Camera Calibration Flashcards
Camera Parameters and Calibration:
What is Luminance (cd/m²)?
- measurement of the photometric luminous intensity per unit area of light
- its the amount of light that goes through a particular area, and falls within a given solid angle
What is Chrominance?
- the way a certain amout of light is distributed among the visible spectrum
- greyscale image has no color information
- any RGB triplet has no chrominance information
- chrominance has no luminance information but is used together with it to describe colored image
How is RGB & YUV defined?
-Y is luminance and is calculated from 3 coefficients and the R, G and B values
-U and V are the chrominance
What is Image Processing Pipeline:
-This processing can be performed on the YUV or RGB
components depending on the system.
-Depending on the system, more or less image parameters may be available for the user to control. Also, some of these parameters (namely brightness, contrast and saturation) are also intrinsic original image characteristics apart from being
externally controllable parameters.
How many Image parameters are there?
6
Numerate the Image parameters
- Brightness:
- Contrast:
- White Balance:
- Saturation:
- Gamma:
- Sharpness:
What defines Brightness? (as an intrinsic original image characteristic)
- average amount of light that is integrated over the image during exposure time
- is a constant (or offset) that can be added (subtracted) from the luminance component of the image (as a controllable parameter)
What is Contrast? (as an intrinsic image characteristic)
- difference in luminance (or color) along the 2D space - most used definition
- difference in the color and brightness of the object in the same field of view - real world
- the faster and higher the luminance changes along the space the higher the contrast is
- gain control function of the luminance of the image
- normally combined with Brightness in a single transfer function
How is Contrast calculated?
luminance difference / average luminance
What is white Balance? (as a controllable parameter)
- global adjustment of the intensities of the colors (red, green and blue primary colors)
- goal is to render specific colors correctly- particularly neutral colors
What is Saturation? (as an intrinsic image characteristic)
- distribution of light accross the spectrum of different wavelengths
- amount of white you have blended into a pure color
-If the light intensity declines, then, as a result, the saturation also decline.
How can the purest (most saturated) color be obtained?
It is obtained when using a single wavelength at a high intensity(laser light is a good example)
How do you remove the saturation of an image? (as a controllable parameter)
By adding white to the original colors (this is the same as changing the gain of the U and V chromatic components).
What is Gamma?
- nonlinear operation used to code and decode luminance values
How is Gamma calculated ?
Vout = A*Vin^δ
where A is a constant and the input and output values are non-negative real values.
In most cases A = 1, and inputs and outputs are typically in the range 0–1.
What is Sharpness? (as a controllable parameter)
-The measure of the energy frequency spatial distribution over the image
-Not all devices provide access to this parameter.
- basically allows the control of the cut-off frequency of a low pass
spatial filter.
In which cases is sharpness very useful?
if the image is afterward intended to be decimated, since it allows to prevent spatial aliases artifacts.
Is Exposure time a controllable parameter of the image device?
It may or may not be
What happens if the brightness is too high?
Overexposure may occur which will white saturate part
or the totality of the image.
What is the maximum possible contrast of an image denominated?
Contrast ratio or dynamic range.
Is it common that contrast and brightness are actually a combined single transfer
function?
Yes
What kind of Camera parameters are there?
Extrinsic:
- define the location and orientation of the camera reference with respect to a known reference
Intrinsic:
- link the image pixel coordinates with the corresponding coordinates in the camera reference frame
How do you accommodate the camera internal
parameters and geometry and its position and
posture in the real worl?
Using geometrical transformations.
Object coordinates (3D) -> World Coordinates (3D) -> Camera Coordinates (3D) -> Image Plane Coordinates (2D) -> Pixel Coordinates (2D, int)
What is the pinhole model?
In this model, the lens is replaced by a very narrow opening
(pinhole) through which lights go through directly into the image acquisition plane.
The point that is stroked by a light ray going through the pin hole in the direction of the lens main axis is called the image plane origin.
