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Assignment #1 Demosaicing

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Programming Assignment #1

Description
The goal of this assignment is to gain an understanding of demosaicing: converting the Bayer pixel pattern into a
RGB representation where each pixel has red, green and blue color channels. Demosaicing is typically the first step in a sequence of non-linear operations of the imaging pipeline that converts
the raw output of the digital camera’s sensor into a JPEG-compressed image which can then be displayed on the
monitor. The figure below shows an example of a Bayer arrangement on the pixel array of an image sensor.
Figure 1: Bayer pattern
Part 1:
Implement a very simple linear interpolation [averaging the 4 or 2 nearest neighbours] approach that converts the
Bayer arrangement to RGB components for each pixel as shown in the figure below [note that the figure is an
example and does not reflect the Bayer arrangement used by the digital camera]. You should create a kernel for
each channel and use cv::filter2D(…) instead of using loops. The “mosaic” image was created by taking the
original color image and keeping only one color component for each pixel, according to the standard Bayer
pattern:
B R B R. . . R G R G. . . B R B R. . .
. . ,
C.POULLIS COMP498G-691G COMPUTER VISION WINTER 2019
This method produces artifacts. To visualize these artifacts compute the image of the squared differences
between the original and reconstructed values for each pixel, summed over the three color
channels.
Original, demosaic, root squared difference. Finally, show a close-up of some patch of the reconstructed image where the artifacts are particularly apparent
and explain the cause of these artifacts. Part 2:
Bill Freeman proposed an improvement of the simple bilinear interpolation approach. Since the R channel is
sampled at a higher rate than the G and B channels, one might expect interpolation to work better for R values. Then it would make sense to use the interpolated R channel to modify the interpolated G and B channels. The
improved algorithm begins with linear interpolation applied separately to each channel, just as you have already
done above. The estimated R channel is not changed, but G and B channels are modified as follows. First, compute the difference images G-R and B-R between the respective interpolated channels. Mosaicing artifacts
tend to show up as small “splotches” in these images. To eliminate the “splotches”, apply median filtering to the
C.POULLIS COMP498G-691G COMPUTER VISION WINTER 2019
G-R and B-R images. Finally, create the modified G and B channels by adding the R channel to the respective
difference images. Implement the above algorithm and visualize the quality of the results in the same way as for Part 1 by displaying
the per-pixel difference image. Compare the output to that of Part 1. Are there visible improvements (especially in
the close-up patch selected in Part 1)?
Hint: Implementing this method should take you six lines of code. Submission (electronic submission through EAS only)
Please create a zip file containing your C/C++ or Python code and a readme text file (.txt). In the readme file document the features and functionality you have implemented, and anything else you want the
grader to know i.e. control keys, keyboard/mouse shortcuts, etc. Additional Information
● Mosaic images and their originals can be downloaded from here:
http://www.poullis.org/courses/2019/Winter/COMP499G-691G/resources/image_set.zip

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