Module 3: Video Sampling Lecture 18: Filtering operations in Camera and display devices. The Lecture Contains: Effect of Temporal Aperture:

Transcription

1 The Lecture Contains: Effect of Temporal Aperture: Spatial Aperture: Effect of Display Aperture: file:///d /...e%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture18/18_1.htm[12/30/2015 4:17:04 PM]

2 Effect of Temporal Aperture: A video camera typically accomplishes a certain degree of prefiltering in the capturing process. The intensity values read out at any frame instant are not the sensed intensity values at that time. In fact they are the averages of the sensed signal over a certain duration, referred to as the exposure time. Consequently, the camera is said to be applying a pre-filter in the temporal domain with an impulse response given by, The frequency response of this filter is, The above function reaches zero at. As is the temporal sampling rate, the ideal prefilter for this purpose will be a low-pass filter with cut off frequency at. By choosing, the camera can suppress the temporal aliasing components, near the sampling rate. However, too large a will blur the signal. In practice, the effect of blurring is sometimes more visible than aliasing. As a result, the exposure time must be chosen to reach a proper trade-off between aliasing and blurring effects. In addition to temporal integration discussed above, the camera also performs spatial integration. file:///d /...e%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture18/18_2.htm[12/30/2015 4:17:04 PM]

3 Spatial Aperture: The value of any pixel obtains from a sensor in a CCD camera is not the optical signal at that point alone. It is a weighted integration of the signals in a small window surrounding it, called the aperture of the camera. The shape of the aperture and the weighting values constitutes the cameras spatial aperture function. The aperture function serves as the spatial prefilter and its Fourier transform is referred to as the modulation transfer function (MTF) of the camera. For most cameras, we can approximate the spatial aperture function by a circularly symmetric Gaussian function given by, The spectrum of this function is also Gaussian given by, where The value of or depends on the size and shape of the aperture. They are usually designed such that the frequency response is 0.5 at half the vertical and horizontal sampling rates. Assuming we obtain file:///d /...e%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture18/18_3.htm[12/30/2015 4:17:04 PM]

4 The overall camera aperture function or prefilter is, with a frequency response given by The impulse response of a camera with and is shown below in figure below. The frequency response is shown for plane at. The frequency response is far from the ideal half band lowpass filter which should have a square pass-band defined by and observing the frequency response, we note that the frequency components inside the designed passband (the Vornoi cell) are attenuated, thereby reducing the signal resolution unnecessarily. In addition, the frequency components in the desired stop band are not completely removed. This results in aliasing in the sampled signal. It has been found that viewers are more annoyed by loss of resolution than aliasing artifacts. This is so because aliasing artifacts are visually noticeable only for images that contain high frequency periodic patterns close to the lowest aliasing frequency. This is rare in natural scene images. It is more preferable to preserve the signal in the passband than its suppression outside this band. Digital filters are often used for more peruse prefiltering. This involves sampling the signal at a rate higher than the desired sampling rate followed by a digital filter to suppress the frequencies outside the desired passband and a down sampler to convert this processed digital signal to the desired sampling rate. Although the use of sharp transition digital filters give better performance in terms of mean square error, it may give rise to Gibb's effect at sharp edges and also require higher order filters for its implementation. This is a serious drawback in video applications. file:///d /...e%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture18/18_4.htm[12/30/2015 4:17:05 PM]

5 Effect of Display Aperture: In a CRT monitor, an electronic gun emits an electronic beam across the screen line by line, striking phosphors with intensities proportional to the intensity of the video signal at corresponding locations. For display of color signal, three beams are emitted by three guns, striking the red, green and blue phosphors with the desired intensity combination at each location. The striking beam thickness plays an important role. It determines the amount of vertical filtering. If the beam is thin, it will make the image look sharp but also cause the perception of scan lines when the observer sits too close to the screen. On the other hand a thick beam will blur the image. Normally to reduce the loss of spatial resolution, thin beams are used so that very little vertical filtering is carried out by the display device. Temporal filtering is determined by the phosphors. The P22 phosphors used in color TVS decays to less than 10% of the peak value in 10ms to 1ms. This is much smaller than field time which is 16-7ms. Thus practically no temporal filtering is performed. Based on the spatio-temporal frequency response of the HVS, the eye performs to some degree the required interpolation. file:///d /...e%20(ganesh%20rana)/my%20course_ganesh%20rana/prof.%20sumana%20gupta/final%20dvsp/lecture18/18_5.htm[12/30/2015 4:17:05 PM]