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# CSCE 489/689 – Computational Photography Programming Assignment 6

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CSCE 489/689 – Computational Photography
Programming Assignment 6

1 Goal
Modern cameras are unable to capture the full dynamic range of commonly encountered real-world scenes.
In some scenes, even the best possible photograph will be partially under or over-exposed. Researchers and
photographers commonly overcome this limitation by combining information from multiple exposures of
the same scene. You will write software to automatically combine multiple exposures into a single high
dynamic range radiance map, and then convert this radiance map to an image suitable for display through
tone mapping.
2 Starter Code
Starter code (in both MATLAB and Python) along with the images can be downloaded from here. The
package includes 2 scenes, which have been captured with two different cameras. “Chapel” is captured
with a Canon 35mm SLR and “Office” is captured with a Canon 5D Mark iv. Note that, some of the
expected results are included in the “Results” folder.
Here, you will calculate the camera response function (CRF) from a set of images captured with different
exposure times. Two scenes are provided in the “Images” folder. You need to obtain the CRFs for both
these cases, and plot the CRFs like Fig. 7 (d) of Debevec’s paper. Generally you have to do the followings
to estimate the CRF:
• Read the images and their corresponding exposures (already done in the starter code).
• Randomly select N pixels to perform the optimization. Note that, you use the same N randomly
selected positions for all the images in the stack. Choosing a very large N could slow down the
optimization process. Therefore, you should choose a reasonable N, e.g., N ≈ 5 × 256/(P − 1),
where P is the number of images in the stack.
• Write the triangle function defined in Eq. 4 of the Debevec’s paper and discussed in the class.
• Perform the optimization. This is provided in the starter code (gsolve.m for MATLAB and
gsolve function for Python). You just have to provide appropriate inputs to the function. Note
that, the dimension of the inputs are as follows: Z (N × P), B (P × 1), l (scalar, the value provided
in the main file), w (256 × 1).
In this task, you use the calculated CRF of each scene to reconstruct the radiance of the scene. This can be
done by implementing Eq. 6 in Debevec’s paper, which was discussed in the class. Once you obtain the
radiance, you need to tonemap it to be able to show the results. For this, you will be implementing a global
and local tonemapper.
Global – Here, you will obtain a tonemapped image using gamma compression as follows:
T =

E
max(E)
γ
(1)
Choose a γ value (less than 1) that produces the best result in each case.
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Local – You will implement a simple local tonemapper (similar to the one discussed in the class) as
follows:
• Compute the intensity (I) by averaging the color channels.
• Compute the chrominance channels: (R/I, G/I, B/I)
• Filter that with a Gaussian filter: B = filter(L). Larger standard deviations result in tonemapped
images with more details. Standard deviation of 0.5 to 2 seem reasonable.
• Compute the detail layer: D = L − B
• Apply an offset and a scale to the base: B0 = (B − o) ∗ s
– The offset is such that the maximum intensity of the base is 1. Since the values are in the log
domain, o = max(B).
– The scale is set so that the range of output base is dR, i.e., s = dR / (max(B) – min(B)). Values
around 4 or 5 for dR should look fine.
• Put back the colors: R0
, G0
, B0 = O ∗ (R/I, G/I, B/I)
• Apply gamma compression. Without gamma compression the result will look too dark. Values
around 0.5 should look fine (e.g. result0.5
).
Your HDR images have zero values which will cause problems with log. You can fix this problem by
replacing all zeros by some factor times the smallest non-zero values.
5 Write Up
For both scenes, you should show the estimated CRF and the global and local tonemapped version of the
HDR image (radiance). Describe how you implemented the assignment. Discuss any problem you faced
when implementing the assignment or any decisions you had to make.
7 Deliverables
Your entire project should be in a folder called “firstname lastname”. This folder should be zipped
up and submitted through e-campus. Inside the folder, you should have the followings:
• A folder named “Code” containing all the codes for this assignment. Please include a README file
to explain what each file does if you add any other files to the starter code.
• A report in the pdf format. Make sure you write your name on top of the report. Also make sure
the pdf file is under 5 MB.
Make sure you exclude all the results and original images from your submission.
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8 Checklist
Make sure you can check all the items below before submitting your assignment. You will lose 5 points for
each item that cannot be checked.
The root folder is called “firstname lastname”. Note between first and last name.
Inside the root folder, there is a folder called “Code” that contains your source code. Also make sure
the report is in the root folder.
The folders “Images” and “Results” are not included (you only submit your codes and a report).
Name written on top of the report.
The report is in pdf format and the file is under 5 MB.
9 Ruberic
Total credit: [100 points]
[30 points] – CRF estimation
[10 points] – Global tonemapping
[30 points] – Local tonemapping
[10 points] – Write up
10 Acknowlegements
This project is partially based on James Hays Computational Photography course with permission.
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CSCE 489/689 – Computational Photography Programming Assignment 6
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