Viewers of the ACD Change Map on Console and the Analytics App and users that download the Change Map Preview Image from Console will see detected changes visualized in red and blue hues. This color coding is set according to an industry-standard pneumonic “Red is Fled, Blue is New.” This colorization, of course, is not applied exclusively to objects within a scene that have the ability to move. It is simply a helpful pneumonic to remember whether the change is related to a decrease (red) or an increase (blue) in amplitude between the two scenes.
Pixel locations that were brighter in image 1 and darker in image 2 will result in values ranging from 0 to –1 on the Change Map. These values are color-coded in shades of red, with more saturated (darker) reds representing values closer to -1 and less saturated (lighter) reds representing values closer to 0. Pixel locations that were darker in image 1 and lighter in image 2 are mapped to values ranging from 0 to 1. These values are a color-coded in shades of blue, with more saturated (darker) blues representing values closer to 1 and less saturated (lighter) blues representing values closer to 0.
Classically, “Red is Fled, Blue is New” means that changes that appear in red can be interpreted as an object that has departed, and areas that appear in blue can be interpreted as a new object that has arrived. The figure below shows two images of an aircraft parking area and the ACD Change Map comparing the two. In the ACD Change Map, the location of the aircraft in image 1 is coded in red since it departed between image 1 and image 2, and the location of the aircraft in image 2 is coded in blue since it arrived between image 1 and image 2.
Certain types of changes—particularly those in relation to water—don't always fit nicely into the “Red is Fled, Blue is New” pneumonic. Nevertheless, the colorization will still help with interpretation of the Change Map. For example, a change due to flooding of a landscape between two images will show increases of water extent as red. Conversely, a change due to water receding will show up as blue. Below is an example of river flooding where the red colors of the Change Map denote changes due to the flooding of the river. Of course, the land didn’t ‘flee’ from the flood, but there is a significant decrease in amplitude that is detected, and hence the change is colored red. Blue changes in the waterway represent increases in surface roughness due to waves and/or wind in the second image.
As described above, Capella’s colorization is further graded in order to give users an understanding of the relative difference in amplitude of the change. More saturated red or blue signatures correspond to changes that have larger differences in amplitude from image 1 and image 2, while less saturated tones show changes that have a smaller difference in amplitude from image 1 and image 2. Consider the example below of some agricultural fields. In this case, there is certainly a change that increases the amplitude in certain plots—perhaps due to soil tillage or crop growth. The change in amplitude, however, is not nearly as significant as the aircraft parking or flooding illustrated above, and therefore shows as a lighter hue of blue.