CS 348B - Computer Graphics: Image Synthesis Techniques

Homework 3 - Camera Simulation

Assigned April 19.   Due May 1.


Many rendering systems approximate the light arriving on the film plane by assuming a pin-hole camera, which produces images where everything that is visible is sharp.  In contrast, real cameras contain multi-lens assemblies with different imaging characteristics such as limited depth of field, field distortion, vignetting and spatially varying exposure.  In this assignment, you'll extend pbrt with support for a more realistic camera model that accurately simulates these effects.  

Specifically, we will provide you with data about real wide-angle, normal and telephoto lenses, each composed of multiple lens elements. You will build a camera plugin for pbrt that simulates the traversal of light through these lens assemblies.  

With this camera simulator, you'll explore the effects of focus, aperture and exposure.  You will empirically characterize the critical points of the telephoto and normal lenses.  Using these data you can  optimize the performance of your simulator considerably.

Step 0

You must have pbrt installed correctly, as in homework 1.  

In addition, you should add . (the local directory) to your PBRT_SEARCHPATH.  

Step 1

Re-read A Realistic Camera Model for Computer Graphics by Kolb, Mitchell, and Hanrahan. 

Step 2: Compound Lens Simulator

Sample Images:


Some conventions:


Step 3: Exploring the way a camera works with your simulator

In this section you'll explore how various choices affect focus, depth of field and exposure in your camera.  Keep copies of your scenes that you use here, as you will email them to us. 

  1. The double gauss and telephoto lenses can be well approximated as a thick lens.  
    1. Your first task is to determine the critical points of these lenses to characterize its thick lens approximation.  Compute these critical points by tracing rays that are parallel to the optical axis from the front and back of the lens stack, as described in the paper.  (These will give you the coordinates for D_P, D_P', D_F and D_F' in the diagram below, which you will report in your write-up).  How do the parameters for the two lenses differ?  
    2. Your second task is to verify your critical point calculation by taking an appropriate picture with the camera. The scene will consist of a depth of field target.  The idea here is to take a picture of a ruler on a 45 degree angle.  If the ruler intersects the plane of focus, then the sharpest point on the image of the ruler lets you deduce the depth of the conjugate plane.  We have created a file, dof_target.pbrt, and a test scene, dof-test.pbrt, that includes this object.  The scene is set up so that when you take a picture of the ruler the point of sharpest focus lets you read off the distance of the focal point from the front of the lens.  For example, the following image shows that the focal point is between 70 and 80 mm of the front of the ruler.  The texture for the ruler is shown on the right.

      Your job is to modify dof-test.pbrt  to produce a picture where the subject is projected at unit magnification onto the film plane (i.e. 1 mm on the subject appears as 1 mm on the film), and show that the focal depth is as expected given your critical point calculation.  Specifically,


      1. Compute the values for D_X and D_X' in the following diagram that will produce an image at unit magnification.  Hint: at unit magnification, W = W'.  (Why is that?)

      2. Modify dof-test.pbrt so that the depth of field target at depth D_X will be visible in the output picture.  You will need to translate the target to get the appropriate focal depth in the frame.  For example, if you calculate that D_X should be at 1000 mm away from the front of the lens (not shown), then you could translate the target away from the camera by 850 mm so that D_X would intersect the middle of the target (around the 15 cm mark).  In addition, you will need to set the film depth so that the film plane intersects your computed D_X'.
      3. Render your scene and verify that the plane in focus in the rendered image and your chosen film plane indeed match your calculations for D_X and D_X'. 

      Note: Taking a picture of a depth of field target in this way is a convenient way to characterize the critical points of a physical camera.  In a real camera it isn't easy to trace parallel rays through the lens and measure where they cross the optical axis (as you did in step (a) above with your ray-tracer).  Instead, the idea is that you move the depth of field target until the plane in focus is at roughly unit magnification.  When this is achieved, the focal setting on the camera gives D_X', and the focal plane on the depth of field target gives D_X.  Along with the observed magnification and focal length of the camera, F, you can calculate all the other variables in the diagram above, giving you the critical points. 

  2. Investigate depth of field.  Use the telephoto lens for this test.  
    1. Set up the camera so that it frames the depth of field target and is focused at 1 meter.  Now take two pictures, one with the aperture set at the maximum radius, and another with it stopped down to 1/2 its radius.  How does the depth of field change?
    2. Now take two pictures with the aperture wide open.  One should be focused at 1 meter and the other at 2 meters.  How does the depth of field change?
  3. Investigate exposure.  Render a scene (any scene you like) with the aperture full open and half open.  How does the exposure change?  Does your ray tracer output the correct exposure?  Why or why not?


Step 4: Web page submission

  1. In the archive that you downloaded in Step 1, you will find a "submission" directory. Copy this directory to your web space, and edit index.html to include your renderings and a description of your approach. The items that you have to replace are marked in green. 
  2. Please send an email to cs348b-spr0405-staff@lists.stanford.edu



Please check this FAQ before sending questions to the staff email list.  We will update this list as common issues arise. 

  1. Q: PBRT complains that it can't find realistic.dll or realistic.so.  What should I do?
    A: Make sure that '.' (the local directory) is in your PBRT_SEARCHPATH environment variable.  In this assignment, you will compile a camera plugin (realistic.so/dll) in your working directory. Note that the path is ':' delimited on Linux and ';' delimited on Windows.
  2. Q: Should we be implementing the thick lens approximation described in the paper or a full lens simulation?
    A: Please implement the full lens simulation, which is less work for you and only slightly less efficient computationally.  Implementing the thick lens approximation is a superset of the work, since you need the full lens simulation in order to calculate the thick lens parameters. 
  3. Q: Why are my images so noisy?
    A: Integrating over the surface of the lens, especially for large apertures, requires multiple samples for accuracy.  Too few samples increases the variance of the integral estimate, resulting in noisy images.  The problem is exacerbated in Step 2 by the fact that you will fire many rays at the lens system that will not make it into the world because they will hit the aperture stop.  To drive down the noise, you can increase the number of samples ("integer pixelsamples" parameter in the scene file).  
  4. Q: How can the value that you read off the depth of field target ruler be equal to the depth?  It's on a 45 degree angle!
    A: Take a look at dof_target.pbrt.  The ruler is scaled by sqrt(2), to allow you to read off the depth in this convenient way.


Miscellaneous Data

You can replace the cones in the rendered scenes with the dragons shown in Figures 6.6-6.9 in the PBRT book.  Copy the dragon.pbrt scene from the book CD, and comment out the appropriate lines in the hw3 directory scenes.  Be warned that this requires a fair amount of memory to render. 



For each step (2 and 3)

** Passes all or almost all our tests.
* Substantial work was put in, but didn't pass all our tests.
0 Little or no work was done.

*** Extra credit may be given on a case-by-case basis for well done extensions (for any part of the project) that produce superior results.


Copyright © 2005 Pat Hanrahan