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Slides on Copying and Transforming Pictures | CS A351, Study notes of Computer Science

University of Alaska AnchorageComputer Science
Prof. Kenrick J. Mock

Material Type: Notes; Professor: Mock; Class: Automata, Algorithms, and Complexity; Subject: Computer Science ; University: University of Alaska - Anchorage; Term: Spring 2009;

Typology: Study notes

2009/2010

Uploaded on 03/28/2010

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Copying and Transforming
Pictures
First, finding the min or max…
• Next homework asks you to write a function to
find the darkest and lightest shade of grey in a
picture
• Here is a similar example to find the value of the
pixel with the largest red component
def findLargestRed(pict):
largestSoFar = -1
for p in getPixels(pict):
r = getRed(p)
if (r > largestSoFar):
largestSoFar = r
return largestSoFar
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pf12

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Download Slides on Copying and Transforming Pictures | CS A351 and more Study notes Computer Science in PDF only on Docsity!

Copying and Transforming

Pictures

First, finding the min or max…

  • Next homework asks you to write a function to find the darkest and lightest shade of grey in a picture
  • Here is a similar example to find the value of the pixel with the largest red component def findLargestRed(pict): largestSoFar = - for p in getPixels(pict): r = getRed(p) if (r > largestSoFar): largestSoFar = r return largestSoFar

Moving pixels across pictures

  • We’ve seen using index variables to track the pixel position we’re working with in a picture.
  • We can copy between pictures, if we keep track of: - The source index variables - Where we’re getting the pixels from - The target index variables - Where we’re putting the pixels at
  • (Not really copying the pixels: Replicating their color.)

What can you do then?

  • What can you do when copying from one

picture to another?

  • Collages: Copy several pictures onto one
  • Cropping: You don’t have to take the whole picture
  • Scaling: Make a picture smaller, or larger when copying it

Copying Barb to a canvas

**def copyBarb():

Set up the source and target pictures

barbf=getMediaPath("barbara.jpg") barb = makePicture(barbf) canvasf = getMediaPath("7inX95in.jpg") canvas = makePicture(canvasf)

Now, do the actual copying

targetX = 1 for sourceX in range(1,getWidth(barb)): targetY = 1 for sourceY in range(1,getHeight(barb)): color = getColor(getPixel(barb,sourceX,sourceY)) setColor(getPixel(canvas,targetX,targetY), color) targetY = targetY + 1 targetX = targetX + 1 show(barb) show(canvas) return canvas**

What’s this naming something to itself?

  • targetX = targetX + 1
  • This isn’t really naming something as itself
    • targetX + 1 is evaluated
      • It will result in the number after targetX
    • targetX = then sets the value of targetX
  • The result is that targetX gets incremented

by 1

Transformation = Small changes in copying

  • Making relatively small changes in this

basic copying program can make a variety

of transformations.

  • Change the targetX and targetY, and you copy wherever you want
  • Cropping: Change the sourceX and sourceY range, and you copy only part of the program.
  • Rotating: Swap targetX and targetY, and you end up copying sideways
  • Scaling: Change the increment on sourceX and sourceY, and you either grow or shrink the image.

Copying into the middle of the canvas

**def copyBarbMidway():

Set up the source and target pictures

barbf=getMediaPath("barbara.jpg") barb = makePicture(barbf) canvasf = getMediaPath("7inX95in.jpg") canvas = makePicture(canvasf)

Now, do the actual copying

targetX = 100 for sourceX in range(1,getWidth(barb)): targetY = 100 for sourceY in range(1,getHeight(barb)): color = getColor(getPixel(barb,sourceX,sourceY)) setColor(getPixel(canvas,targetX,targetY), color) targetY = targetY + 1 targetX = targetX + 1 show(barb) show(canvas) return canvas**

Copying: How it works 3

  • After yet another increment of sourceY and targetY:
  • When we finish that column, we increment sourceX and targetX, and start on the next column.

Copying: How it looks at the end

  • Eventually, we copy every pixel

Making a collage

  • Could we do something to the pictures we copy in? - Sure! Could either apply one of those functions before copying, or do something to the pixels during the copy.
  • Could we copy more than one picture! - Of course! Make a collage!

