Custom Camera Calibrations

note

This feature is new in version 1.8.0 of kognic-io and some parts require optional dependencies. Run pip install kognic-io[wasm] to install it.

If your calibration is not in the list of supported standard camera calibrations, you can provide a custom calibration. The table below show the attributes of the CustomCameraCalibration object. The wasm_base64 attribute is a base64 encoded string of the WebAssembly module that implements the calibration. The test_cases attribute is a list of TestCase objects that are used to validate the calibration during creation. It is recommended to provide a few test cases to make sure that the calibration is correct.

Key	Value	Parameters
rotation_quaternion	A RotationQuaternion object	w, x, y, z
position	A Position object	x, y, z
image_width	Integer	NA
image_height	Integer	NA
wasm_base64	String	NA
test_cases	A list of TestCase objects	point3d, point2d

The CustomCameraCalibration object can conveniently be instantiated directly from the binary or a wasm-file:

calibration_file = CustomCameraCalibration.from_file("/path/to/calibration.wasm", ...)
calibration_binary = CustomCameraCalibration.from_bytes(b"...", ...)

We provide a set of utilities that will make it easier to work with the WebAssembly module. More specifically we provide validation code and compilation code from a few different languages to WebAssembly. The code is available both as python functions and via the kognicutil cli.

The WebAssembly module

The WebAssembly must follow a strict interface where the module exports a function called project_point_to_image. The function must take three arguments of type float64 and return two values of type float64. Thus, the WebAssembly text representation of this interface is func (param f64 f64 f64) (result f64 f64). The three arguments are the x, y and z coordinates of the 3D point. The two return values are the x and y coordinates of the projected point in the image plane. WebAssembly doesn't support multiple return values by default but this can be enabled with the multi-value proposal.

If the point is not within the field of view, the function should return NaN for both the x and y coordinates.

Validation

note

This requires wasmtime to be installed which is an optional of dependency kognic-io. Run pip install kognic-io[wasm] to install it.

We provide validation code both as python functions and via the kognicutil cli. We validate things such as but not limited to

1. The module can be loaded
2. The function exists and has the correct signature
3. That a point can be projected using the module
4. That points are projected correctly if test cases are provided

In python there are three different ways to validate a calibration

import kognic.io.tools.calibration.validation as wasm_validation
from kognic.io.model.calibration.camera.custom_camera_calibration import CustomCameraCalibration, Point2d, Point3d, TestCase

test_cases = [
    TestCase(
        point3d=Point3d(x=1.0, y=2.0, z=3.0),
        point2d=Point2d(x=2.0, y=5.6)
    ),
    TestCase(
        point3d=Point3d(x=1.0, y=1.0, z=-1.0),
        point2d=Point2d(x=float("nan"), y=float("nan")) # point is outside field of view
    )
]

wasm_file = "/path/to/calibration.wasm"
calibration = CustomCameraCalibration.from_bytes(wasm_file, test_cases=test_cases, ...)
wasm_bytes = calibration.to_bytes()

# Validate the calibration object
wasm_validation.validate_custom_camera_calibration(calibration, test_cases=test_cases)

# Validate the wasm file
wasm_validation.validate_wasm_file(wasm_file, test_cases=test_cases)

# Validate the wasm binary
wasm_validation.validate_wasm_bytes(wasm_bytes, test_cases=test_cases)

The kognicutil cli can be used as follows

kognicutil wasm validate calibration.wasm

Compilation

note

It is recommended to keep the wasm file as small as possible. Try to avoid dependencies that are not needed. For example, it may be preferred to implement some mathematical functions yourself instead of using the standard library.

As stated above the WebAssembly module must follow a strict interface and compilation requires the multi-value proposal. We provide a set of utilities that will make it easier to compile the WebAssembly module from a few languages, see table below.

Language	Target	Compilation tool
Rust	*.rs	rustc
Rust (Cargo)	Cargo.toml	cargo
C++	*.cc, *.cpp	emscripten
C	*.c	emscripten

The utilities are available both as python functions and via the kognicutil cli. From Python, you can compile the module with

from kognic.io.tools.calibration.compilation import compile_to_wasm
wasm_binary = compile_to_wasm("path/to/source")

The returned binary can then be used to create a CustomCameraCalibration object. If the output_wasm parameter is passed, the binary will be saved to the specified path. The kognicutil cli can be used as follows

kognicutil wasm compile path/to/source path/to/output.wasm

Note that, validation is run by default after compilation. This can be disabled with the --skip-validation flag.

Calibration parameters have to be embedded in the binary so that they can be used by the WebAssembly module. Try to pre-compute as much as possible to increase the speed of the projection function at runtime.

Below follows examples of a simplified version of the pinhole calibration in a few different languages.

Example: Rust

note

Rust needs to be installed with the wasm32-wasi target for this. Install Rust according to the instructions here and then add the target with rustup target add wasm32-wasi.

A Rust file can be compiled with

kognicutil wasm compile path/to/source.rs path/to/output.wasm

Note that panics are not supported and compilation will fail if the code contains it.

Rust with Cargo

note

Rust and Cargo need to be installed with the wasm32-wasi target for this. Install Rust and Cargo according to the instructions here and then add the target with rustup target add wasm32-wasi.

A Rust module with Cargo can be compiled with

kognicutil wasm compile path/to/source/Cargo.toml path/to/output.wasm

Note that it is important to specify that the library is a cdylib and it is also recommended to set strip = true to reduce the size of the WebAssembly module. This is done by adding the following to the Cargo.toml file

[lib]
crate-type = ["cdylib"]

[profile.release]
strip = true

Example: C++

note

Emscripten needs to be installed for this, which can be done according to the instructions here.

A C++ file can be compiled with

kognicutil wasm compile path/to/source.cc path/to/output.wasm

or with

kognicutil wasm compile path/to/source.cpp path/to/output.wasm

Example: C

note

Emscripten needs to be installed for this, which can be done according to the instructions here.

A C file can be compiled with

kognicutil wasm compile path/to/source.c path/to/output.wasm