1 What is a Raster image?

• An image is a 2D rectilinear array of pixels

1.1 Raster is a German word for grid**.**

1. output device
   • we see a raster image via a raster display — screen.
   • 大多数打印机也是raster device，没有物理上的grid，而是通过逐行scan实现
2. input device
   1. digital cameras

raster: grid, matrix, 2d array, tensor

1.2 An example

from numpy import random as r
I = r.rand(256,256);

• What is this? What does it look like?
• Which values does it have?
• How many are there?
• — from Danny Alexander

from matplotlib import pyplot as p
from numpy import random as r
I = r.rand(256,256);
p.imshow(I);
p.show();

1. This code will actually produce an image like this:
2. matplotlib assumes the data is ‘scientific’ and applies a colourmap.
3. Note: matplotlib and colour maps.

from matplotlib import pyplot as p
from numpy import random as r
I = r.rand(256,256);
p.imshow(I,cmap=‘gray’);
p.show();

Next issue: matplotlib will normalize input e.g., map the lowest value to 0, and the highest to 1. Even though we multiplied by 0.5, the overall brightness is the same.

from matplotlib import pyplot as p
from numpy import random as r
I = r.rand(256,256)*0.5;
p.imshow(I,cmap=‘gray’);
p.show();