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3.11.13. It\'s a palindrome!. Announcements. March USACO 2nd to last round last day to take it if busy, can start late too because of CAHSEE tomorrow. Harker Programming Invitational (3/16) Cool prizes! This Saturday. Announcements (registration). Stanford ProCo (5/19) Cool prizes!

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3 11 13

3.11.13

It\'s a palindrome!

announcements
Announcements
  • March USACO
    • 2nd to last round
    • last day to take it
      • if busy, can start late too because of CAHSEE tomorrow.
  • Harker Programming Invitational (3/16)
    • Cool prizes!
    • This Saturday
announcements registration
Announcements (registration)
  • Stanford ProCo (5/19)
    • Cool prizes!
    • Signups are in progress
    • 6 team max per school
  • Google Codejam
    • Somewhat similar to USACO
    • Series of qualification rounds
    • Rounds get rapidly more difficult
    • Registration begins tomorrow
example
Example

Source

Target

image manipulation
Image Manipulation
  • Images are made of pixels
    • Colors are formed using red, green, and blue subpixels
  • Brightness
    • Calculated most simply as sum of red, green, and blue channels
  • Gradient
    • Calculated from neighboring pixels
    • Take differences in brightness
    • Is a vector:
      • Magnitude
      • Direction
example cont
Example (cont.)

Gradient

Final

general idea
General Idea
  • To seemingly blend a pixel into another image, its neighboring pixel values must be known
  • Need to consider how pixels change with respect to their neighbors
    • Gradient
  • Alter the boundary pixels of the foreground image to be equal to corresponding pixels of the background image
    • Need to make an improved version of the foreground image that blends with the background image
slide9
cont.
  • Copies regions from a source image to a target image
  • If one of the neighbors is a boundary pixel, its value is fixed
    • Keep the transition smooth
  • Preserve the gradient of the foreground image pixels
    • The sum of the differences between a pixel and its neighbors
  • If one of the neighbors is out of bounds of the selection, it is excluded
    • Done for all pixels within the improved foreground
slide10
cont.
  • Assume there are n pixels in foreground
  • Need to solve for every pixels
  • System of linear equations in n variables
    • n by n matrix
  • N is very large
  • Note that the matrix is very sparse
  • Can use Gauss Jordan Elimination (RREF)
    • too slow
  • Use conjugate gradient
slide12
PotW

Have fun at Harker Proco~

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