© Copyright 1995 Malin Space Science Systems, Inc.

The "Face on Mars"

Processing the Viking Orbiter Images

One advantage of digital imaging is the ability to process the data in order to improve its visibility. In the figures below, the general flow of processing for Viking Orbiter data is outlined, illustrated with the best two images of the "Face"(Figure 1). Note that in all of these figures the images have been enlarged by a factor of 3 for better viewing. Pixel-replication (i.e., taking each pixel and using the same value for each 3 X 3 area in the enlargement) was used for most of the images. Those that were enlarged using bicubic interpolation are so noted.

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Figure 1: Raw (top) and contrast-enhanced raw (bottom) enlargements of 035A72 (left) and 070A13 (right).

Bit-error correction

The communications link between the Viking Orbiter spacecraft and NASA's Deep Space Network antennas in California, Spain, and Australia did not incorporate error-correcting encoding. As a result, random errors were introduced into the data stream as bit errors. Sometimes errors were in the high bits, resulting in bright or dark pixels; at other times these random errors were in the low bits, and the resulting incorrect picture elements were less easily distinguished from correct picture. A relatively simple but remarkably effective means to radically improve the appearance of an image is to replace any value that differs from its neighbors by more than a specified value with the average of its nearest neighbors. The differences between Figure 1 bottom (left and right) and Figure 2 top (left and right) illustrate the application of this technique.

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Figure 2: Bit-error corrected (top) and reseau- removal (bottom) enlargements of 035A72 (left) and 070A13 (right). These images were contrast enhanced so the effects of processing could be seen.

Reseau Removal

The Viking Orbiter cameras used vidicon television tubes to acquire their images. These tubes operate much like a CRT or television picture tube in reverse. Rather than emitting light proportional to the strength of an electron beam shot at a phosphorescent surface (which is how a TV tube works), charge builds up on a vidicon's surface proportional to the amount of light falling on it, and is "read" by the change in current of an electron beam scanned across the surface. If one holds a magnet near a TV tube, one sees large distortions of the image. Because of unknown magnetic fields that might be encountered in space after launch, the Viking vidicons had small dots painted on their face-plates at regular intervals, which were carefully measured before launch. These fiducial marks, called reseaus or reseaux, can be used to correct for camera- induced geometric distortions. Once they have been measured, however, they may also be removed for cosmetic purposes, using a technique similar to that used to remove bit-errors, but covering more than one pixel. Figure 2 bottom (left and right) shows the effect of removing reseaus. Note that, in 035A72, there are no reseaus near the "face," but that in 070A13, there is a reseau on it.

Contrast Enhancement

The quality of an image is affected by many things, but probably the most important are the brightness and contrast. Contrast and brightness adjustment, called contrast enhancement, is easily applied to Viking images to improve the visibility of specific features of scientific interest, often with impressive results. For example, bland images without shadows can be transformed to dramatic versions by simply increasing the contrast by darkening dark areas and brightening light areas.

Figure 3 illustrates the effect on the "face" of applying a very simple, linear contrast enhancement over a smaller and smaller range of values. Note that with this processing, the shaded (but not shadowed) side of the hill in 070A13 becomes darker and darker, until little remains visible, and the surface appears shadowed.

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Figure 3: Increasing the contrast of the "face" images (035A72 (left) and 070A13 (right)).


Just as brightness and contrast adjustments on a broad scale can improve the overall visibility of features in an image, increasing the contrast between adjacent but dissimilar pixels tends to increase the apparent clarity of an image. Spatial frequency filtering or sharpening basically changes the value of a given pixel based on the values of its neighbors, multiplied by appropriate factors that scale, for example, with the distance of these neighbors. These techniques increase very local contrast, creating new adjacent pixel values that differ more from each other than they did originally. For data with significant pixel-to-pixel variations (e.g., noisy or aliased images), sharpening often worsens the appearance. Figure 4 shows the effects of applying two slightly different techniques. In normal application, sharpening is applied before contrast enhancement, which is then used to adjust the resulting image to visually appropriate brightness and contrast.

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Figure 4: Spatial frequency enhancement, enlarged using bicubic interpolation. These images were contrast enhanced so the effects of processing could be seen. The top row (left to right) shows 070A13 after bit-error correction and reseau removal, laplacian-sharpening filtering, and, alternatively, unity-weight "box" filtering with a 3 X 3 kernel and 30% addback. The bottom row (left to right) show these same three images, contrast enhanced. Note that the two filters amplify the pixel-to-pixel variations in the image, creating "speckle."

Image Processing of Images of the "Face on Mars": A Cautionary Tale

The "Face on Mars" images have been processed and reproduced many times by people with a wide range of experience and available software. However, it is important to remember that one cannot extract more information from the data than was there at the beginning. The following example, taken from the headlines of a popular "supermarket" tabloid, shows some of the pitfalls of image processing.

Figure 5 shows how the "Face on Mars" acquired teeth. Examination of the raw (upper left) and bit-error/reseau corrected (upper right) images, near the "mouth" of the "Face" (arrow), shows two attributes that play a role in creating the appearance of "teeth" in later-processed images: first, the bit error correction "filled in" several bit errors (both above and below the "mouth") with values averaged from neighboring pixels, creating a sharper contrast in the image. Second, the jaggedness of the boundary was accentuated by this process. The lower left image shows how, with the application of a laplacian-sharpening filter, subtle contrast on a pixel-to-pixel level is greatly enhanced, creating a few pixels much brighter than their immediate neighbors, and much brighter than they were previously. In this image, however, the same process can be seen in many areas; the individual pixels are clearly seen and compared to other jagged features created by the pixelation of the image. Many people use interpolation when they enlarge images, however, and interpolation "fuzzes" the individual pixels with their neighbors, thus "smoothing" somewhat the sharp differences between pixels. The lower right image was processed exactly the same as the lower left image, but enlarge using bicubic interpolation rather than pixel replication. The result: the "Face" looks like it has teeth.

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Figure 5: The "Face on Mars" gains "teeth" by harsh, pixel-scale processing.

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