Echo Bat

Hans Scharler
/
Echo Bat
on 12 Nov 2026
8
40
12
4
8689
game.m
function g=game()
    % ECHO BAT: a bat in a cave it cannot see. A tap is a wingbeat and a chirp.
    g.init=@init; g.update=@update; g.render=@render;
end
function s=init(ax)
    ax.Color=[0 0 0];
    G.im=image(ax,[2 802],[2 598],zeros(150,201,3),'Interpolation','bilinear','HitTest','off','PickableParts','none');
    G.bat=patch(ax,nan,nan,[.92 .88 .8],'EdgeColor','none','HitTest','off','PickableParts','none');
    % HUD: distance, a meter for how loud the next chirp will be, moths, best.
    line(ax,[330 470],[584 584],'Color',[.2 .25 .3],'LineWidth',6,'PickableParts','none');
    G.ch=line(ax,[330 330],[584 584],'LineWidth',6,'PickableParts','none');
    h=txt(ax,322,584,9,'right'); h.String='CHIRP';
    G.sc=txt(ax,20,584,14,'left'); G.hr=txt(ax,780,584,11,'right'); G.zn=txt(ax,400,545,12,'center');
    G.t1=txt(ax,400,430,44,'center');
    G.t2=txt(ax,400,330,12,'center'); G.t3=txt(ax,400,215,15,'center');
    % Sponge at the left and right edges, with a little loss everywhere.
    x=1:201; s.sp=.996*min(1,.86+min(x,202-x)/70);
    [a,b]=meshgrid(-3:3); s.G=exp(-(a.^2+b.^2)/2);
    s.g=G; s.best=0; s.nb=0; s=reset(s,18); s.mode=0;
end
function h=txt(ax,x,y,sz,al)
    h=text(ax,x,y,'','Color',[.8 .9 .95],'FontSize',sz,'FontWeight','bold','HorizontalAlignment',al,...
        'HitTest','off','PickableParts','none');
end
function s=reset(s,w)
    % w widens the cave. The title screen uses a wide chamber and adds the
    % name of the game as rock, so that the chirps light it.
    s.w=w; s.rot=0; s.mode=1; s.age=0; s.ph=6*rand(1,3); s.col=0; s.d=0; s.a=0; s.tp=0;
    s.y=300-60*(w>0); s.vy=0; s.wing=0; s.n=0; s.q=1;
    s.u=zeros(150,201); s.v=s.u; s.B=s.u; s.E=s.u;
    s.N=strata(1:201); s.R=cave(s,1:201); L=zeros(150,201);
    if w
        T=['### ### # # ###  ##   #  ###';'#   #   # # # #  # # # #  # ';'##  #   ### # #  ##  ###  # ';...
            '#   #   # # # #  # # # #  # ';'### ### # # ###  ##  # #  # ']>32;
        L(96:115,45:156)=kron(flipud(T),ones(4)); s.R=s.R|L;
    end
    s=mask(s); s.W=max(s.W,L);
    s.mx=[150 230 310]; s.my=[75 75 75]; s.mb=[0 0 0]; s=ping(s,1.5);
end
function [r,c,h]=cave(s,X)
    % Rock for world columns X: a winding tunnel that narrows with distance,
    % with a spike from the roof or the floor every 90 columns.
    e=min(1,X/150);
    c=75+e.*(22*sin(X/90+s.ph(1))+7*sin(X/37+s.ph(2)));
    h=max(20,31-X/160)+s.w+2*sin(X/4+s.ph(3));
    k=.6*h.*exp(-((mod(X,90)-45)/5).^2).*(X>180);
    t=mod(floor(X/90),2);
    r=(1:150)'>c+h-k.*t+2*sin(X/3.3+s.ph(1))+1.5*sin(X/1.9)|...
        (1:150)'<c-h+k.*(1-t)+2*sin(X/2.9+s.ph(2))+1.5*sin(X/2.3);
end
function n=strata(X)
    % Bands of harder and softer rock, tilted and folded along the cave.
    n=.7+.3*sin(1.6*(1:150)'+5*sin(X/31)+X/4);
end
function s=mask(s)
    % S zeroes the wave inside rock and absorbs part of it next to rock.
