250
7 ENVELOPE CORRELATION
251
In this section we deal with the third performance limitation mentioned in the
introduction -- the loss of fine image detail where no coarse image data is
present.
Progressive matching of the images from coarse-to-fine detail is only possible
if all of the bands contain correlatable image data. If data is missing from
one band then the discriminator for the next higher band may not be within its
capture range and the pro-
cess may come to a halt.
While such occurrences are
not frequent with terrain
imagery, a human operator
must monitor the very fast
Gestalt Photomappers and
take corrective action when
necessary. The function of
Envelope Correlation is to
eliminate the problem by pro-
viding correlatable envelope
data in bands where direct
signal data is missing.
Figure 3 illustrates the dif-
ference between a composite
signal and a modulation
envelope.
The left column of figures
SIGNAL
SPECTRA
WAVEFORMS A >
HIGH-PASS
FILTERED
PASS
LOW-
FILTERED
ADDITION
Low- Pass, y" High- pass
LL pecora:
£^
VS
ACA
A
NAYS
fol M p
MODULATION
Low- ie Vei ci pass
Sx"
| zd s | | |
0 i 6 S 49
se a REP La
FULL WAVE QU EO
RECTIFIED
LOW-PASS
FILTERED
are a
shows how band pass filters
separate low and high fre-
quency signals prior to cor-
relation. The right column
Figure, 3
SIGNAL ADDITION vs. MODULATION
shows how to extract a low frequency envelope signal from a modulated high fre-
quency signal that contains no direct low frequency data.
Figure 4 shows how frequency
changes in a constant ampli-
tude signal can give rise to
low frequency envelopes in
several higher frequency
bands,
Envelope Correlation is an
easy addition to multiband
Gestalt type stereo correl-
ators and increases greatly
the reliability and speed of
the correlation process. In
photogrammetry the advantage
of envelope correlation would
be particularly useful in
finely textured areas without
shading and in tree covered
INPUT
BAND II
1075
ENVELOPES FROM FREQUENCY
Figure 4
CHANGE
areas under high sun conditions where leaf detail is strong but shading is weak.
