Conceal Brainfuck in ASCII artwork

Brainhider.rb

#!/usr/bin/env ruby

brainfuck = File.open("helloworld.bf", "rb").read

ascii_art = File.open("animaniacs.art", "rb").read

if brainfuck.length > ascii_art.gsub(" ","").length
	abort "The supplied brainfuck will not fit inside the ASCII art."
end

output = ""
ascii_art.split("").each do |i|
	if brainfuck != ""
		if i != " " and i != "\n"
			i = brainfuck[0,1]
			brainfuck = brainfuck[1..-1]
		end
	end
	output << i
end

File.open( "hidden.bf", "w" ) do |file|
	file.write( output )
end

I rewrote the script to include faking brainfuck code to make it less obvious, and only replace certain characters in the ascii art.
It's good for things like this where you only want to replace ":" for example:

                                                      lllllll                                                 iiii  kkkkkkkk
                                                      l:::::l                                                i::::i k::::::k
                                                      l:::::l                                                 iiii  k::::::k
                                                      l:::::l                                                       k::::::k
    cccccccccccccccc   ooooooooooo      ooooooooooo    l::::lyyyyyyy           yyyyyyy mmmmmmm    mmmmmmm   iiiiiii  k:::::k    kkkkkkk eeeeeeeeeeee
  cc:::::::::::::::c oo:::::::::::oo  oo:::::::::::oo  l::::l y:::::y         y:::::ymm:::::::m  m:::::::mm i:::::i  k:::::k   k:::::kee::::::::::::ee
 c:::::::::::::::::co:::::::::::::::oo:::::::::::::::o l::::l  y:::::y       y:::::ym::::::::::mm::::::::::m i::::i  k:::::k  k:::::ke::::::eeeee:::::ee
c:::::::cccccc:::::co:::::ooooo:::::oo:::::ooooo:::::o l::::l   y:::::y     y:::::y m::::::::::::::::::::::m i::::i  k:::::k k:::::ke::::::e     e:::::e
c::::::c     ccccccco::::o     o::::oo::::o     o::::o l::::l    y:::::y   y:::::y  m:::::mmm::::::mmm:::::m i::::i  k::::::k:::::k e:::::::eeeee::::::e
c:::::c             o::::o     o::::oo::::o     o::::o l::::l     y:::::y y:::::y   m::::m   m::::m   m::::m i::::i  k:::::::::::k  e:::::::::::::::::e
c:::::c             o::::o     o::::oo::::o     o::::o l::::l      y:::::y:::::y    m::::m   m::::m   m::::m i::::i  k:::::::::::k  e::::::eeeeeeeeeee
c::::::c     ccccccco::::o     o::::oo::::o     o::::o l::::l       y:::::::::y     m::::m   m::::m   m::::m i::::i  k::::::k:::::k e:::::::e
c:::::::cccccc:::::co:::::ooooo:::::oo:::::ooooo:::::ol::::::l       y:::::::y      m::::m   m::::m   m::::mi::::::ik::::::k k:::::ke::::::::e
 c:::::::::::::::::co:::::::::::::::oo:::::::::::::::ol::::::l        y:::::y       m::::m   m::::m   m::::mi::::::ik::::::k  k:::::ke::::::::eeeeeeee
  cc:::::::::::::::c oo:::::::::::oo  oo:::::::::::oo l::::::l       y:::::y        m::::m   m::::m   m::::mi::::::ik::::::k   k:::::kee:::::::::::::e
    cccccccccccccccc   ooooooooooo      ooooooooooo   llllllll      y:::::y         mmmmmm   mmmmmm   mmmmmmiiiiiiiikkkkkkkk    kkkkkkk eeeeeeeeeeeeee
                                                                   y:::::y
                                                                  y:::::y
                                                                 y:::::y
                                                                y:::::y
                                                               yyyyyyy

This is what Hello World (with a lot of garbage end data) looks like in ascii art of my name:

