cs 101: introduction to computing
DESCRIPTION
CS 101: Introduction to Computing. Programming picture manipulations. Developed by Mark Guzdial, Georgia Institute of Technology, 2003–2004; modified by Robert H. Sloan, University of Illinois at Chicago, 2005, for educational use. Demonstrating: Manipulating Colors. >>> print color - PowerPoint PPT PresentationTRANSCRIPT
CS 101:Introduction to Computing
Programming picture manipulations
Developed by Mark Guzdial, Georgia Institute of Technology, 2003–2004; modified by Robert H. Sloan, University of Illinois at Chicago, 2005, for educational use.
Demonstrating: Manipulating Colors>>> print getRed(pixel)168>>> setRed(pixel,255)>>> print getRed(pixel)255>>> color=getColor(pixel)>>> print colorcolor r=255 g=131 b=105>>> setColor(pixel,color)>>> newColor=makeColor(0,100,0)>>> print newColorcolor r=0 g=100 b=0>>> setColor(pixel,newColor)>>> print getColor(pixel)color r=0 g=100 b=0
>>> print colorcolor r=81 g=63 b=51>>> print newcolorcolor r=255 g=51 b=51>>> print distance(color,newcolor)174.41330224498358>>> print colorcolor r=168 g=131 b=105>>> print makeDarker(color)color r=117 g=91 b=73>>> print colorcolor r=117 g=91 b=73>>> newcolor=pickAColor()>>> print newcolorcolor r=255 g=51 b=51
We can change pixels directly…>>> file="/Users/sloan/mediasources/MauryBaby.jpg">>> pict=makePicture(file)>>> show(pict)>>> setColor(getPixel(pict,10,160), blue)>>> setColor(getPixel(pict,11,160), blue)>>> setColor(getPixel(pict,12,160), blue)>>> setColor(getPixel(pict,13,160), blue)>>> repaint(pict)
But that’s really tedious…Manipulating pictures more cleverlyis our next topic!
How do you find out what RGB values you have? And where?
Use the MediaTools!
(especially useful whentesting and debugging…)
Use a loop!Our first picture recipe
def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
Used like this:>>> file="/Users/sloan/Mediasources/Tonks.jpg">>> pic=makePicture(file)>>> show(pic)>>> decreaseRed(pic)>>> repaint(pic)
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Note: the name picture takes on the value of pic
How loops are written for is the name of the command An index variable is used to represent the different values
in the loop The word in A function that generates a sequence
The index variable will be the name for each value in the sequence, each time through the loop
A colon (“:”) And, another block that is indented e.g. for x in range(1,100):
print x
What happens when a loop is executed
The index variable is set to the next (or first, at the beginning) item in the sequence
The block is executed The variable is often used inside the block
Then execution returns to the for statement, where the index variable gets set to the next item in the sequence
Repeat until the sequence is exhausted.
for x in range(1,100): print x
output: 1 2 3 ...
getPixels returns a sequence of pixels Each pixel knows its color and its location in the
picture Change the pixel, you change the picture So the loop below assigns the index variable p to
each pixel in the picture picture, one at a time.
def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
Do we need value?
Not really: Remember that we can swap names for data or functions that are equivalent.
def decreaseRed(picture): for p in getPixels(picture): setRed(p, getRed(p) *0.5)
Let’s walk through that slowly…def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
Here we get a picture object in as input and give it the variable name picture
picture
Now, get the pixelsdef decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
We get all the pixels from the picture, then make p be the name of each one one at a time
picture
Pixel, color r=168 g=131 b=105
Pixel, color r=160 g=131 b=105
Pixel, color r=168 g=132b=106
…
p
getPixels()
Get the red value from pixeldef decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
We get the red value of pixel p and name it value
picture
Pixel, color r=168 g=131 b=105
Pixel, color r=168 g=132b=106
…
pvalue = 168
Pixel, color r=160 g=131 b=105
Now change the pixeldef decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
Set the red value of pixel p to 0.5 (50%) of value
picture
Pixel, color r=84 g=131 b=105
Pixel, color r=168 g=132b=106
…
pvalue = 168
Pixel, color r=160 g=131 b=105
Then move on to the next pixeldef decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
Move on to the next pixel and name it p
picture
Pixel, color r=84 g=131 b=105
Pixel, color r=168 g=132b=106
…
pvalue = 168
Pixel, color r=160 g=131 b=105
Get its red valuedef decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
Change value to the new red value at the new p
picture
Pixel, color r=84 g=131 b=105
Pixel, color r=168 g=131 b=105
Pixel, color r=168 g=132b=106
…
value = 160p
Get its red valuedef decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5)
Change value to the new red value at the new p
picture
Pixel, color r=84 g=131 b=105
Pixel, color r=168 g=132b=106
…
p
Pixel, color r=160 g=131 b=105
And change this red value
def decreaseRed(picture): for p in getPixels(picture): value=getRed(p) setRed(p,value*0.5) Change the red value at
pixel p to 50% of valuepicture
Pixel, color r=84 g=131 b=105
Pixel, color r=80 g=131 b=105
Pixel, color r=168 g=132b=106
…
value = 160p
And eventually, we do all of the pixels...
We go from this… to this!
“Tracing/Stepping/Walking through” the program What we just did is called “stepping” or “walking
through” the program You consider each step of the program, in the order that the
computer would execute it You consider what would specifically happen there You write down what values each variable (name) has at each
point.
It’s one of the most important debugging skills you can have. And everyone has to do a lot of debugging, especially at first.
Did that really work?How can we be sure? Sure, the picture looks different, but did we
actually decrease the amount of red? By as much as we thought?
Let’s check it!
>>> file = pickAFile()>>> print file/Users/sloan/Mediasources/Tonks.jpg>>> pict = makePicture(file)>>> pixel = getPixel(pict,145,17)>>> print pixelPixel, color=color r=236 g=98 b=147>>> decreaseRed(pict)>>> print pixelPixel, color=color r=236 g=98 b=147 Still the old pixel>>> newPixel = getPixel(pict, 145,17)>>> print newPixelPixel, color=color r=118 g=131 b=105>>> print 236 * 0.5118.0
Checking it in the MediaTools
If you make something you like…
writePictureTo(picture, “filename”) Writes the picture out as a JPEG Be sure to end your filename as “.jpg”!
e.g. myPic.jpg
If you don’t specify a full path,will be saved in the same directory as JES.