Chemotaxis in NetLogo - gradient

Question

I have made a gradient, and I am quite unsure as to how I can make my turtle(s) attracted to one end and also behave a different way. For instance, from light blue to dark blue. Dark blue would be where the food is (chemoattractant), and in this zone, the bacterias would prefer to run randomly or be more energetic. While, in the light blue area, there would be fewer turtles and their movements would be slow or "tumbling."
Overall, how do I make the turtles sense the chemoattractant or the gradient in the patch?

Answer 1

If pcolors were floating point you could just search the immediate neighbors and use
ask turtles [ uphill pcolor ]
but that won't work if the gradient is subtle. The turtles might just sit there.
If you store a floating-point patch variable "exact_color" , say, the next code will work but the angle of motion is always some multiple of 45 degrees.
ask turtles [ uphill exact_color]
So if you generalize the search to a larger radius to determine the local gradient,
this would work:
let neighborhood patches with [ distance myself <= search_radius]
let p max-one-of neighborhood [exact_color]
if [exact_color] of p > exact_color [ face p move-to p]
Here's a working implementation that assigns the color gradient then tracks it.
globals [ scentXC scentYC search_radius ] ;; location of center of maximum scent
patches-own [ exact_color ];
to setup
clear-all
print ("This assumes the world does not wrap");
;; decide to put center of attraction at ( 14, 7 )
set scentXC 14;
set scentYC 7;
;; search a larger radius for gradient since it may be low
set search_radius 5;
color_code_patches
;; next line can visually check work so far
;; ask N-of 20 patches [ set plabel pcolor]
create-turtles 10 [ setxy random-pxcor random-pycor set size 2 set shape "arrow" pen-down];
reset-ticks
end
to go
;; ask turtles [ uphill pcolor ] ;; fails because the gradient is too low
;; following works but the angle of motion is always a multiple of 45 degrees
;; ask turtles [ pen-down uphill exact_color] ;; move one step uphill
;; the following searches a larger neighborhood for the uphill direction
ask turtles
[
;; set search_radius 1.45 to simply search neighbors, ie "uphill exact_color"
let neighborhood patches with [ distance myself <= search_radius]
let p max-one-of neighborhood [exact_color] ;; or neighbors4
if [exact_color] of p > exact_color [ face p move-to p]
]
end
to color_code_patches
no-display
ask patches [
let blueness 0 ;; initialize to make the interpreter happy
let dist distancexy scentXC scentYC
ifelse (dist <= 1) [ set blueness 250 ]
[ set blueness (250 / dist ) ]
set exact_color blueness;
set blueness round blueness;
;;set pcolor [ 0 0 blueness] ;; for unknown reason this doesn't work so more complex
let color_assign ( word "set pcolor [ 0 0 " blueness " ] ")
run color_assign
]
display
end