There are some limits to target size. You do not want the targets to be too large or too small.
Circular targets that are photographed at a large angle to the camera appear as ellipses. This is not a problem unless the target is very large in the photograph because it is close to the camera. In this case, the center of the ellipse (which is what PhotoModeler sub-pixel marker finds) is not the projection of the center of the circular target. Usually you only need to be concerned if the target image is larger than an 1/8th the size of the photograph.
Note that if using Coded Targets the requirement for size of the center dot is similar to what is described here but see the Coded Targets section, Target Size and Appearance to verify your sizing. Note also that the sizes computed below are diameters where Coded Targets uses radius. They differ just by a factor of two.
The sub-pixel marker will accept a target down to five pixels in diameter in the digital image, provided they are very high contrast (e.g. retro-reflective). However if you do not take into account targets that appear in the distance, and/or the camera angles, you may end up with background targets that are too small to be marked. For good accuracy you will usually want the target to be eight or more pixels across, and higher if the images are ‘noisy’ (i.e. contain high contrast items that are not targets).
The chart below shows typical target size minimums for various target types, for use in the formula below when printing targets.
|
Target Type |
Minimum Target Diameter (in pixels) |
|
High contrast / retro-reflective |
6 |
|
Circular Targets, naturally lit |
10 |
|
Coded Targets (center dot portion) |
12 |
|
Manually Marked (e.g. Survey targets) |
10 |
You will need to plan how big to make the physical targets. Here is a calculation you can do to get an estimate:
Target Diameter = Minimum Target Diameter in pixels (see chart) * maxDistance * formatSizeWidth / ( imageSizeWidth * focalLength)
Where:
- “Minimum Target Diameter in pixels” is from the chart above
- “maxDistance” is the farthest a target will be from the camera
- “formatSize” is the horizontal size of the image format
- “imageSize” is the number of pixels in the image
- “focalLength” is the focal length of the lens.
Making sure your units are consistent, let us do two examples:
I. Laser Printed Targets (naturally lit) on a Building using a Digital camera
- maximum distance from camera: maxDistance = 15m = 15,000mm
- target diameter = 10pixels
- formatSize = 12mm
- focalLength = 15mm
- imageSize = 1200 pixels
therefore minimum target diameter = 10 * 15000 * 12 / (1200 * 15) = 100mm. Print out a 16cmx16cm black area with a 10cm diameter white circle in center for the target. Note this is really the smallest the target should be and if you can use bigger targets, do so.
II. Removable coded targets on a Car using a Film Camera and Scanner
- maximum distance from camera: maxDistance = 7m = 7,000mm
- target diameter = 12pixels
- formatSize = 36mm
- focalLength = 50mm
- imageSize = 3600 pixels
therefore minimum target diameter =12 * 7000 * 36 / (3600 * 50) = 16.8mm. So print coded targets such that the center circular dot is 16.8mm (using contrasting color to the car).
Note: The suggested sizes shown are starting points. You might find for highest accuracy you need the targets to be a bit larger than shown here. When a target is viewed at a shallow angle it will need to be bigger to be marked correctly. The numbers above are for targets that are perpendicular to the camera. For this reason, erring a bit on the larger size might be advisable.