Average of Time Series

This facility is specifically designed to efficiently compute image time averages over medium to long times. It performs the averaging process at full resolution for as long a sequence as required. The main restriction is that samples contributing to the average are spaced ten video fields apart. This is to allow time for the ALU to perform the necessary manipulations and the look up tables to be changed.

The basic operation of the routine is as follows:

           Acquire image to buffer 0.
           Add buffer 0 to buffer 2, placing bit 9 in buffer 1.
           Add buffer 1 to buffer 3, placing result in buffer 3.
           Add buffer 0 to buffer 2, placing result in buffer 2.

This strategy accumulates 16 bit totals using three ALU operations. After 255 samples have been accumulated it is necessary to make a copy of the Carry buffer (buffer 3) and reinitialise it to prevent overflow. There will be a short gap in the sampling while this copy is made.

If an ensemble rather than temporal average is required, or averaging with a sample spacing of less than 10 video fields, [;RA Average of buffers] should be used instead.

The routine may operate with either manual or computer controlled VTR operation.

Average video signal over what period ?

This entry specifies the period over which the incoming video signal is to be averaged. If this period is 1275 frames (51s for PAL or 42.5s for NTSC) or less, then the average may be computed using only buffers 0 to 3. If a longer period is required, then one additional buffer will be required for each additional group of up to 1275 frames.

The requested sample will require the use of buffer 0 to ##

The current contents of these buffers will be lost... Do you wish to continue ?

Calculating the time average using this technique requires the specified range of buffers.

Place time averaged intensity in what buffer ?

This entry specifies the buffer into which the final time average is to be placed. This may be any valid buffer, including one of those used by the averaging process as the final image is not created until after all bit planes and blocks have been processed. The buffer specified is made the current output buffer.

Manual or Computer controlled VTR ?

The VTR may be controlled either manually (<M>), or automatically by the computer (<C>). The latter choice will be required for operation from a command file.

{If VTR controlled by computer}

The VTR will be controlled by the computer. Please ensure the VTR is in "remote" mode. Start time (offset from current position) ?

The VTR will be controlled by the computer throughout the averaging process. This entry specifies the start point of the time series relative to the current position of the tape. The time offset uses the normal DigImage time format of minutes:seconds (see the general help facility in <shift><f1> for more details). A negative value implies time before the current point.

{If VTR controlled by computer}

On completion Reposition video tape or Leave ?

The response to this question determines what is to be done with the video tape when the time average has been determined. If <R> is specified, then the tape will be returned to its starting point (ie. where it now is). If <L> is specified, then the VTR will be left in pause mode with the tape wherever it is when the averaging process is completed.

{If VTR controlled manually}

Ready to start averaging Press any key to continue (or <escape> to terminate current activity) ...

The synchronisation of the averaging process is provided manually (even if overall control is from a command file). The averaging commences from when a key is pressed.

Warning Message:
Time Average aborted at user request

The time averaging procedure was terminated at the request of the user.

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Stuart Dalziel, last page update: 19 February 1996