How to add image channel with torch?

Question

I have loaded a jpg image with 3 channels, how to add one last channel to generate an image tensor with 4 channels?

Answer 1

Let's say you have an image with 3 channels, height height and width width:

a = torch.Tensor(3, height, width) -- this is your image
b = torch.Tensor(1, height, width) -- the channel you want to add
c = torch.cat(a,b,1)

A working example:

th> a = torch.Tensor(3,3,3):fill(1)                                                               
th> b = torch.Tensor(1,3,3):fill(0)
th> c = torch.cat(a,b,1)                                                                   
th> c
(1,.,.) = 
  1  1  1
  1  1  1
  1  1  1

(2,.,.) = 
  1  1  1
  1  1  1
  1  1  1

(3,.,.) = 
  1  1  1
  1  1  1
  1  1  1

(4,.,.) = 
  0  0  0
  0  0  0
  0  0  0
[torch.DoubleTensor of size 4x3x3]