def createCollage():flower1=makePicture(getMediaPath("flower1.jpg")) print flower1flower2=makePicture(getMediaPath("flower2.jpg")) print flower2canvas=makePicture(getMediaPath("640x480.jpg")) print canvas#First picture, at left edge targetX=1for sourceX in range(1,getWidth(flower1)): targetY=getHeight(canvas)-getHeight(flower1)-5for sourceY in range(1,getHeight(flower1)): px=getPixel(flower1,sourceX,sourceY)cx=getPixel(canvas,targetX,targetY) setColor(cx,getColor(px))targetY=targetY + 1 #Second picture, 100 pixels overtargetX=targetX + 1 targetX=100for sourceX in range(1,getWidth(flower2)): targetY=getHeight(canvas)-getHeight(flower2)-5for sourceY in range(1,getHeight(flower2)): px=getPixel(flower2,sourceX,sourceY)cx=getPixel(canvas,targetX,targetY) setColor(cx,getColor(px))targetY=targetY + 1 targetX=targetX + 1

#Third picture, flower1 negatednegative(flower1) targetX=200for sourceX in range(1,getWidth(flower1)): targetY=getHeight(canvas)-getHeight(flower1)-5for sourceY in range(1,getHeight(flower1)): px=getPixel(flower1,sourceX,sourceY)cx=getPixel(canvas,targetX,targetY) setColor(cx,getColor(px))targetY=targetY + 1 #Fourth picture, flower2 with no bluetargetX=targetX + 1 clearBlue(flower2)targetX= for sourceX in range(1,getWidth(flower2)):targetY=getHeight(canvas)-getHeight(flower2)- for sourceY in range(1,getHeight(flower2)):px=getPixel(flower2,sourceX,sourceY) cx=getPixel(canvas,targetX,targetY)setColor(cx,getColor(px)) targetX=targetX + 1targetY=targetY + 1 #Fifth picture, flower1, negated with decreased reddecreaseRed(flower1) targetX=400for sourceX in range(1,getWidth(flower1)): targetY=getHeight(canvas)-getHeight(flower1)-5for sourceY in range(1,getHeight(flower1)): px=getPixel(flower1,sourceX,sourceY)cx=getPixel(canvas,targetX,targetY) setColor(cx,getColor(px))targetY=targetY + 1 show(canvas)targetX=targetX + 1 return(canvas)

Exactly from book

Scaling the picture down

**def copyBarbSmaller():

Set up the source and target pictures

barbf=getMediaPath("barbara.jpg") barb = makePicture(barbf) canvasf = getMediaPath("7inX95in.jpg") canvas = makePicture(canvasf)

Now, do the actual copying

sourceX = 1 for targetX in range(100,100+(getWidth(barb)/2)): sourceY = 1 for targetY in range(100,100+(getHeight(barb)/2)): color = getColor(getPixel(barb,sourceX,sourceY)) setColor(getPixel(canvas,targetX,targetY), color) sourceY = sourceY + 2 sourceX = sourceX + 2 show(barb) show(canvas) return canvas**

Scaling Up: Growing the picture

  • To grow a picture,

we simply duplicate

some pixels

  • We do this by

incrementing by 0.5,

but only use the

integer part.

**>>> print int(1) 1

print int(1.5) 1 print int(2) 2 print int(2.5) 2**

Scaling the picture up

**def copyBarbLarger():

Set up the source and target pictures

barbf=getMediaPath("barbara.jpg") barb = makePicture(barbf) canvasf = getMediaPath("7inX95in.jpg") canvas = makePicture(canvasf)

Now, do the actual copying

sourceX = 1 for targetX in range(10,10+(getWidth(barb)2)): sourceY = 1 for targetY in range(10,10+(getHeight(barb)2)): color = getColor(getPixel(barb,int(sourceX),int(sourceY))) setColor(getPixel(canvas,targetX,targetY), color) sourceY = sourceY + 0. sourceX = sourceX + 0. show(barb) show(canvas) return canvas**

Scaling up: How it works

  • Same basic setup as copying and rotating:

Scaling up: How it works 4

  • And twice…

Scaling up: How it works 5

  • The next “column” (x) in the source, is the same “column” (x) in the target.

Scaling up: How it ends up

  • We end up in the same place in the source, but twice as much in the target.
  • Notice the degradation: - Curves get “choppy”: Pixelated

Described in the text, but skipping here. Good things to try:

  • Can you come up with general copy,

rotate, copy, and scale functions?

  • Take input pictures and parameters
  • Return the canvas the correct transformation applied
  • Also think about generalizing the

transformations:

  • Scaling up and down by non-integer amounts
  • Rotating by something other than 90 degree increments

Blending through Averaging

Chromakey

  • What the weather person does
  • Pose in front of a blue or green screen
  • Swap all “blue” or “green” for the background

Example Solution

def chromakey2(source,bg): for p in getPixels(source): if (getRed(p)+getGreen(p) < getBlue(p)): setColor(p, getColor(getPixel(bg, getX(p),getY(p)))) return source