April 6, 2014

Fiction

I was reading a story on the plane today. It was the tale of a terrible war, a battle between two civilizations bent on the destruction of the other. It spoke of barbaric acts, of unspeakable horrors, of cruelty and pain on such a magnitude it could only exist in a place devoid of morality.

None of it is real. No one is really dying, no one is having their heart broken, no one's lives are really being destroyed. And yet, it bothers me. It bothers me because I know such tales of war were not composed in a vacuum. The power that story holds over me does not come from the imaginary characters it paints, but of the real people it is based off of. The lives that really were lost, the tragedies that tore us apart, the chapters of human history most of us would prefer to forget.

I sometimes find it difficult to keep reading, to discover the horrors I know are lying in wait for our beloved protagonist. With each tragedy that befalls them, I find myself feeling sorry for the character in the story, even though they aren't real. Yet, I'm also feeling sorry for the millions of people I have never met who suffered the same fate. It's difficult to continue because every chapter reminds me of the perils of human existence. I'm not sure there is a happy ending to this tale, or if there is, what the tremendous costs might be.

Perhaps we seek happy endings in our stories because we care about these imaginary characters that have been invented for our benefit. Or perhaps it is because we desperately want to believe that our lives also have a happy ending. We project our troubles and battles into what we read, wanting to believe that we can be the protagonist in our own tales.

This story speaks of tremendous struggles, of soldiers who lost everything and fought to save their nation from being lost to the sands of time. It is brutally effective in reminding us of what we truly hold dear, of what really matters in the end. It puts you in the armor of a new recruit who is suddenly left wondering if he should have spent more time enjoying life now that his could very well be extinguished in a moment. You watch a soldier die by the hand of his own commander simply because he refused to obey a command he knew was wrong.

You don't need a sword to tear the life out of your closest friend. A drunk driver could do the same. A rare disease. Cancer. We may frame heartbreak in many different ways, but the emotion stays the same. The pain of loss is something that transcends mere words, and we can feel it's power even when we're reading about people who never existed.

Sometimes, stories are hard to read because they remind us too much of the world we were trying to escape in the first place. They remind us of everything—and everyone—we could lose.

And those... those are the most precious stories of all.

March 18, 2014

The Problem With Photorealism

Many people assume that modern graphics technology is now capable of rendering photorealistic video games. If you define photorealistic as any still frame is indistinguishable from a real photo, then we can get pretty close. Unfortunately, the problem with video games is that they are not still frames - they move.

What people don't realize is that modern games rely on faking a lot of stuff, and that means they only look photorealistic in a very tight set of circumstances. They rely on you not paying close attention to environmental details so you don't notice that the grass is actually just painted on to the terrain. They precompute environmental convolution maps and bake ambient occlusion and radiance information into level architecture. You can't knock down a building in a game unless it is specifically programmed to be breakable and all the necessary preparations are made. Changes in levels are often scripted, with complex physical changes and graphical consequences being largely precomputed and simply triggered at the appropriate time.

Modern photorealism, like the 3D graphics of ages past, is smoke and mirrors, the result of very talented programmers and artists using tricks of the eye to convince you that a level is much more detailed and interactive than it really is. There's nothing wrong with this, but we're so good at doing it that people think we're a heck of a lot closer to photorealistic games then we really are.

If you want to go beyond simple photorealism and build a game that feels real, you have to deal with a lot of extremely difficult problems. Our best antialiasing methods are perceptual, because doing real antialiasing is prohibitively expensive. Global illumination is achieved by deconstructing a level's polygons into an octree and using the GPU to cubify moving objects in realtime. Many advanced graphical techniques in use today depend on precomputed values and static geometry. The assumption that most of the world is probably going to stay the same is a powerful one, and enables huge amounts of optimization. Unfortunately, as long as we make that assumption, none of it will ever feel truly real.

Trying to build a world that does not take anything for granted rapidly spirals out of control. Where do you draw the line? Does gravity always point down? Does the atmosphere always behave the same way? Is the sun always yellow? What counts as solid ground? What happens when you blow it up? Is the object you're standing on even a planet? Imagine trying to code an engine that can take into account all of these possibilities in realtime. This is clearly horrendously inefficient, and yet there is no other way to achieve a true dynamic environment. At some point, we will have to make assumptions about what will and will not change, and these sometimes have surprising consequences. A volcanic eruption, for example, drastically changes the atmospheric composition and completely messes up the ambient lighting and radiosity.

Ok, well, at least we have dynamic animations, right? Wrong. Almost all modern games still use precomputed animations. Some fancy technology can occasionally try to interpolate between them, but that's about it. We have no reliable method of generating animations on the fly that don't look horrendously awkward and stiff. It turns out that trying to calculate a limb's shortest path from point A to point B while avoiding awkward positions and obstacles amounts to solving the Euler-Lagrange equation over an n-dimensional manifold! As a result, it's incredibly difficult to create smooth animations, because our ability to fluidly shift from one animation to another is extremely limited. This is why we still have weird looking walk animations and occasional animation jumping.

The worst problem, however, is that of content creation. The simple fact is that at photorealistic detail levels, it takes way too long for a team of artists to build a believable world. Even if we had super amazing 3D modelers that would allow an artist to craft any small object in a matter of minutes (which we don't), artists aren't machines. Things look real because they have a history behind them, a reason for their current state of being. We can make photorealistic CGI for movies because each scene is scripted and has a well-defined scope. If you're building GTA V, you can't somehow manage to come up with three hundred unique histories for every single suburban house you're building.

