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.