to compress the dictionary.
FUNCTION RETURN VALUE
The returned value for this function is a pointer to the newly compressed
cxPyrDictCompress makes a compressed version of its input
pyramid dictionary, which can be used to convert a pyramid compressed
at one layer to one compressed at a lower layer. Such an
operation is useful in creating a module that operates on compressed
pyramids of all layers, but has a preferred layer of operation (while
it is possible to expand the input pyramid, it may be very inefficient
to do so; converting the compressed input to the desired level may be
more efficient). However, the returned cxPyramidDictionary
structure is not a valid dictionary and must not be used
as part of a compressed pyramid.
Given a pyramid compressed at one layer, there are three steps to
creating an equivalent pyramid compressed at a lower layer. For
instance, consider the case of recompressing a solid element
pyramid compressed at layer 3 to a planar element pyramid compressed
at layer 2.
The first step is to compute the representation of each layer 3
reference element (the solid elements in the 3D dictionary) as a
collection of layer 2 elements; each 3D element is then stored as a
pyramid compressed at layer 2.
The second step is to merge all of the layer 2 elements into a single
2D pyramid dictionary, updating the references of the compressed 3D
elements so that they all refer to this single pyramid dictionary.
The third step is to rewrite the original 3D compressed pyramid in
terms of the new 2D pyramid dictionary, converting each reference to a
3D dictionary element into several references to 2D dictionary
cxPyrDictCompress performs the first two steps of this
operation, creating a cxPyramidDictionary structure having a
compressed pyramid for each uncompressed pyramid in the src
dictionary; all the compressed reference pyramids refer to the same
layer dictionary. Given the returned cxPyramidDictionary
structure, it is straightforward to construct the recompressed pyramid
at the new layer (step 3). For instance, the modules ShrinkPyr
and ExteriorPyr use this routine to perform the layer conversion on
It is important to note that the returned cxPyramidDictionary
structure is not valid for use in a compressed pyramid; it is only a
convenient form in which to represent the translation between the old
compression layer and the new layer.
If layer is greater than or equal to the compression layer of
src, a NULL pointer is returned.
The returned pyramid dictionary is a reference counted structure owned
by the caller. After using the structure, it may be deleted with
Last modified: Mon Nov 6 16:33:43 GMT 2000
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© The Numerical Algorithms Group Ltd, Oxford UK. 1996