CS 248: Introduction to Computer Graphics

Pat Hanrahan

4. The following questions concern signal processing and its application to computer graphics.

4A. (10 points) Define the following three terms, and dicuss how they are related.

Reconstruction.

The process of recovering an analog signal from discrete samples of that signal.

Interpolation.

A reconstruction method that creates an analog signal that interpolates all of the samples. An example is linear interpolation, that simply connects successive samples the straight lines.

Magnification.

Increasing the size of a signal (e.g. an image). Increasing the sampling rate of a signal. Since this may involve replicating samples, interpolation is often used to smooth out these changes.

Scoring:

3 points each definition

1 point for relating them in some way

C(x) and Y(x) in the Fourier domain
This is a plot of the maginitude of the combined spectrum

How might the functions C(x) and Y(x) be extracted from the combined signal?

First, let's assume that C(x) and Y(x) are bandlimited - i.e. they don't contain frequencies larger than w/2 or smaller than -w/2 in this case. To extract Y(x), multiply Y(w) by a box function of width w centered at the origin. Apply the inverse Fourier transform to get the result.

To extract C(x), either apply a high pass filter to eliminate Y, or apply a low-pass filter (i.e. a box) that has been shifted to w. This will yeild a copy of C(w) centered at w. Convolve this with FT(sin(wx)). This yeilds a copy of C(w) at the origin, a copy at C(w), and a copy at C(2w). Multiply this by a box centered at the origin and do an inverse Fourier transform to recover C(x).

Multiplying by a shifted box to get one copy of C(w) centered at w

Convolving with FT(sin(wx)) and multiplying by a box to get a copy of C(w) at the origin.

Suppose the function Y(x) is the luminance of an image, and the function C(x) is the color (don't worry about what color space). What happens to the extracted function C(x), if Y(x) contains frequencies greater than 1/2 the carrier frequency? How would this appear to the observer?

First, let's assume that we always use a box of width w for a reconstruction filter (so it extends from -w/2 to w/2 if centered at the origin). This is reasonable since we don't know in advance the bandwidth of either C or Y, so we just choose to use the same reconstruction filter for both signals. Thus, if Y(x) contains frequencies greater than w/2, then Y(w) will extend past w/2, and thus not all of it will be captured by the reconstruction filter for Y. Worse, some of the spectra of Y will now be in C's reconstruction filter, and so high Y frequencies will masquerade as C frequencies. The result: high frequency changes in luminance will have distorted color. The best example of this is looking at someone wearing a shirt with thin stripes on a television. The stripes will appear colored because of the artifact described here.

Some special notes:

• Convolving by sin(wx) yeilds two copies of the spectrum of C(x) in the first part. This is the essense of the first part.
• The Fourier Transform is a linear operator. Thus, FT(f(x)+g(x)) = FT(f(x)) + FT(g(x))
• If you want to extract either C or Y from the combined signal, you need to make sure to both multiply by a box in the Fourier domain and do the inverse Fourier on the result!
• This problem has absolutely nothing to do with the Nyquist rate or Shannon's Theorem. They both deal with sampled signals. There are no sampled signals in this problem.

• 5 points per section
• Important items for first part: diagram, Fourier transform of sin(wx), C(w) shifted to w
• Important items for second part: low pass/high pass followed by inverse Fourier transform, shifting of C(w) before transform
• Important items for third part: aliasing, high frequency luminance appears as color artifacts, nothing to do with sampling frequency (since we aren't sampling)