1. CS1315: Introduction to Media Computation
How the transformations worked, and how to make them better
This is an exploration lecture – things to explore:
How to walk through mirroring more carefully and how to debug for understanding.
An additional copying transformation – rotations
How we can use blurring to get scaling to look better
If time permits:
Talk about other kinds of blurring (weighting so that you don’t mess up edges)
Talk about how to do rotations OTHER THAN right angles

2. Understanding and using image manipulations better
Let’s walk through mirroring in more detail
Use the temple fixing example
What was really going on there?
More on what we can do with copying: Rotating
Let’s talk about how to avoid the degradation of scaling

3. Program to mirror the temple
def mirrorTemple():
source = makePicture(getMediaPath("temple.jpg"))
mirrorpoint = 277
lengthToCopy = mirrorpoint - 14
for x in range(1,lengthToCopy):
for y in range(28,98):
p = getPixel(source,mirrorpoint-x,y)
p2 = getPixel(source,mirrorpoint+x,y)
setColor(p2,getColor(p))
show(source)
return source

4. Did it really work?
It clearly did the mirroring, but that doesn’t create a 100% realistic image.
Check out the shadows: Which direction is the sun coming from?
Consider other reasons why this worked as well as it did. Clouds in the sky?

5. Understanding the Temple Fix
What is the very first transfer of pixels from and to? Which (x,y) pixel from? Which (x,y) pixel to?
What is the second set of pixels?
How many pixels get copied?
Try to get people to make guesses here, first, before going on.

6. Adding print statements to see what’s happening
def mirrorTemple():
source = makePicture(getMediaPath("temple.jpg"))
mirrorpoint = 277
lengthToCopy = mirrorpoint - 14
for x in range(1,lengthToCopy):
for y in range(28,98):
print "Copying color from",mirrorpoint-x,y
print "to",mirrorpoint+x,y
p = getPixel(source,mirrorpoint-x,y)
p2 = getPixel(source,mirrorpoint+x,y)
setColor(p2,getColor(p))
show(source)
return source

7. First pixels are either side of the mirrorpoint, then moving down
>>> p2=mirrorTemple()
Copying color from to
Copying color from to
Copying color from to

8. Counting pixels def mirrorTemple():
source = makePicture(getMediaPath("temple.jpg"))
mirrorpoint = 277
lengthToCopy = mirrorpoint - 14
count = 0
for x in range(1,lengthToCopy):
for y in range(28,98):
p = getPixel(source,mirrorpoint-x,y)
p2 = getPixel(source,mirrorpoint+x,y)
setColor(p2,getColor(p))
count = count + 1
show(source)
print "We copied",count,"pixels"
return source

9. Counting pixels >>> p2=mirrorTemple() We copied 18340 pixels
Where did that come from?
How many rows? Y goes from 28 to 98
= 70 rows of pixels
How many columns? X goes from 1 to =263
= 262 columns of pixels
70 * 262 = 18340

10. Rotating the copy def copyBarbSideways():
# 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)+1):
targetY = 1
for sourceY in range(1,getHeight(barb)+1):
color = getColor(getPixel(barb,sourceX,sourceY))
setColor(getPixel(canvas,targetY,targetX), color)
targetY = targetY + 1
targetX = targetX + 1
show(barb)
show(canvas)
return canvas
Here’s another example of a transformation.

11. Rotating: How it works
We increment the same, but we use targetX for the Y coordinate and targetY for the X coordinate

12. Rotating: How it works 2
We increment sourceY and targetY just the same.

13. Rotate: How it ends
Same amount of increment, even same values in the variables, but a different result.

14. Did this really “Rotate”?
No, it flipped along the main diagonal.
What do we need to do to really rotate (either clockwise or counter-clockwise?)

15. Rotating the copy def rotateBarbSideways():
# 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
width = getWidth(barb)
for sourceX in range(1,getWidth(barb)+1):
targetY = 1
for sourceY in range(1,getHeight(barb)+1):
color = getColor(getPixel(barb,sourceX,sourceY))
setColor(getPixel(canvas,targetY,width-targetX+1), color)
targetY = targetY + 1
targetX = targetX + 1
show(barb)
show(canvas)
return canvas
Here’s another example of a transformation.

16. What to do about scaling?
How do we clear up the degradation of scaling up?
Variety of techniques, but mostly following the same basic idea:
Use the pixels around to figure out what color a new pixel should be, then somehow (e.g., by averaging) compute the right color.
Different techniques look at different pixels and compute different averages in different ways.

17. A blurring recipe
def blur(pic,size):
for pixel in getPixels(pic):
currentX = getX(pixel)
currentY = getY(pixel)
r = 0
g = 0
b = 0
count = 0
for x in range(currentX - size,currentX + size):
for y in range(currentY - size, currentY + size):
if(x<=0) or (y<=0) or (x > getWidth(pic)) or (y > getHeight(pic)):
pass # Skip if we go off the edge
else:
r = r + getRed(getPixel(pic,x,y))
g = g + getGreen(getPixel(pic,x,y))
b = b + getBlue(getPixel(pic,x,y))
count = count + 1
newColor = makeColor(r/count,g/count,b/count)
setColor(pixel,newColor)
We’ll see pass and else later, but you can probably get a sense here of what’s going on.

18. Blurring out the pixelation