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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


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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


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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



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PotW

Have fun at Harker Proco~


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