Interpolation

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    from PIL import Image

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Table of Contents

We spent a fair amount of time this semester thinking about sampling (converting a CT signal into a DT signal). In this problem, we'll explore a couple of different methods of interpolation, which we'll use to approximate an underlying CT signal from a given set of DT samples.

Zero-Order Hold

One method for approximating the underlying continuous waveform is called zero-order hold (ZOH), which is illustrated in the following plot.

In this scheme, we use samples separated by T=1/sample_rate (left figure) to create a function of continuous time (right figure) by assigning the sample value to all times t in the interval nT\le t\lt(n\!+\!1)\,T. The advantage of this scheme is that it is easy to program (and easy to implement in hardware).

Write a function zoh(samples, sample_rate) that takes as arguments a list of samples and a sampling rate (Hz) that characterize x[n]. zoh should return a function. The returned function should take an arbitrary value of time t (in seconds) as input, and it should return the value of x_h(t) at that time t. You may assume that the first sample (n=0) corresponds to t=0 seconds, and you only need to worry about values of t that fall in the range associated with the given samples.

Enter your function in the box below and check that it works properly.

Error on line 15 of question tag.
    csq_tests.append({'code': 'func = zoh(%r, %d)\nans = func(%f)' % (samples, sr, t), 'check_function': checknum})

NameError: name 'checknum' is not defined

Linear Interpolation

It is often the case that the method described above does not produce a particularly good approximation of the underlying CT function. We can do better by using a better interpolator (i.e., to change the way we estimate the underlying continuous waveform from the discrete samples we are given). A good choioce is linear interpolation (sometimes referred to as a first-order hold), which is described by the figures below.

Write a function linear(samples, sample_rate) that returns the value of x_l(t) for an arbitary time t. Enter you function in the box below to check that it works properly.

Error on line 21 of question tag.
    csq_tests.append({'code': 'x_l = linear(%r, %d)\nans = x_l(%f)' % (samples, sr, t), 'check_function': checknum})

NameError: name 'checknum' is not defined

One neat thing we can do with the interpolators from above is to use them to generate resampled versions of a signal. We can do this by first approximating the underlying continuous function using one of the methods above, and then re-sampling that continuous function at a different sampling rate.

In general, if we are given N_1 samples of a DT signal sampled at sampling rate f_{s1} samples per second, then we can consider the underlying CT signal to have a length of N_1/f_{s1}. When we resample this CT function to generate a sequence of length N_2, we should sample every f_{s2} = (N_2/N_1)f_{s1} seconds.

In fact, we can apply the same idea in 2-D as well. An interesting way to implement these 2D interpolators is as repeated applications of a 1D interpolator. First apply a 1D interpolator to all of the rows of the image and stack these results together to get an image with width W_o and height H_i. Then apply the same 1D interpolator to all of the columns to get the final image.

ZOH

Write a function called zoh1D to do a 1D interpolation using a zero-order hold. The inputs to zoh1D should be

• sequence – which is a list of values,
• new_length – which is an integer specifying the length of the desired output

and the output should be a new list of values.

If it is helpful, you may assume that your zoh function from above is defined for you in the box below:

Error on line 39 of question tag.
    i['check_function'] = check_list

NameError: name 'check_list' is not defined

Next write a function called zoh2D to do a 2D interpolation using the zero-order hold. The inputs to zoh2D should be

• array – which is an array (list of lists) of values organized as a list of rows,
• new_width – which is an integer specifying the width of the desired output, and
• new_height – which is an integer specifying the height of the desired output

and the output should be a new list of lists (as opposed to a numpy array).

In this code box, zoh1D has been defined for you, so you may use it in your implementation of zoh2D.

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    'transform_output': block},

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We have provided a test image called zebra.png here.

Use your zoh2D program to create three new images and upload them below. Note that you can use arr.tolist() to convert a numpy array to a list-of-lists, and you can use numpy.array(lis) to convert a list-of-lists into a numpy array.

• z_tall.png with width of 100 and height of 500,
• z_short.png with width of 500 and height of 100,
• z_zoom.png with width of 500 and height of 500.

z_tall.png:

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z_short.png:

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z_zoom.png:

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Bilinear Interpolation

Write a function called linear1D to do a 1D interpolation using linear interpolation. The inputs to linear1D should be

• sequence – which is a list of values,
• new_length – which is an integer specifying the length of the desired output

and the output should be a new list of values.

You may assume that your linear function from above is defined for you.

Error on line 24 of question tag.
    {'code': 'ans = linear1D([0,1,0], 3)', 'check_function': check_list},

NameError: name 'check_list' is not defined

Next write a function called bilinear2D to do a 2D interpolation using biliear interpolation. The inputs to bilinear2D should be

• array – which is an array of values organized as a list of rows,
• new_width – which is an integer specifying the width of the desired output, and
• new_height – which is an integer specifying the height of the desired output

and the output should be a new list-of-lists, organized as a list of rows.

In this code box, linear1D has been defined for you, so you may use it in your implementation of bilinear2D.

Error on line 34 of question tag.
    'transform_output': block_pretty, 'check_function': check_2D_list},

NameError: name 'block_pretty' is not defined

Use your bilinear2D program to create three new images based on zebra.png and upload them below:

• l_tall.png with width of 100 and height of 500,
• l_short.png with width of 500 and height of 100,
• l_zoom.png with width of 500 and height of 500.

l_tall.png:

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l_short.png:

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l_zoom.png:

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