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Python: Working with pixels

Python: Working with pixels. Reminder: Conditionals. if age < 18: showInformation (“Sorry, not allowed to vote yet.”) else : showInformation (“Please select candidate.”). Relational Operators. a > b a < b a == b (equality) a <= b (“less than or equal”)

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Python: Working with pixels

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  1. Python: Working with pixels

  2. Reminder: Conditionals if age < 18: showInformation(“Sorry, not allowed to vote yet.”) else: showInformation(“Please select candidate.”)

  3. Relational Operators • a > b • a < b • a == b (equality) • a <= b (“less than or equal”) • a >= b (“greater than or equal”) • != (inequality) Note: The order of signs matters: =! =< => do not work! There should be no spaces inside the relational operator, so “a < = b” also does not work (See also Appendix A.4, p362)

  4. Getting the leading zero into the filename: if n < 10: filename = “swatch0” + str(n) + “.jpg” else: filename = “swatch” + str(n) + “.jpg”

  5. Getting the leading zero into the filename: if n < 10: filename = “swatch0” + str(n) + “.jpg” else: # here we know n >= 10 filename = “swatch” + str(n) + “.jpg” What if you need more frames? if n < 10: filename = “swatch00” + str(n) + “.jpg” else: # here we know n >= 10 if n < 100: # here n is in the range of 10 … 99 filename = “swatch0” + str(n) + “.jpg” else: # here we know n >= 100 filename = “swatch” + str(n) + “.jpg”

  6. Examples from lab 11 • Be sure to review these! • Open examples.py in JES • Load • In the command area, try the functions • Here are links to small images you can use to test your program (or use your own):  eye.jpg, luca.jpg

  7. Getting at the pixels • getPixel(picture,x,y) • Gets a single pixel – returns pixel at position x,y of picture • getPixels(picture) • gets all the pixels – returns an array containing all the pixels of picture

  8. Reminder: Manipulating Pictures >>> pic1 = makeEmptyPicture(200,100) >>> seafoam = makeColor(153, 255, 204) >>> setAllPixelsToAColor(pic1, seafoam) >>> addText(pic1,30,50,“hello") >>> show(pic1) >>> pic2 = makePicture(pickAFile()) >>> show(pic2)

  9. red=108 green=86 blue=142 y = 9 Color:(108,86,142) Position: (12,9) >>> p = getPixel(picture,12,9) >>> print p Pixel, red=108 green=86 blue=142 x = 12

  10. What can we do with a pixel p? • getRed(p), • getGreen(p) • getBlue(p) • setRed(p, value) • setGreen(p, value) • setBlue(p, value) functions that take a pixel (p) as input and return a value between 0 and 255 functions that set the value of pixel (p) to a given value between 0 and 255

  11. We can also get, set, and modify Colors • getColor(p) takes a pixel as input and returns a Color object with the color at that pixel • setColor(p, c) sets the color of pixel (p) as input and a color (c), then sets the pixel to that color. • We also have functions that can makeLighter(c) and makeDarker(c) an input color • Last time we saw that we can also create colors: • makeColor(r,g,b) takes red, green, and blue values (in that order) between 0 and 255, and returns a Color object • pickAColor() lets you use a color chooser and returns the chosen color

  12. red=108 green=86 blue=142 y = 9 Color:(108,86,142) Position: (12,9) >>> pixel=getPixel(picture,12,9) >>> print pixel Pixel, red=108 green=86 blue=142 >>> value = getRed(pixel) >>> setRed (pixel, value+50) >>> setGreen(pixel, 0) >>> setBlue(pixel, getBlue(pixel)/2) x = 12

  13. red=108 red=158 green=86 Green=0 blue=142 Blue=121 y = 9 y = 9 Color:(108,86,142) Position: (12,9) Color:(158,0,121) Position: (12,9) >>> pixel=getPixel(picture,12,9) >>> print pixel Pixel, red=108 green=86 blue=142 >>> value = getRed(pixel) >>> setRed (pixel, value+50) >>> setGreen(pixel, 0) >>> setBlue(pixel, getBlue(pixel)/2) x = 12

  14. red=108 green=86 blue=142 y = 9 Color:(108,86,142) Position: (12,9) >>> pixel=getPixel(picture,12,9) >>> print pixel Pixel, red=108 green=86 blue=142 >>> redValue = getRed(pixel) >>> greenValue = getGreen(pixel) >>> blueValue = getBlue(pixel) >>> newColor = makeColor(redValue+50, 0, getBlue(pixel)/2) >>> setColor(pixel, newColor) x = 12

  15. red=158 Green=0 Blue=121 y = 9 Color:(158,0,121) Position: (12,9) >>> pixel=getPixel(picture,12,9) >>> print pixel Pixel, red=108 green=86 blue=142 >>> redValue = getRed(pixel) >>> greenValue = getGreen(pixel) >>> blueValue = getBlue(pixel) >>> newColor = makeColor(redValue+50, 0, getBlue(pixel)/2) >>> setColor(pixel, newColor) x = 12

  16. Repeating an action for all the pixels in a picture Example: for p ingetPixels(picture):    value = getRed(p)setRed(p, value*0.5)

  17. Repeating an action for all the pixels in a picturedecreaseRed() Example: def decreaseRed(picture): for p ingetPixels(picture):    value = getRed(p)setRed(p, value*0.5)

  18. More examples: More examples (link)   - you can copy and paste these to save time • decreaseGreen() • decreaseBlue() • clearBlue() • lighten() • darken() • negative() • grayScale()

  19. “Posterize” function • For each pixel, if red<128, we set red=0, otherwise we set red=255 • Similarly for green, blue

  20. Assignment: Create a python function for each of the following: • Decrease red by 20% • Decrease green by 20% • Decrease blue by 20% • Increase red by 20%, if possible (i.e., if it does not exceed 255) • Similarly for increasing blue and green • “Posterize” • Think of another way to change an image and implement a function to do that

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