Working with Images: Special Effects

Transcription

1 Power of Choice Working with Images: Special Effects CSC121, Introduction to Computer Programming conditional processing is a powerful tool for creating special effects with digital images choosing alternative actions based on measuring or evaluating current data e.g., posterizing displaying an image using a small number of different tones Understanding Images natural images are usually comprised of continuously varying tones (intensity, color) an image histogram reveals these properties a graph depicting the frequency distribution of selected image values Understanding Images an image histogram is a graph depicting the frequency distribution of selected image values (e.g., 0-127, , , etc.) 1

2 Histograms reveal dynamic range posterized Programming a Posterizing Effect The programming amounts to this choosing how many color bins will we reassign each color intensity? Suppose we choose four how can we divide the dynamic range into four intervals? full dynamic range limited dynamic ranges Programming a Posterizing Effect Programming a Posterizing Effect The programming amounts to this choosing how many color bins will we reassign each color intensity? Suppose we choose four how can we divide the dynamic range into four intervals? Choose a representative value in each range for the reassignment in the example, four values are chosen for five intervals (leaving the middle range empty) in each channel, the colors matching the interval are reassigned 2

3 2/14/18 #loop through the pixels for p in getpixels(picture): #get the RGB values r = getred(p) g = getgreen(p) b = getblue(p) #check and set red values if(r < 64): setred(p, 31) if(r > 63 and r < 128): setred(p, 95) if(r > 127 and r < 192): setred(p, 159) if(r > 191 and r < 256): setred(p, 223) #check and set green values if(g < 64): setgreen(p, 31) if(g > 63 and g < 128): setgreen(p, 95) if(g > 127 and g < 192): setgreen(p, 159) if(g > 191 and g < 256): setgreen(p, 223) #check and set blue values if(b < 64): setblue(p, 31) if(b > 63 and b < 128): setblue(p, 95) if(b > 127 and b < 192): setblue(p, 159) if(b > 191 and b < 256): setblue(p, 223) Example: the garden image (original) NOTE: #comments explain logic Example: the garden image (posterized) Works with grayscale images too 3

4 Works with grayscale JPEG images too Creating Sepia Tones in Images Older photo prints were often tinted due to the silver emulsion printing process We can simulate these monochromatic sepia tones digitally Histograms, again Programming Sepia Tones convert the image to monochromatic or grayscale we ve done this before the standard sepia RGB values are (112, 66, 20) strategy: boost for red values and reduce blue values greens can be ignored (due to our green- dominant vision) work in three areas separately shadows, midtones, highlights 4

5 user- defined helper function Programming Sepia Tones def sepiatint(picture): grayscale(picture) #loop through picture to tint pixels red = getred(p) blue = getblue(p) #tint shadows if (red < 63): red = red*1.1 blue = blue*0.9 #tint midtones if (red > 62 and red < 192): red = red*1.15 blue = blue*0.85 #tint highlights if (red > 191): red = red*1.08 if (red > 255): red = 255 blue = blue*0.93 #set the new color values setblue(p, blue) setred(p, red) return (picture) Example: sepia tones Review: The If statement We have used the If statement in two forms if (condition): statement block if (condition): statement block- 1 else: statement block- 2 one- way decision two- way decision def choice(): choice = input("enter your choice: 1 or 2: ") if choice == 1: print "Your choice is 1 elif choice == 2: print "Your choice is 2 else: print "Invalid choice" 5

6 #loop through the pixels #get the RGB values r = getred(p) g = getgreen(p) b = getblue(p) #check and set red values if(r < 64): setred(p, 31) if(r > 63 and r < 128): setred(p, 95) if(r > 127 and r < 192): setred(p, 159) if(r > 191 and r < 256): setred(p, 223) How could we rewrite the posterize( ) function with an elif structure? #loop through the pixels #get the RGB values r = getred(p) g = getgreen(p) b = getblue(p) #check and set red values if(r < 64): setred(p, 31) if(r > 63 and r < 128): setred(p, 95) if(r > 127 and r < 192): setred(p, 159) if(r > 191 and r < 256): setred(p, 223) How could we rewrite the posterize( ) function with an elif structure? If we execute the first if, do we need to consider any of the others? #loop through the pixels #get the RGB values r = getred(p) g = getgreen(p) b = getblue(p) #check and set red values if(r < 64): setred(p, 31) if(r > 63 and r < 128): setred(p, 95) if(r > 127 and r < 192): setred(p, 159) if(r > 191 and r < 256): setred(p, 223) How could we rewrite the posterize( ) function with an elif structure? If we execute the first if, do we need to consider any of the others? If we get this far, do we need another test? 6