Image Examples
Let's take a look at what PEP's image processing does to images. We'll start with a region of Mars around Latitude -22, Latitude 21. (You can download this data yourself using "Go To Location" feature.)
Without any image processsing, this area of Mars looks like this:
This image is around 90 km across. You easily see four or five cirular impact craters that likely formed when small asteroids crashed into the planet. In the lower left quadrant is something that looks like a river fan network. But, in general, the image looks washed out. Everything is a medium shade of gray; the image is low in contract.
So, if contrast is greatly increased (using the Adjust Contrast slider), the image changes to this:
Personally, I think that helps a lot. Much more detail is visible and several small craters jump right out. You'll find data from each planetary probe needs different levels of brightness and contrast to produce the best image.
Now, if we reset contrast and create a sharpen filter, we get this image of limited value:
Notice the white horizontal line that divides the jumble in the top half from the reasonable image in the bottom half. PEP constructed this image using two NASA data files. A probe in orbit around a planet gathers data in narrow strips running mostly north/south (that is, the probe is in a polar orbit). NASA takes these strips and creates individual files that cover a rectangular region of a planet. Data in different files and even a single file was gathered at different times under different conditions. The data in the top half of this image has more noise in it that the data in the lower half. If you look very closely, or increase the amount of sharpening, you can find a very low noise part of the image in the lower lefthand corner. This diagonal strip traces out a small part of a Viking probe's orbit.
The white horizontal strip is the boundry between the two files. PEP is written so the image processing algorithms are applied to each file indvidually and then combined. The tends to create these kinds of lines.
If we delete the Sharpen filter and make a Blur one, we get this image:
In this case, I don't think blur adds much. However, it has a few uses. One is when you zoom way in on an image and the speckle from the noise becomes annoying. Another is to use it before the edge detection filter. Each speckle of noise can look line an edge. Bluring an image can help eliminate these "false edges".
Speaking of edge detection, here is an edge detected view of the same region of Mars:
So far, this discussion focused on processing images but all of it applies to elevation data as well. Initally, PEP displays elevation data after an edge detection filter has been applied. The yellow lines indicate where there is a sharpe change in elevation. Instead of edge detection, here's an image created by just applying a false color filter to MOLA data. The lower elevation appear in red and higher ones in green and blue.
Here, ancient river bed and particularly low impact crater basins show up in red. To create an image like this you have to select the Elevation layer for image processing, using "Select Layer". Then, delete the Sobel Gradient and Threshold filters. Finally, on the False Color filter select "Rainbow".
By adjusting transparency (via a slider on the False Color filter), you can see both elevation and imagery data at the same time. Here, lower elevation is dark and it covers the entire screen. Four NASA imagery files are blending into the elevation data.
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