The object of computer vision is a digital image.
So, first of all, we should understand what the digital image is.
Let's start from optical image.
Optical image is 2D representation of a physical object,
obtained by propagation of light through some optical system,
depicting object contours and features.
As you can see from these examples,
optical systems can be very different.
The first image is obtained through this common camera.
Second, there's a fisheye lens camera and capture 180 degrees by 360 degrees hemisphere.
The last image is example of
optical tomography slight based the construction of an object.
But mostly, they are working the simplest model,
which is called the pinhole camera model.
This is probably the first optical system,
which allow humans to capture the shape of objects from projected light.
The idea was already known to Ancient Greeks. You have a dark room.
There's a small hole in one wall.
A light come through the small hole and fall into the other hole, making an image.
Human artist then can draw the contour of an object.
Pinhole camera captures pencils of rays.
All rays pass to a single point.
This point is called the center of projection or focal point.
The image is formed on the hole which is called image plane.
During this perspective projection,
the distortion of some properties of real-world objects are introduced.
For example, the vertical lines are convergent instead of being parallel.
The exterior columns in the old same columns appear larger.
Such distortions come not from flaws of our optical system,
but are inherent properties of perspective projection.
This problem was first pointed out by Leonardo da Vinci himself.
During image capturing, we obtained 2D images from 3D world.
Thus, some information is discarded. What have we lost?
We have lost angles and distances.
The square table is not quite in the image.
The main property which is retained is linearity.
If three points are located on the single line in 3D space,
then their projections also lie on a single way.
But in practice, this property also can be violated due to optical effects in lenses.
Such effect is called radial distortions.
And as a result, straight lines are projected into slightly curved lines,
or not so slightly if fisheye lens are used.
The modern digital camera is based on the same principle as pinhole model,
but is complex optical system and digital sensor array instead of wide wall.
Digital sensory gives us discrete representation of optical image.
First, the spatial sampling is performed.
Sensory consist of a set of photosensitive elements on the rectangular grid.
Each elements measure the total light fallen into the senior element.
So, instead of continuous representation of optical image,
we have only a set of samples on the grid.
Then, the continuous measurement of light energy is also discretized.
So, digital grayscale image is a 2D array,
where each value measures image brightness.
Usually, brightness value weigh in interval from zero to one.
We can code brightness value with one byte.
So, b is usually lie in interval from 0 to 255.
We can also obtain better precision by coding b is more bits.
In modern camera, usually from 10 to 16 bits are used.