    % W is how strongly each rock cell lights up: full at the surface, fading
    % over eight cells of depth, with strata that are fixed to the rock.
    A=+~s.R;
    s.S=A.*s.sp.*(1-.5*min(1,conv2(+s.R,ones(3),'same')/3));
    o=ones(1,17);
    s.W=s.R.*max(min(1,conv2(o,o,A,'same')/90).^1.5.*s.N,conv2(A,ones(3),'same')>0);
    % Past 200 m, stretches of moss answer the sound only faintly.
    X=s.col+(1:201); s.W=s.W.*(1-.7*(sin(X/45)>.7&X>2e3));
end
function s=ping(s,amp)
    i=round(s.y/4)+(-3:3); i=min(max(i,1),150); j=47:53;
    s.u(i,j)=s.u(i,j)+amp*s.G; s.v(i,j)=s.v(i,j)+amp*s.G;
end
function s=update(s,dt,p)
    s.age=s.age+dt; s.wing=max(0,s.wing-3*dt);
    if s.mode==1
        % A rested bat chirps louder: the loudness grows with the time since
        % the last wingbeat.
        s.q=s.q+dt;
        if p, s.vy=160; s.wing=1; s=ping(s,min(1.5,.4+s.q)); s.q=0; end
        s.vy=s.vy-440*dt; s.y=s.y+s.vy*dt;
        s.d=s.d+(110+min(60,s.d/130))*dt;
        while s.d/4>s.col+1
            % Scroll one cell: shift every field left and add a cave column.
            s.col=s.col+1; z=zeros(150,1);
            s.u=[s.u(:,2:end) z]; s.v=[s.v(:,2:end) z];
            s.B=[s.B(:,2:end) z]; s.E=[s.E(:,2:end) z];
            s.N=[s.N(:,2:end) strata(s.col+201)];
            s.R=[s.R(:,2:end) cave(s,s.col+201)]; s=mask(s);
        end
        % Moths: lit by sound, caught by touch, respawned ahead.
        for k=1:3
            x=s.mx(k)-s.col; y=s.my(k)+3*sin(3*s.age+2*k);
            if x<2||abs(x-50)<4&&abs(y-s.y/4)<4
                if x>2, s.n=s.n+1; s=ping(s,2.5); end
                X=s.col+205+60*rand; [~,c,h]=cave(s,X);
                s.mx(k)=X; s.my(k)=c+(h-9)*(2*rand-1); s.mb(k)=0;
            end
        end
        i=round(s.y/4);
        if i<3||i>148||any(s.R(i+(-1:1),50))
            s.nb=s.d>s.best; s.mode=2; s.age=0; s.tp=0; s.vy=0; s.best=max(s.best,s.d);
        end
    elseif p&&s.age>.8
        s=reset(s,0);
    elseif s.mode==0
        s.y=240+12*sin(2*s.age);
        if s.age>s.tp, s.tp=s.tp+1.5; s.wing=1; s=ping(s,1.5); end
    else
        % The crash: four shrieks, each quieter, while the bat tumbles to the floor.
        if s.age>s.tp&&s.tp<.4, s=ping(s,3-5*s.tp); s.tp=s.tp+.13; end
        i=round(s.y/4);
        if i>3&&~s.R(i-2,48), s.vy=s.vy-440*dt; s.y=s.y+s.vy*dt; s.rot=s.rot+9*dt; end
    end
    % Wave equation, leapfrog in time, 5-point Laplacian in space.
    s.a=s.a+194*dt; s.E=abs(s.u);
    while s.a>1
        s.a=s.a-1; u=s.u;
        L=u([2:end end],:)+u([1 1:end-1],:)+u(:,[2:end end])+u(:,[1 1:end-1])-4*u;
        s.u=(2*u-s.v+.45*L).*s.S; s.v=u; s.E=max(s.E,abs(s.u));
    end
    % Rock surfaces light up where the sound is, and fade. The deeper the
    % cave, the shorter the reach of the light and the faster it fades. The
    % title is lit more strongly. After a crash the light stays, so you see
    % what you hit.
