如何进行随机步行读取背景图像？

Question

I'm trying to get my random walk to invoke function z on each step. Whenever the step ends on a red pixel in the background image variable CS=1. If the step ends on a black pixel then CS=0. Right now it's just a randomly generated value. Don't worry about the other stuff, I provide it for you to reproduce the issue, but those parts work fine. The critical part is variable CS in function step.prob.

Main question: How do I make the walk function read the background image?<-- solved

New issue: I'm receiving a "subscript out of bounds" error in the step.prob() function. Why and how do I fix it?

so far it looks like this:

You will need package EBImage, and this image saved as testmap2.png in your working directory to run this code. The code is currently:

library(tiff)
library("EBImage")

pic<-readImage("testmap2.png",all=TRUE,package="EBImage")
display(pic, method="raster")
dim(pic)
pic[,,1]


par(bg="black",col="black",col.axis="white",new=T)

P<-30
step.max=125
s<-step.max
pic<-readImage("testmap2.png",all=TRUE,package="EBImage")
walkW <- function(n.times=125,
               xlim=c(524058,542800),
               ylim=c(2799758,2818500),
               start=c(542800,2815550),
               stepsize=c(4000,4000)) {
display(pic, method="raster")
par(bg="black",col="black",col.axis="white",new=T)

    plot(c(0,0),type="n",xlim=xlim,ylim=ylim,
           xlab="Easting",ylab="Northing",col="white",col.lab="white")#arena 
    x <- start[1]
    y <- start[2]
    steps <- 1/c(1,2,4,8,12,16)
    steps.y <- c(steps,-steps,0)
    steps.x <- c(steps[c(1,5,6)],-steps,0)
    points(x,y,pch=16,col="green",cex=1)

for (i in 1:n.times) {
        repeat {
           xi <- stepsize[1]*sample(steps.x,1)
           yi <- stepsize[2]*sample(steps.y,1)
           newx <- x+xi
           newy <- y+yi
           if (newx>xlim[1] && newx<xlim[2] &&
               newy>ylim[1] && newy<ylim[2]) break
        }
        lines(c(x,newx),c(y,newy),col="white")
        x <- newx
        y <- newy

####the following function returns an "subscript out of bounds" error
 step.prob<-function(n.times=step.max){
CS<-pic[x,y,1]
CS.max<-1
step.num<-15
SP<-(((CS/CS.max)*(1-(step.num/step.max))+(step.num/step.max))*100)
}
z<-step.prob(1)
##end the function that returns an error
if(z>P)break
else

if(step.max){points(newx,newy,pch=16,col="yellow",cex=1)
}

 }
}
set.seed(101)
walkW(s)

Answer 1

A color png or jpg is just a multidimensional array for position and RGB color.

library(png) # same idea for EBImage 
pic<-readPNG("testmap2.png")
dim(pic) # 615 683   3 (3 for the RGB levels)
pic[1:5,1:5,1] # where is it red?

Returns:

     [,1] [,2] [,3] [,4] [,5]
[1,]    0    0    0    1    1
[2,]    0    0    0    1    1
[3,]    0    0    0    0    0
[4,]    0    0    0    0    0
[5,]    0    0    0    0    0

So to test a point on the walk for red:

x <- 2; y <- 2
pic[x,y,1] # 0, not red

x <- 2; y <- 5
pic[x,y,1] # 1 , red