Processing an Image (Brightness, Contrast, and Gamma, and Topographic Correction)

When acquiring an image (either an SED or ABS video, or Elemental region), AugerMap displays the image in 256 scales of grey, with a linear relationship between the data and the grey scale. You can change this relationship by either modifying the thresholds (cropping the range of data used to create the image), or modifying the gamma (changing the linearity of the data used to create the image). These three controls will affect the overall brightness and contrast of the image.

Note that processing the image is different from applying image enhancements, in that processing determines how the image will be created from the data. Image enhancements modify the image itself. It is because of this relationship (creating the image from the raw data) that threshold controls are used instead of standard brightness and contrast.

Also, because changing the processing recreates the image from the raw data, any changes made here will remove any enhancements and filtering. It will not, however, affect color assignments.

To change the processing of an image, select that image and click the Processing button from the Image menu. This will display the Image Processing Dialog.

Black and White Threshold

Adjust these sliders to change the limits that map the acquired data to the black and white levels of the image. The percentages shown at the bottom of the sliders indicate the range of data that will be ignored (cropped) when creating the image.

For example, setting the Black threshold to 5% means that the lowest 5% of the data range will be ignored, and that the darkest black of the image will be mapped starting at 5% of the data range. Any thing below that will be pure black.

Similarly, changing the White threshold will determined where the whitest level of the image will be mapped to. For example, adjusting the White threshold to 90% means the whitest part of the image will map up to 90% of the data, the remaining 10% will be displayed as pure white.

After experimenting with the thresholds, it will become more obvious how they function. Normally, adjusting the sliders closer together (so the Black threshold is higher than 0% and the White threshold is less than 100%) will produce higher contrast in the image, since the full grey-scale range is being used to represent a smaller range of data. Moving both sliders closer to 0% tends to brighten the image (whereby a large portion of the data remains white), and moving both closer to 100% tends to darken the image (whereby a large portion of the data remains black).

Tips for Using the Threshold Controls

Adjusting the thresholds can be very helpful in negating the affect of noise spikes, or other areas that effectively reduce contrast. For example, if there are a few areas where there is a very high count rate, these will appear as very bright pixels, reducing the contrast of the rest of the data. To counter this, try adjusting the white threshold below 100%. This should help improve contrast in the surrounding image.

When using the threshold controls, keep in mind that, if the original data is elemental in nature, changing the thresholds results in an image that does not represent the full range of data.

Gamma Control

The Gamma control controls the gamma function of the data as it is mapped to the image. Computer monitors all have built in gamma functions, which are normally corrected for by your video hardware. Gamma is most useful to correct non-linearity in color monitors, but it can be useful for black+white images also.

Adjusting the gamma changes the linearity of the image as it is mapped from the data. A gamma correction of 1 is the default, and leaves the relationship between the data and image linear. Experiment with the gamma to improve the brightness and contrast, especially when an image appears washed-out, or “shadowy”. Also, you may find that an image looks fine on your PC monitor, but does not print well. Adjusting the gamma slightly may help

Topographic Correction

The Apply Topographic Correction checkbox allows you the option of correcting an elemental image for sample topography This is done by dividing the peak-background by the square of the background. Only elemental images that have acquired background data can be corrected.

Reset

Click the Reset button to reset all values to their defaults. The Black threshold will be set to 0%, the White threshold to 100%, and the Gamma to 1.