Plane stacking, refers to a technique of stack multiple images of planes either taking off of departing from an airport into a single image. Examples can be found at Planes at Suvarnbhumi Airport.
Photoshop has a number of stacking options, but they all degrade the image of the planes. Another alternative in Photoshop is to simply cut-and-paste all the planes into a single image, which can be very time-consuming and is not very elegant.
It occurred to me, that an algorithm for "Plane Stacking" would actually be similar to Star-trail stacking. Star-trail stacking takes the brightest part of each photo and adds it to the stack, where-as Plane Stacking would analyse individual color components and based on certain color thresholds, would only add certain pixels to the stack.
To make a "plane stack", get the StarView software and load all the images under "Load Image Files" and select the check-boxes on all the photos to be stacked. Next go to the Stacking Menu and Select a "Plane stack" and click stack. All the input photos must have a consistent background, which means the camera must not have moved while taking a sequence of shots; ie. use of a tripod is mandatory.
As mentioned above, the process is activated by a single click, but internally, the algorithm is a two step process. First all the images are averaged together. If the number of images are large enough, then the planes will be effectively removed from the averaged image. This average is done using a mean stack, but it may be upgraded to a median stack sometime later which will help when the number of input images is low. The second step is compare each photo to the average photo. If a pixel has a different color intensity from the average, then it is added to the stack; this process is done for each of the 3 colors, subject to different thresholds. The process is repeated similar to the star-trail stacking process. This means that color consistency of the background is also an important criteria. Things that can change the color consistency of the background include (a) moving clouds, and (b) a long sequence of photos taken in the late afternoon or early morning when the sun is near the horizon.
In summary, the criteria for obtaining good input photo's for plane stacking are:
- No camera movement. Use a tripod and remote shutter release.
- No moving clouds in the background.
- Avoid late afternoon or early morning, where the white balance will be changing as the sun nears the horizon.
- Use manual exposure and manual focusing.
- A small number of input images may not process optimally, such as less than 5. (t.b.d.)