                                                      lllllll                                                 iiii  kkkkkkkk
                                                      l+++++l                                                i++++i k+++++[k
                                                      l>++++l                                                 iiii  k+<-]>+k
                                                      l+.--<l                                                       k++++++k
    cccccccccccccccc   ooooooooooo      ooooooooooo    l[>++lyyyyyyy           yyyyyyy mmmmmmm    mmmmmmm   iiiiiii  k+++<-k    kkkkkkk eeeeeeeeeeee
  cc]>+.+++++++..++c oo+.>++++++[>oo  oo+++++<-]>++oo  l.--<l y+++++y         y+++++ymm+[>++++m  m+<-]>++mm i.<<.+i  k++.--k   k----.kee--------.>>>ee
 c+++++[>++++++<-]>co+++.+>>-++-+>-+oo>>>->>+-->+-+->o l>->>l  y-++>>y       y>>->-ym>>>->--+-+mm++>-+>>+->m i>-+>i  k+->>-k  k>>>>+ke--+->+eeeee+-+->ee
c+>>+>>-cccccc+>>>+co>+++-ooooo>+>--oo>+>--ooooo+->--o l---+l   y+>>--y     y>->>>y m>>+>++>>>-+->>++>--->>m i>+++i  k>>>++k k>-+>-ke--++>>e     e---++e
c>--+>>c     ccccccco>--+o     o-+--oo-->>o     o>+++o l+>>-l    y-+---y   y--++-y  m-+>>-mmm>>-+>+mmm-+++-m i+>>+i  k--+--+k>+-->k e>-->>>>eeeee+-->++e
c->->+c             o-+>-o     o-+>+oo-+>+o     o->->o l->--l     y++--+y y->>+>y   m>>++m   m>-+>m   m->>-m i+-->i  k+--+>>+>>+-k  e>>>+++>->+>+++++-e
c++->-c             o--+-o     o-+->oo>+->o     o+--+o l+>-+l      y+->++y>----y    m++->m   m++-+m   m>->>m i++-+i  k+>-+>+-->->k  e+->>>-eeeeeeeeeee
c-+>>++c     ccccccco>->+o     o>-++oo-+>-o     o--++o l-->>l       y>+->+-->>y     m+++>m   m>++-m   m>>-+m i->+-i  k-+--+-k+>+>-k e>>->->>e
c+-+>-+-cccccc+->>>co->>--ooooo+>+-+oo->++>ooooo->-->ol>>+-++l       y-+++->+y      m+>-+m   m+-++m   m--->mi--+>->ik>---+-k k+>->-ke-+->-+->e
 c>++>>-+--->>>>+-+co++>-+---+++++++oo--+>++->+>--->+ol+>>--+l        y>+->+y       m->++m   m>>++m   m>-++mi+>--->ik+>-->+k  k+->--ke++>>--+-eeeeeeee
  cc>++++-->->+>-+>c oo>++-+>>+->>oo  oo--+>++->>--oo l->+-->l       y+->--y        m+>>>m   m+++>m   m->+-mi>-+-+>ik+>->-+k   k>>->-kee-+>++++->-+->e
    cccccccccccccccc   ooooooooooo      ooooooooooo   llllllll      y>>+>>y         mmmmmm   mmmmmm   mmmmmmiiiiiiiikkkkkkkk    kkkkkkk eeeeeeeeeeeeee
                                                                   y+-+>>y
                                                                  y++->-y
                                                                 y+++-+y
                                                                y->+++y
                                                               yyyyyyy

Way less obvious where the code ends that way, and still runs just fine (just takes a bit longer to halt and uses slightly more memory for programs that don't loop themselves).

You can find the script in this github repo

This is really cool. How do you guarantee that the fake BF padding at the end doesn't do anything unexpected?

By only using the >, +, and - characters. if a program finishes running it doesn't need those values anymore, it doesn't generate syntax errors, and it doesn't go out of bounds left. The only thing that can happen is that it goes out of bounds right. It always halts after running those instructions because it doesn't create a loop, and it doesn't freeze because it's never asking for user input. It also doesn't mess up the output because it never prints anything.