Let's get practical!

We are going to transform a 100 x 100-pixel image to a greyscale. This means that there are therefore $100\times100=10000$ pixels to modify. You can access the image used in this example at the following address:
http://xlogo.tuxfamily.org/pics/transfo.png
This is how we are going to proceed: first, we will refer to the top left corner of the image as [0 0]. Then the turtle will examine the first 100 pixels of the first line, followed by the first 100 of the second line, and so on. Each time, the colour of the pixel will be retrieved with findcolor, and the colour will then be changed to the average of the three [r g b] values. Here is the relevant code:
(Don't forget to change the filepath in the transform procedure!)

to pixel :list
# return the average of three numbers [r g b]

make "r first :list
make "list bf :list
make "g first :list
make "list bf :list
make "b first :list
make "b round (:r+:g+:b)/3
output se list :b :b :b  
end

to greyscale :c
if :y=-100 [stop]
if :c=100 [make "c 0 make "y  :y-1] 
# We assign the "average" colour of the following pixel to the pen
setpc pixel fc liste :c :y
# We turn the dot to greyscale
dot list :c :y
greyscale :c+1
end

to transform
# You must change the path to the image transfo.png
# Eg: setdir "c:\\my_images loadimage "transfo.png] 

cs ht setdir "/home/loic loadimage "transfo.png make "y 0 greyscale 0
end

Figure: XLOGO fait de la retouche d'images....