CSC1401 Viewing a picture as a 2D image 1

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CSC1401Viewing a picture as a 2D image - 1
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Review from the last week

• We used a getPixels() to get all of the pixels of
  a picture
• But this has been somewhat misleading
• The pixels are in a 2-dimensional array
• Getting the pixels has been placing them into a
  1-dimensional list

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What is a two-dimensional array?

• The pixels in a picture are really stored in a
  two-dimensional array
• Each pixel has a x value (horizontal location)
• Each pixel has a y value (vertical location)
• pictureObj.getPixel(x,y) returns the pixel at
  that location

x
y
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Example Two-Dimensional Arrays

• Maps
• That street is in D-5
• Battleship
• Try I-5
• Hit or miss
• Chairs at a theater or game
• Row C seat 20

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Nested Loop

• How would you get all the pixels in a picture
  using their x and y values
• First row x0 and y0, x1 and y0, x2 and y0,
  
• Second row x0 and y1, x1 and y1, x2 and
  y1,
• Third row x0 and y2, x1 and y2, x2 and y2,
  
• We need to have one loop inside another
• The outer loop counts y from 0 to height - 1
• The inner loop counts x from 0 to width 1
• We could also nest the loops the other way around

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Recall Alice
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Nested Loop Template

• // loop through the rows
• for (int y 0 y lt this.getHeight() y)
• // loop through the columns
• for (int x 0 x lt this.getWidth() x)
• // get the current pixel
• pixelObj this.getPixel(x,y)
• // do something to the color
• // set the new color
• pixelObj.setColor(colorObj)

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Mirroring

• What if we want to pretend to place a mirror in
  the middle of the picture
• We would see the left side of the picture
  mirrored on the right side

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Mirroring
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• If we just think of a number at each x and y
  location instead of a color
• The mirroring would look like this
• Can you find the algorithm to do this?

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What is the Mirror for this?
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• Try the solve the problem for small samples
• If you cant solve it on a small sample
• You cant write a program to solve it

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Mirror Algorithm

• Loop through all the rows (y starts at 0,
  increments by 1, and is less than the picture
  height)
• Loop with x starting at 0 and x less than the
  midpoint (mirror point) value
• Get the left pixel at x and y
• Get the right pixel at width 1 - x
• Set the color for the right pixel to be the color
  of the left pixel

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Mirror Algorithm to Code

• We are going to need the midpoint
• int midpoint this.getWidth() / 2
• Loop through the rows (y values)
• for (int y 0 y lt this.getHeight() y)
• Loop through x values (starting at 1)
• for (int x 0 x lt midpoint x)
• Set right pixel color to left pixel color
• Pixel leftPixel this.getPixel(x, y)
• Pixel rightPixel this.getPixel(this.getWidth()
  - 1 - x, y)
• rightPixel.setColor(leftPixel.getColor())

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Mirror Method

• public void mirrorVertical()
• int mirrorPoint this.getWidth() / 2
• Pixel leftPixel null
• Pixel rightPixel null
• // loop through the rows
• for (int y 0 y lt this.getHeight() y)
• // loop from 0 to just before the mirror
  point
• for (int x 0 x lt mirrorPoint x)

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Mirror Method - Continued

• leftPixel this.getPixel(x, y)
• rightPixel this.getPixel(this.getWidth()
  1 x, y)
• rightPixel.setColor(leftPixel.getColor())

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Mirroring in the other direction

• What about mirroring around a mirror held
  horizontally in the vertical center of the
  picture?
• Like a reflection in a lake?

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Mirror Horizontal Exercise

• Write the method to mirror the top half of the
  picture to the bottom half.
• This is a motorcycle redMotorcycle.jpg
• How about mirroring bottom to top?

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Summary

• A two-dimensional array has columns and rows
• Use nested loops to work with 2-d arrays
• One loop for the x direction and one for the y
• for (int x 0 x lt this.getWidth() x)
• // loop through the rows (y direction)
• for (int y 0 y lt this.getHeight() y)
  

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Assignment

• Review Media Computation Chapter 5, Section 1