------------------------------------------------------------------ Please write your answer below each question ------------------------------------------------------------------ ------------------------------------------------------------------ Part B.1 Questions (you must answer questions 1-4 to get any credit for Part B.1!!) ------------------------------------------------------------------ 1. What camera model did you use to capture these photos 2. What camera settings did you use (if any)? 3. What was the order in which the photos were captured? (eg. "comp1.jpg,comp2.jpg,...") 4. In 2-3 sentences, describe your image acquistion procedure (eg. "I put the camera on a table, then put the object in front of a piece of cloth, and took a picture; then ...") 5. If you consider your experiment unsuccessful, explain why it was not possible for you to capture a good set of photos for the matting algorithm ------------------------------------------------------------------ Part B.2 Questions: Note that you need to answer two of the three questions to get full credit for Part B.2. If you answer all three, you will receive EXTRA credit. ------------------------------------------------------------------ First, run the algorithm on the following input: Background 1: test_images/small/flowers-backA.jpg Background 2: test_images/small/flowers-backB.jpg Composite 1: test_images/small/flowers-compA.jpg Composite 2: test_images/small/flowers-compB.jpg Then, run the algorithm with the following input: Background 1: test_images/small/flowers-backC.jpg Background 2: test_images/small/flowers-backD.jpg Composite 1: test_images/small/flowers-compC.jpg Composite 2: test_images/small/flowers-compD.jpg You will notice that in the second run both the computed Alpha Matte and the Object image has many "artifacts". ------------------------------------------------------------------ Question 1 (10 points) The Object image computed in the second run contains a lot of pixels that should really be part of the background, but aren't. Equivalently, even though the Alpha Matte should be zero at those pixels, the algorithm assigned non-zero values. Take, for example, pixel (I,J) = (50,375) with intensity (243,243,243). Its alpha value is (243/255), i.e., close to 1, even though it should really be 0. To take a better look at individual pixels in that region, enter the following values in the view control boxes on both panels (Zoom: 10; Center I: 50; Center J: 375) Explain why this pixel has a non-zero alpha. Be as concrete as possible. ------------------------------------------------------------------ Question 2 (10 points) Conversely, there are many pixels that should be fully opaque (ie. part of the foreground), but aren't. Take, for example, pixel (I,J) = (332,164). Even though that pixel is on a flower petal, which is on the foreground, the intensity of its Alpha component is (200,200,200), meaning that its alpha value (=200/255) is less than 1. The effect is that a portion of the flower is taken to be partially transparent. Explain why this pixel has non-unit alpha. Be as concrete as possible. Hint: examine the linear system of equations that must be solved for that pixel (and the associated matrix). ------------------------------------------------------------------ Question 3 (10 points) Now let's go back to the first run above (ie. the Alpha Matte and Object computed from the *A.jpg and *B.jpg images). Looking closely at the computed Alpha Matte, you will notice that the alpha values are either zero or near zero on the right size of the vase. For example, pixel (I,J)=(515,819) has intensity (0,0,0). Use Zoom=1; (I,J) = (515,819) for better viewing... On the other hand, pixels on the *left* side of the vase have non-zero (albeit small) alpha values. For example, pixel (I,J)=(50,816) has alpha equal to 0.1=25/255, suggesting that there is there is an "object" in front of the background in that part of the photo, with a transparency of about 90%. The effect may be slight, but it is noticeable (Use Zoom-1; (I,J)=(50,816) for better viewing). Why does that pixel (and many others on the left side of the vase) have non-zero alpha? Be as concrete as possible. ------------------------------------------------------------------