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Perspective Rectifier 3.3

The functions in this section use a so-called pinhole camera model. The view of a scene is obtained by projecting a scene's 3D point \(P_w\) into the image plane using a perspective transformation which forms the corresponding pixel \(p\). Both \(P_w\) and \(p\) are represented in homogeneous coordinates, i.e. as 3D and 2D homogeneous vector respectively. You will find a brief introduction to projective geometry, homogeneous vectors and homogeneous transformations at the end of this section's introduction. For more succinct notation, we often drop the 'homogeneous' and say vector instead of homogeneous vector.

perspective rectifier 3.3


In some cases, the image sensor may be tilted in order to focus an oblique plane in front of the camera (Scheimpflug principle). This can be useful for particle image velocimetry (PIV) or triangulation with a laser fan. The tilt causes a perspective distortion of \(x''\) and \(y''\). This distortion can be modeled in the following way, see e.g. [138].

The function converts points homogeneous to Euclidean space using perspective projection. That is, each point (x1, x2, ... x(n-1), xn) is converted to (x1/xn, x2/xn, ..., x(n-1)/xn). When xn=0, the output point coordinates will be (0,0,0,...).

However, if not all of the point pairs ( \(srcPoints_i\), \(dstPoints_i\) ) fit the rigid perspective transformation (that is, there are some outliers), this initial estimate will be poor. In this case, you can use one of the three robust methods. The methods RANSAC, LMeDS and RHO try many different random subsets of the corresponding point pairs (of four pairs each, collinear pairs are discarded), estimate the homography matrix using this subset and a simple least-squares algorithm, and then compute the quality/goodness of the computed homography (which is the number of inliers for RANSAC or the least median re-projection error for LMeDS). The best subset is then used to produce the initial estimate of the homography matrix and the mask of inliers/outliers.

The function computes the rectification transformations without knowing intrinsic parameters of the cameras and their relative position in the space, which explains the suffix "uncalibrated". Another related difference from stereoRectify is that the function outputs not the rectification transformations in the object (3D) space, but the planar perspective transformations encoded by the homography matrices H1 and H2 . The function implements the algorithm [97] .

Power supplies designed for worldwide use were once equipped with an input voltage selector switch that allowed the user to configure the unit for use on local power grid. In the lower voltage range, around 115 V, this switch is turned on changing the power grid voltage rectifier into a voltage doubler in delon circuit design. As a result, the large primary filter capacitor behind that rectifier was split up into two capacitors wired in series, balanced with bleeder resistors and varistors that were necessary in the upper input voltage range, around 230 V. Connecting the unit configured for the lower range to a higher-voltage grid usually resulted in an immediate permanent damage. When the power-factor correction (PFC) was required, those filter capacitors were replaced with higher-capacity ones, together with a coil installed in series to delay the inrush current. This is the simple design of a passive PFC.

A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as amplitude modulation detectors (envelope detectors) of radio signals. Rectifiers are most commonly made using solid state diodes but other type of components can be used when very high voltages or currents are involved.When only a single diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is simply one of usage. The term rectifier describes a diode that is being used to convert AC to DC. Most rectifier circuits contain a number of diodes in a specific arrangement to more efficiently convert AC power to DC power than is possible with only a single diode.

A full-wave rectifier converts both the positive and negative halves of the input waveform to a single polarity (positive or negative) at its output. By using both halves of the AC waveform full-wave rectification is more efficient than half wave.

When a simple transformer with out a center tapped secondary is used, four diodes are required instead of the one needed for half-wave rectification. Four diodes arranged this way are called a diode bridge or bridge rectifier as shown in figure 6.2. The bridge rectifier can also be used for translating a DC input of unknown or arbitrary polarity into an output of known polarity. This is generally required in electronic telephones or other telephony devices where the DC polarity on the two phone wires is unknown. There are also applications for protecting against accidental battery reversal in battery-powered circuits.

