Cover by Toby Segaran, Jeff Hammerbacher

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Passing the Image: Communication Among the Three Tasks

Image data fresh from one of the cameras was written into a slot by one of the camera tasks. After performing any required pixel correction or subframe operations, the camera task notified the ICS that a new image was available for processing. The ICS would then perform any commanded compression (either lossy or lossless) in place on the image within its slot, and then package and hand off the data for downlink. Only after the downlink was complete would the slot be released and become available for a new image. The sequence of events from exposure to image hand-off for the SSI camera is shown in Figure 3-7.

The entire sequence of events shown in Figure 3-6 was contingent on the availability of an image slot. If a slot was not available, the camera task would wait for a configurable period of time to allow the ICS to finish compressing and downlinking an image, which would result in a slot becoming available. If the ICS didn't release a slot within that period of time, the instrument task would generate an error message for the operators back on Earth and drop the image on the floor (there really wasn't any place else to drop it).

Once one of the camera instrument tasks obtained a slot, it "owned" that slot until it was handed off to the ICS, which then became the owner. Ownership verification was based on the slot ID (its number) assigned by the slot manager when the slot was initially allocated, an image ID code, and the camera ...

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