The Contrast Adjustment options are used to adjust the contrast of imagery. Most raw photographic imagery data requires some contrast adjustment to optimize the visible contrast between the most frequent data values. The data values within the image need to be represented in the RGB colorspace of the monitor, which contains values from 0-255 (one byte for each of the color bands).
No Contrast Adjustment
directly colors pixels on screen based on the data values. With no contrast adjustment, 8-bit values (0 to 255) are mapped directly into the RGB colorspace. 16-bit values (0-65535) are divided by 256 to get a range from 0-255.
Linear (Percentage Stretch)
This contrast adjustment method applies a standard deviation contrast adjustment to each color channel in the image. The visible color range is therefore optimized to show contrast for the majority of the data. Any outlying values beyond the specified standard deviation will be assigned the lowest or highest color value (0 or 255). The default of 2 standard deviations is generally recommended.
This setting is particularly useful for improving the display of dark or satellite imagery. Dark water or highly reflective urban and bare earth surfaces that have outlying values are grouped into the lowest or highest values, therefore allowing more color contrast for pixels closer to the average reflectance such as fields, forest, etc.
Linear (Minimum/ Maximum Stretch)
The method finds the minimum and maximum values in each color channel and stretches that range to a range of 0 to 255. For most imagery with 8-bits or less per color channel this will have little effect, but can produce a good result for high-color imagery.
This checkbox allows the user to specify that the calculated contrast adjustment used should be based on all loaded raster layers that have contrast adjustment enabled, rather than the color histogram for this single layer. This is enabled by default and provides consistent results when adjusting the contrast for multiple mosaiced images.