CPSC120B - Day 31 Notes

No Reading Questions

No Quiz

PPM Module

We’ve already done some image manipulation
But it was per-pixel so it was possible to read treat the PPM file as a 1-dimensional list
Today we are going to do more complicated image manipulation that requires pixel operations using neighboring pixels
This requires a 2D list, or matrix, of pixel data
You know how to read PPM files and how to create matrices
But to save you some time, you can use a module I created that does that for you
```
import ppm

image = ppm.read_image("glorious_image.ppm")
pixel = image[0][0]
```
The read_image function returns a 2D list of pixels

Each pixel is represented with a list of three integers, for the red, green, and blue channels

import ppm

image = ppm.read_image("glorious_image.ppm")
pixel = image[0][0]
red = pixel[0]
green = pixel[1]
blue = pixel[2]

Modifying Images

If you want to do something for every pixel in the image, need to use nested loops to iterate over all combinations of row and column values

import ppm

image = ppm.read_image("glorious_image.ppm")
for row in ppm:
    for col in ppm:
        pixel = image[row][col][1] = 0
        pixel = image[row][col][2] = 0

Writing Images

Writing a 2D list of pixels to a PPM file is as easy as reading
```
import ppm

ppm.write_image("boring_image.ppm", image)
```
Just specify the file name and the 2D list of pixels
Warning: the ppm module does no imput verification, you must specify a 2D list of lists containing 3 integers, anything else will cause it to fail in unclear ways

Copying Images

Sometimes you need to create a new image, without modifying the old one

Be careful, you can not copy a list with an assignment statement due to aliasing

image = ppm.read_image("glorious_image.ppm")
image_copy = image # DANGER! This is an alias, not a copy

To actually copy an image, you need to create a new 2D list

image = ppm.read_image("glorious_image.ppm")
num_rows = len(image)
num_cols = len(image[0])
new_image = []
for row in range(num_rows):
    new_row = []
    for col in range(num_cols):
        pixel = image[row][col]
        new_pixel = [pixel[0], pixel[1], pixel[2]]
        new_row.append(new_pixel)
    new_image.append(new_row)

Lab

lab31