This page describes the process in which the Viking Orbiter imagery of the Face and other objects on Mars is processed with particular attention to enhancement of fine structure (i.e., features that are at or slightly above the resolution limit of the sensor). The overall process used to remove noise, enhance contrast, and magnify selected portions of the data for analysis is described in Carlotto (1988, 1991).
The raw Viking imagery contained a great deal of "salt-and-pepper" noise caused by data transmission errors. The first processing step is to "clean up" the imagery. A 3x3 pixel Laplacian filter is used first to detect outliers (i.e., pixels whose values differ from the local mean by more than a specified threshold). The values of these pixels are then replaced by the local median value computed in a 3x3 window centered about the pixel. The local median is used because it maintains edge structure and thus better preserves subtle detail in noisy areas of the image. The threshold value is selected to reduce the magnitude of the noise without significantly distorting the fine-scale detail in the image.
For visual interpretation, the Viking imagery has been enhanced using several techniques (Carlotto 1988, Carlotto and Stein 1990, Carlotto 1991). Over large areas a local contrast stretch algorithm is used to remove shading variations due to illumination and albedo variations across the imagery, and increase the local contrast while maintaining the overall tonal balance of the imagery. Over small areas and for isolated features the contrast is enhanced using a global "clip-and-stretch" which assigns pixels below a minimum value to zero, pixels above a maximum value to 255, and pixels in between to the range 1-254.
Two enhanced views of the Face are shown below from 35A72 (left) and 70A13 (right) after noise removal and clip and stretch. The images were magnified using a cubic spline technique which uses the values of pixels in a 5x5 window to interpolate intermediate pixel locations. A visual comparison of different pixel interpolation techniques can be found in Carlotto (1991).
Figure 1 Face from 35A72 (left) and 70A13 (right)
In addition to the overall organization and symmetry of the Face a number of subtle features or embellishments have also been observed. They include several thin lines that intersect above the eyes, four broad stripes across the face, and fine structure in the mouth area that appear to some as teeth. Each is discussed below.
A number of broad lateral stripes can be seen in the image of the Face from Viking frame 35A72. These features are emphasized as shown in Figure 2 in which the image was first rotated so that the stripes are in the horizontal direction and then enhanced using a horizontal averaging operation. These features are not aligned in the same direction as the scan lines in 35A72 and so cannot be caused by either the sensor or subsequent digital processing. These stripes are not evident in 70A13 (higher sun angle) which suggest that they may be due to subtle topographic variations rather than by differences in albedo. (Subtle variations in topographic relief are more evident at lower sun angles.)
Figure 2 Enhancement of Lateral Stripes in 35A72
Several thin lines which cross in the forehead area can be seen in both 35A72 and 70A13. These lines in 35A72 (the horizontal line in particular) shown in Figure 3 were enhanced using the same process as described above. These features are not aligned in the same direction as the scan lines in either 35A72 or 70A13 and so cannot be caused either by the sensor or subsequent digital image processing. In addition as seen in Figure 1 they appear in both images further increasing the likelihood that they are real features.
Figure 3 Enhancement of Lines in 35A72
Perhaps the most controversial feature of the Face are the "teeth." In actuality the teeth are fine-scale structures that appear in the mouth area. As seen in Figure 4 these features cannot be dismissed as noise in the imagery or artifacts of the processing since they appear in both 35A72 and 70A13 taken about 35 days apart.
Figure 4 Features in mouth in 35A72 (left) and 70A13 (right)
Figure 5 is the result of adding the two images in Figure 4 and dividing by two in order to emphasize the features that are present in both images.
Figure 5 Fine-scale features on left side of Face
It has been suggested that the teeth are nothing more than noise which has been emphasized through the improper use/interpretation of image enhancement techniques (Malin 1996). Malin's teeth which are caused by noise in 70A13 are identified in our enhancement (Figure 6). However as noted by McDaniel (1996) the features identified by Malin are not the features presently under study. Malin's teeth are on the right shadowed side of the Face and are present in only one of the images while the features under study are on the left sunlit side of the Face and are present in both images (Figures 4 and 5).
Figure 6 Malin's "teeth" are on the right side of the Face
Carlotto, M.J., "Digital imagery analysis of unusual Martian surface features," Applied Optics, Vol. 27, pp 1926-1933, 1988.
Carlotto, M.J. and Stein, M.C, "A method for searching for artificial objects on planetary surfaces," Journal of the British Interplanetary Society, Vol. 43, pp 209-216, 1990.
Carlotto, M.J. , The Martian Enigmas: A Closer Look, North Atlantic Books, Berkeley CA, 1991.
(Page last updated 7/4/96)
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