Image Size vs. File Size. gif File Compression. 1 Review. Slides05 - Pixels.key - September 28, 2015

Transcription

1 1 Review What is a pixel? What is RGB? Image Size vs. File Size The Picture use 857 * 1280 * 3 bytes 3.3 MB (MB = megabytes = millions of bytes) of memory The jpeg file uses 201 KB (KB = kilobytes = thousands of bytes) of memory How can that be??? jpeg, gif, png, etc. files are compressed gif File Compression 3 Custom palette of 256 colors Image associates a color index in range with each pixel Records number of consecutive pixels of same color

2 Get the components of a color in Python -1 getred ( <pixel> ) getgreen ( <pixel> ) getblue ( <pixel> ) Each of these returns a number between 0 and 255 to represent the redness / greenness / blueness of the color Get the components of a color in Python -2 getred ( <pixel> ) getgreen ( <pixel> ) getblue ( <pixel> ) Each of these returns a number between 0 and 255 to represent the redness / greenness / blueness of the color Changing a color with Python 5 setred ( <pixel>, <value> ) setgreen ( <pixel>, <value> ) setblue ( <pixel>, <value> ) Changes the redness / greenness / blueness of a single pixel in an image to the new value

3 6 Before After 7 for loops Suppose we want to remove the red from all the pixels? We need to execute the function to remove red millions of times! for loop example 8 def removered ( picture ) : # Loop over all of the pixels for pixel in getpixels ( picture ) : # Remove the red from each pixel setred ( pixel, 0 )

4 for loops 9 Syntax: for <variable> in <array> : <instructions> Semantics: Execute the instructions sequentially, once for each value in the array. Each time through the loop, the variable is assigned the next value in the array. Simple for loop example 10 # Initialize an array # For each number in the array, print the number # and the square of the number 11-1

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7 11-8 print 12-1 print 12-2 print

8 12-3 print 12- print 12-5 print

9 12-6 print 13-1 print print

10 print 13-6 print print 6

11 print 6 36 for loop example revisited 1 def removered ( picture ) : # Loop over all of the pixels for pixel in getpixels ( picture ) : # Remove the red from each pixel setred ( pixel, 0 ) Writing Test Functions 15 A test function is a function whose job it is to run another function to make sure it works. Test functions make it easy for us to re-run a function after we ve changed it without doing a lot of typing in the command area. def removered ( picture ) : for pixel in getpixels ( picture ) : setred ( pixel, 0 ) def testremovered ( filename ) : mypict = makepicture ( filename ) removered ( mypict ) explore ( mypict ) Function we are trying to write. Should work on any picture. Test function. Calls our function on 1 specific picture.

12 Test Function def removered ( picture ) : for pixel in getpixels ( picture ) : setred ( pixel, 0 ) The main function modifies the image def testremovered ( filename ) : mypict = makepicture ( filename ) removered ( mypict ) explore ( mypict ) The test function: Reads the image from the file Calls the function we are working on Displays the modified image Summary of new functions 17 getpixels ( <picture> ) - returns an array of all the pixels getred ( <pixel> ) - returns a number between 0 and 255. Also getgreen and getblue setred ( <pixel>, <value> ) - changes the redness of a pixel to the value, which should be between 0 and 255. Also setgreen and setblue Summary of new Python 18 for <variable> in <array> : <instructions> Execute the instructions sequentially, once for each value in the array. Each time through the loop, the variable is assigned the next value in the array.