calc_diameter: Calculate diameter

In rcaiman: CAnopy IMage ANalysis

Calculate diameter

Description

Calculate the diameter in pixels of a 180 deg fisheye image.

Usage

calc_diameter(lens_coef, radius, angle)

Arguments

lens_coef	numeric vector. Polynomial coefficients of the lens projection function. See lens().
radius	numeric vector. Distance in pixels from the zenith.
angle	numeric vector. Zenith angle in degrees.

Details

This function is useful when the recording device has a field of view smaller than 180 deg. Given a lens projection function and data points consisting of radii (pixels) and their corresponding zenith angles (\theta), it returns the horizon radius (i.e., the radius for \theta equal to 90 deg).

When working with non-circular hemispherical photography, this function helps determine the diameter that a circular image would have if the equipment recorded the whole hemisphere, required to build the correct zenith image to use as input for expand_noncircular().

The required data (radius–angle pairs) can be obtained following the instructions in the user manual of Hemisfer software. A slightly simpler alternative is:

1. Find a vertical wall and a leveled floor, both well-constructed.

2. Draw a triangle of 5 \times 4 \times 3 meters on the floor, with the 4-meter side along the wall.

3. Place the camera over the vertex 3 meters away from the wall, at a chosen height (e.g., 1.3 m).

4. Make a mark on the wall at the chosen height over the wall-vertex nearest to the camera vertex. Make four more marks at 1 m intervals along a horizontal line. This creates marks for 0, 18, 34, 45, and 54 deg \theta.

5. Before taking the photograph, align the zenith coordinates with the 0 deg \theta mark and ensure the optical axis is level.

The line selection tool of ImageJ can be used to measure the distance in pixels between points on the image. Draw a line and use the menu Analyze > Measure to obtain its length.

For obtaining the projection of a new lens, see calibrate_lens().

Value

Numeric vector of length one. Estimated diameter in pixels, rounded to the nearest even integer (see zenith_image() for details).

Examples

# Nikon D50 and Fisheye Nikkor 10.5mm lens
calc_diameter(lens("Nikkor_10.5mm"), 1202, 54)

rcaiman documentation built on Sept. 9, 2025, 5:42 p.m.