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anode-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings are required on the transformer secondary to obtain the same output voltage compared to the bridge rectifier above. This is not as efficient from the transformer perspective because current flows in only one half of the secondary during each positive and negative half cycle of the AC input.

If a second pair of diodes is included as in figure 6.4 then both positive and negative polarity voltages with respect to the transformer center tap can be generated. One can also view this arrangement to be the same as adding a center tap to the secondary winding in the full-wave bridge rectifier from figure 6.2.

Half-wave or full-wave rectification does not produce a constant-voltage DC as we have seen in the previous figures. In order to produce a steady DC voltage from a rectified AC source, a filter or smoothing circuit is needed. In the simplest form this can be just a capacitor placed across the DC output of the rectifier. There will still remain an amount of AC ripple voltage where the voltage is not completely smoothed. The amplitude of the remaining ripple depends on how much the load discharges the capacitor between the peaks of the waveform.

Sizing of the filter capacitor, C1, represents a tradeoff. For a given load, RL, a larger capacitor will reduce ripple but will cost more and will create higher peak currents in the transformer secondary and in the supply feeding it. In extreme cases where many rectifiers are loaded onto a power distribution circuit, it may prove difficult for the power distribution grid to maintain a correctly shaped sinusoidal voltage waveform.

For a given tolerable ripple the required capacitor size is proportional to the load current and inversely proportional to the supply frequency and the number of output peaks of the rectifier per input cycle. The load current and the supply frequency are generally outside the control of the designer of the rectifier system but the number of peaks per input cycle can be affected by the choice of rectifier design. The maximum ripple voltage present for a Full Wave Rectifier circuit is not only determined by the value of the smoothing capacitor but by the frequency and load current, and is calculated as:

A half-wave rectifier, figure 6.5(a) will only give one peak per cycle and for this and other reasons is only used in very small power supplies and where cost and complexity are of concern. A full wave rectifier, figure 6.5(b) achieves two peaks per cycle and this is the best that can be done with single-phase input. For three-phase inputs a three-phase bridge will give six peaks per cycle and even higher numbers of peaks can be achieved by using transformer networks placed before the rectifier to convert to a higher phase order.

A more usual alternative to a filter, and essential if the DC load requires a very smooth supply voltage, is to follow the filter capacitor with a voltage regulator which we will discuss in section 6.3. The filter capacitor needs to be large enough to prevent the troughs of the ripple getting below the drop-out voltage of the regulator being used. The regulator serves both to remove the last of the ripple and to deal with variations in supply and load characteristics. It would be possible to use a smaller filter capacitor (which can be large for high-current power supplies) and then apply some filtering as well as the regulator, but this is not a common design strategy. The extreme of this approach is to dispense with the filter capacitor altogether and put the rectified waveform straight into an inductor input filter. The advantage of this circuit is that the current waveform is smoother and consequently the rectifier no longer has to deal with the current as a large current pulse just at the peaks of the input sine wave, but instead the current delivery is spread over more of the cycle. The downside is that the voltage output is much lower - approximately the average of an AC half-cycle rather than the peak.

The simple half wave rectifier can be built in two versions with the diode pointing in opposite directions, one version connects the negative terminal of the output direct to the AC supply and the other connects the positive terminal of the output direct to the AC supply. By combining both of these with separate output smoothing capacitors it is possible to get an output voltage of nearly double the peak AC input voltage, figure 6.7. This also provides a tap in the middle, which allows use of such a circuit as a split rail (positive and negative) supply.

A variant of this is to use two capacitors in series for the output smoothing on a bridge rectifier then place a switch between the midpoint of those capacitors and one of the AC input terminals. With the switch open this circuit will act like a normal bridge rectifier with it closed it will act like a voltage doubling rectifier. In other words this makes it easy to derive a voltage of roughly 320V (+/- around 15%) DC from any mains supply in the world, this can then be fed into a relatively simple switched mode power supply.


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