    m=s.mode==0; o=ones(1,17); f=(.5+m-min(.25,s.d/6e4))*conv2(o,o,s.E,'same');
    s.B=max(s.B*exp(-dt/(m+max(.2,.6-s.d/3e4))*(s.mode<2)),s.W.*min(1,f));
    for k=1:3
        x=min(max(round(s.mx(k)-s.col),2),200);
        s.mb(k)=max(s.mb(k)*exp(-dt/.5),min(1,f(round(s.my(k)),x)));
    end
end
function render(~,s)
    G=s.g; z=@(v)reshape(v,1,1,3);
    % Sound is a blue ring, and red after a crash. Lit rock fades from white
    % through teal to indigo, and the afterglow color changes every 100 m.
    c=[.2 .5 .85]; if s.mode==2, c=[1 .3 .1]; end
    C=min(1,(26*abs(s.u)).^2).*z(c)+s.B.^z(circshift([3 1.3 .6],floor(s.d/4e3))).*z([.9 1 .7]);
    for k=1:3
        x=round(s.mx(k)-s.col); y=round(s.my(k)+3*sin(3*s.age+2*k));
        if x>3&&x<198
            i=y+(-3:3); j=x+(-3:3);
            C(i,j,:)=C(i,j,:)+s.mb(k)*s.G.*z([1 .7 .25]);
        end
    end
    % The scene dims behind the end text. A dark band sits behind the HUD,
    % and the picture shakes on impact.
    C=C*(1-.7*min(1,max(0,2*s.age-1.6))*(s.mode==2));
    C(143:150,:,:)=C(143:150,:,:)/4;
    f=s.d/4-s.col;
    set(G.im,'CData',uint8(255*C),'XData',[2 802]-4*f*(s.mode>0)+8*exp(-6*s.age)*sin(50*s.age)*(s.mode==2));
    % The bat brightens as it rests, and the meter shows the next chirp.
    w=9*sin(9*s.wing^2); q=min(1,s.q/1.1);
    x=1.3*[-15 -8 -3 0 3 8 15 7 0 -7]; y=1.3*[w 3+w/3 2 5 2 3+w/3 w -2 -5 -2];
    set(G.bat,'XData',198+x*cos(s.rot)-y*sin(s.rot),'YData',s.y+x*sin(s.rot)+y*cos(s.rot),'FaceColor',[.92 .88 .8]*(.5+.5*q));
    set(G.ch,'XData',[330 330+140*q],'Color',[.4 .8 1]+[.6 .2 0]*(q==1));
    G.sc.String=sprintf('%.0f m',s.d/40);
    G.hr.String=sprintf('moths %d     best %.0f m',s.n,s.best/40);
    k=floor(s.d/4e3); b=k>0&&mod(s.d,4e3)<400&&s.mode==1; G.zn.Visible=b;
    if b
        N={'THE ECHOES FADE FASTER','MOSS AHEAD: SOME ROCK BARELY ANSWERS','THE DEEP DARK'};
        G.zn.String=sprintf('%d m     %s',100*k,N{min(k,3)});
    end
    v=s.mode==0||s.mode==2&&s.age>.8;
    G.t1.Visible=v&&s.mode==2; G.t2.Visible=v; G.t3.Visible=v&&mod(s.age,1)<.7;
    if s.mode==0
        G.t2.String={'You are a bat in a cave you cannot see.';...
            'Every wingbeat is also a chirp, and the sound lights the rock it touches.';...
            'Rest between taps to chirp louder. Catch the moths.'};
        G.t3.String='TAP TO FLY';
    elseif v
        N={'',' That is a new best.'};
        G.t1.String='SILENCE';
        G.t2.String={sprintf('You flew %.0f m and caught %d moths.%s',s.d/40,s.n,N{1+s.nb});'';...
            sprintf('Best this session: %.0f m',s.best/40)};
        G.t3.String='TAP TO FLY AGAIN';
    end
end
Play Game
Preview

Play Game opens this in MATLAB Drive. Click Run this app there to play.