Getting the Picture: Image Download and Processing
A lot went on between the time an image exposure occurred and the eventual hand-off to the ICS. Each camera in the system had its own control electronics to process commands, convert the analog signals from a CCD into 12-bit digital values, and then store the data in a hardware buffer until the flight software could download it into an image slot. All this occurred under the control of logic embedded in radiation-hardened programmable gate array devices.
Once an image was acquired from a camera and written into an image slot, it was subjected to various forms of processing, all of which occurred in-place within the confines of an image slot. No additional large (image-sized) buffers were used for the processing or the results thereof, and only a few small buffers were necessary to hold intermediate results. The use of in-place processing was a key factor in the design of the imaging software, and allowed the three tasks to maintain a small memory footprint in the overall system. Figure 3-9 shows a comparison between a multiple-buffer approach and the single buffer (i.e., slot) in-place design used for the Phoenix imaging flight software.
Figure 3-9. Multiple data buffers versus a single data buffer.
This was another design trade-off that was made early on in order to meet the image processing requirements and still stay within the amount ...