SUNY Geneseo Department of Mathematics

An Algorithm for Chroma Keying

Friday, April 10

Math 230 02
Spring 2015
Prof. Doug Baldwin

Misc
- Move test to Apr. 17?
  - No, keep on Monday Apr. 20
  - Material since 1st hour exam (e.g., control structure: for, while, if, vectorization; algorithms: bisection, chroma key; etc.)
  - Rules & format similar to 1st exam
  - I’ll give you sample exam
- Colloquium
  - “Quantum Messages and Quantum Games”
  - Monday, April 13, 4:00 - 4:50, Newton 201
  - Tom Cooney, Louisiana State University
  - Faculty candidate!
  - Extra credit for summaries
Questions?

Chroma Key

Algorithm

for each source pixel
    if R,G,B intensities in range for “green” screen
        copy background image pixel to output
    else
        copy source pixel to output

How to vectorize this for Matlab?

Comparisons?

Comparison operators do basic comparisons

Combine comparison results in several ways, but clearest is & operator

>> A = rand( 4 )
A =
    0.8147    0.6324    0.9575    0.9572
    0.9058    0.0975    0.9649    0.4854
    0.1270    0.2785    0.1576    0.8003
    0.9134    0.5469    0.9706    0.1419
>> low = 0.25;
>> hi = 0.75;
>> low < A
ans =
     1     1     1     1
     1     0     1     1
     0     1     0     1
     1     1     1     0
>> A1 = ans
A1 =
     1     1     1     1
     1     0     1     1
     0     1     0     1
     1     1     1     0
>> A2 = A < hi
A2 =
     0     1     0     0
     0     1     0     1
     1     1     1     0
     0     1     0     1
>> A1+A2
ans =
     1     2     1     1
     1     1     1     2
     1     2     1     1
     1     2     1     1
>> low < A & A < hi
ans =
     0     1     0     0
     0     0     0     1
     0     1     0     0
     0     1     0     0
>> A1 & A2
ans =
     0     1     0     0
     0     0     0     1
     0     1     0     0
     0     1     0     0
>> A1 .* A2
ans =
     0     1     0     0
     0     0     0     1
     0     1     0     0
     0     1     0     0
>> A
A =
    0.8147    0.6324    0.9575    0.9572
    0.9058    0.0975    0.9649    0.4854
    0.1270    0.2785    0.1576    0.8003
    0.9134    0.5469    0.9706    0.1419
>> mask = low < A & A < hi
mask =
     0     1     0     0
     0     0     0     1
     0     1     0     0
     0     1     0     0

“Copy pixel” steps?

“Mask” produced by comparisons can be used to select elements from a matrix in several ways, clearest of which is logical indexing

Logical indexing uses a vector or array of logical values as an index, and selects elements wherever the logical array is true

>> A .* mask
ans =
         0    0.6324         0         0
         0         0         0    0.4854
         0    0.2785         0         0
         0    0.5469         0         0
>> A( mask )
ans =
    0.6324
    0.2785
    0.5469
    0.4854
>> out = zeros(4)
out =
     0     0     0     0
     0     0     0     0
     0     0     0     0
     0     0     0     0
>> A
A =
    0.8147    0.6324    0.9575    0.9572
    0.9058    0.0975    0.9649    0.4854
    0.1270    0.2785    0.1576    0.8003
    0.9134    0.5469    0.9706    0.1419
>> out(mask) = A(mask)
out =
         0    0.6324         0         0
         0         0         0    0.4854
         0    0.2785         0         0
         0    0.5469         0         0

Next
- Code chroma key

Next Lecture