Vignetting Effects Illustrated
In general photography, the term ‘vignetting’ refers to a gradual darkening of an image around the image’s periphery. The title graphic above is a good illustration of this phenomenon as the four corners are noticeably darker than the image’s center. Almost all images have some degree of vignetting whether they are seen by the naked human eye or not. There are several causes of this vignetting but for the purposes of aerial photography (where our goal is to create orthorectified mosaics) we are really only concerned with one cause; the sun’s position relative to the drone. The resulting vignetting from the sun’s position has been coined as ‘solar vignetting.’
Below is a graphic of what happens when a drone flies over a field at solar noon. In simplest terms, solar noon is the time of day when the sun is at its highest point in the sky; this is usually not the same as 12:00 noon, and the offset depends on the date and latitude of a specific flight. If we were to fly over mountains or a dense city, the sun’s light would reflect off the surface of the earth in several different directions but because most agricultural fields are flat and have some degree of uniformity, they often act as a lake or ocean would and reflect the light back up to the sky.
When flying at or close to solar noontime, the sun’s reflection will be at its most intense point time-wise, and therefore the center of the photograph will be intensely bright and the periphery will gradually darken.
The next pair of images below show what happens when a drone flies over a field at mid day. In our case ‘mid day’ is a concept rather than a specific range of hours as the effects of mid day flights will vary with calendar days, latitude, and even your drone camera’s specific characteristics.
The images above shows how mid day flights produce vignetting; the apex of the sun reflection will be off center and less intense than it would be at solar noon. In these photos the drone’s shadow may or may not be visible, and the photo will be at its darkest opposite the apex of the sun reflection. In the image above, the darkest region of the image are the upper and lower right-hand corners.
The next two images below show an optimal set of conditions for flying agricultural drones. Here the sun is causing some residual amount of solar vignetting but this time the apex of the sun’s reflection (and the drone’s shadow) are both outside the camera’s view angle.
In this case, we still have solar vignetting as the left side of the photo is brighter than the right side, yet these image types will have an easier time stitching than the previous two types (discussed above) and so it is a good idea to consider time of day when flying an agricultural drone. The easiest way to do this is to periodically conduct a short test flight wherein the drone flies a box pattern (north, east, west, and south) at the normal speed and altitude it would run on a typical mission. If the apex of the sun’s reflection is visible, the flight time is too close to solar noontime. This does not mean you absolutely cannot fly during these mid day hours, but that if you do, you will need to take other steps to ensure your photoset will produce adequate results. These steps include:
• Making sure you have an oversampling of 10. that is, that each square centimeter of your field appears in at least 10 different individual photographs. Oversampling is a function of sidelap and overlap. Many Agribotix clients can get good results with an oversampling number of between 7.5 and 8 but we always ask for 10 to ensure good results. If you fly a drone at solar noon, and if your camera produces an ample amount of solar vignetting as a result, you will need to ensure you have an oversampling of 10 or else your final stitched image will have ample stitching artifacts.
• Making sure you have adequate sidelap in your flight path. If you have an oversampling of 10 in your photo set but all 10 photos of a particular landmark are in a single row, you will still get vignetting bands. In example, if you fly over a center pivot (or any recognizable landmark somewhere in the middle of your field) and all 10 photos of the center pivot were taken in sequential order as the drone made a single pass over the center pivot, you will still get vignetting bands. This is almost always a problem on quadcopters and never a problem with fixed wing drones because fixed wing drones fly too fast for this to occur. As a rule of thumb, if you have a landmark such as a center pivot, it should be visible in 3 separate passes over the field. This means when you look at your photo set you should see your landmark in three separate groups of photos, and not just in 1 or 2 groups.
• Ensuring your camera does not produce blurry images.
• Ensuring your camera does not change its white balance settings mid flight.
• Submitting images that are geotagged.
If a photo set is submitted to Agribotix and the majority of those photos have intense solar vignetting, we usually can stitch them and get accurate results. If the vignetting does produce stitching artifacts we can usually spot it (even if faint) and let our customers know.
Below are a couple of illustrations showing the effects of flying under an overcast canopy.
As the image above shows, there is hardly any solar vignetting visible in the photograph at all. The irony here is that while flying in overcast conditions eradicates solar vignetting all together, it brings a new series of problems in the form of cloud effects.
Below are three fields that show the results of image vignetting. Although solar vignetting (as discussed above) is the main culprit producing these stitching artifacts, there are other types (i.e. causes) of vignetting that will be unique to your specific camera’s make & model.