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[121] This series of pictures illustrates the way in which so called raw data can be recovered and modified to produce a picture o] optimum quality and usefulness.
Data loss can occur at a number of points as information is transmitted from Lander to Orbiter, from Orbiter to antennae girdling Earth, or from the receiving stations to JPL. Early in the mission all critical data were transmitted redundantly to protect against loss.
In figure 145 the data loss occurred during the Lander to Orbiter relay. As the Orbiter rose above the horizon, the received signal from the Lander became progressively stronger. The initially weak signal caused a large number of "bit" errors. In many instances the computer was unable to locate the beginning of a line in the telemetry stream. Reprocessing of the data located most of the image data that was present but misaddressed (fig. 146).
There remains a scattering of isolated picture elements with incorrect brightness numbers due to infrequent, but statistically significant, errors in transmission. This is termed "noise," analogous to the background static that interferes with a conventional radio broadcast. Noise is removed by assigning the spurious pixels values intermediate between those of neighboring pixels. Although the apparent quality of the picture is dramatically improved there is only a small gain in actual information.
The contrast of a picture can be increased. In figure 147 the brightness values of constituent pixels have been "stretched" to encompass a larger range of gray levels. Most of the brightness numbers recorded by the camera were clustered between 8 and 40. To increase contrast the limits of the distribution were reassigned values of 0 and 63, and intervening brightness numbers were stretched linearly. The result is that the bright pixels are brighter while the dark pixels are darker.
Another technique for increasing discriminability involves use of a mathematical construction called a box filter. Individual pixel brightness values are compared to the average brightness within a square array of neighboring pixels. The difference between the two is then increased by an arbitrary factor. Increasing the contrast of fine detail while leaving unchanged or removing larger scale variations has the visual effect of "focusing" the scene.