Even if we did invent a way to render photorealistic graphics, it would all be for naught until we figured out a way to generate obscene amounts of content at incredibly high levels of detail. Older games weren't just easier to render, they were easier to make. There comes a point where no matter how many artists you hire, you simply can't build an expansive game world at a photorealistic level of detail in just 3 years.

People always talk about realtime raytracing as the holy grail of graphics programming without realizing just what is required to take advantage of it. Photorealism isn't just about processing power, it's about content.

February 20, 2014

Success Is Relative

It seems that many people view success as some sort of absolute value, usually inextricably tied to how much money you make. Ignoring the fact that we should not be tying success to monetary value, we should also not be treating success as some sort of absolute value based on where you end up. What really matters is where you started from.

Climbing a mountain is not that impressive when you start half a mile from the peak. On the other hand, if you start all the way at the bottom, climbing the whole thing is very impressive, indeed. A blogger once said that being born as a middle-class straight white male is like playing life on easy mode. Minorities have many cultural barriers they must break down to reach the same level as a white male, often because they are born poor.

Poor people have a huge range of difficulties that are hard to appreciate unless one experiences them first-hand. To the middle-class, overdraft fees, parking tickets and unexpected taxes are nothing more than an annoyance. To the poor, they are the stuff of nightmares. A single slipup, a single misstep, and they will lose all the spending money they had for the next 3 months, and possibly not be able to buy dinner. They must work demanding, low-paying jobs that leave them physically and mentally exhausted, which itself makes it even harder for them to land a promotion or pursue their hobbies.

Robbed of the mental energy to work on things they find interesting and fun, poor people often slip into depression, or self-destructive lifestyles, because there literally is no way out. The only way out is for them to somehow be less stressed, which requires a better job, which requires them to have more time to spend bettering themselves, which requires them to be less stressed, which requires a better job...

It's the worst kind of catch-22, and it's one whose existence is repeatedly and blatantly denied by a middle-class oblivious to the struggles of the poor. They do not understand the herculean efforts it takes to do things that seem simple and routine for themselves. They do not appreciate how impressive it is when a poor person elevates themselves to a position that they might consider demeaning.

What's worse is that they over-inflate their own progress, and then act as though a poor person could have done the same thing. "Just teach yourself how to do it," they say. "Use the internet," they say. They don't understand that without a decent education, poor people either do not understand the importance of these things and shouldn't be expected to, or they have so much to learn they can't possibly find the time to do so in conjunction with the soul-sucking job that occupies most of their time. Being poor is not a slight disadvantage, it's like starting a race half a mile behind your opponent with a lead weight attached to your ankle, and your opponent has a motorcycle.

Many of my acquaintances, when they hear of my 6-figure salary, automatically assume that I am successful. That I have somehow beaten the odds through concentrated effort and landed an amazing job they can only dream of. The truth is that what I have accomplished is hardly anything special. I won the genetic and environmental lottery. I was gifted with talents that gave me a jump start on skills that happened to be wildly in-demand in today's economy. I went to excellent schools, and had attentive parents that never shielded me from my own failures, but encouraged me to learn from them. I worked just as hard as everyone else, but I didn't even need to. My efforts in making music and building games never accomplished anything. All I had to do was dutifully finish my schoolwork until I graduated with a degree in applied mathematics, and poof, I had a fantastic, cushy job many people would kill for.

This is hardly impressive. I have simply traveled the path that was set up for me, and every attempt I made to excel, to do more than simply walk down the road in front of me me has been met with abject failure. I was given a gift, a free ride halfway up the mountain, and I haven't made it very far at all. The people at the bottom look up to me and see success, without realizing that I have climbed the same distance they have.

They are just as successful as I am - they just started farther down.

When I see an artist just barely managing to make ends meet, I see success. When I see a musician living in a run-down apartment and paying rent, I see accomplishment. When I see a writer making ends meet with a few odd commissions, I see tenacity. Our culture heaps scorn on those who do what they love and barely manage to make a living out of it without realizing how brutally difficult this is to do. Simply managing to feed yourself by following your dream is an accomplishment on par with finishing a master's degree.

I am not writing this to belittle myself, or others, but to elevate those who have accomplished a great deal, and get comparatively little recognition for it. I write this because too often artists set absurd goals for themselves without knowing that the ones they idolize didn't climb nearly as many steps as themselves. Artists look around and see themselves barely surviving off of their art, and wonder if they are a failure, when in fact simply being able to sustain oneself like that is a great accomplishment. Sadly, it seems that it is an accomplishment that does not come with a diploma, or a trophy, or even respect.

I hope that artists, and poor people in general, will eventually realize that they are just as successful as everyone else - they just have more steps to climb.

December 14, 2013

My SteamOS Experience

SteamOS

After quite a bit of fighting, I have managed to create a stable, working SteamOS installation, on which I am writing this blog post. In the interest of science! I will be detailing the steps I took while installing the OS, along with all the issues I had and how I solved them. To help anyone trying to troubleshoot things, here are a list of things that went wrong. If you want to know how they were fixed, keep reading:

  • Initial steam shortcut failed to start the program
  • Recovery partition failed to install and wouldn't shut down properly
  • SteamOS cannot be used with more than 1 monitor!
  • Upon login, steam requires a confirmation code. That's sent to your e-mail. Which you need a web browser for.
  • Steam doesn't have any default repositories
  • Sound doesn't work

To install SteamOS, I essentially followed the steps outlined here, but they weren't all nicely organized for me. I downloaded the zip, extracted it into a FAT32 usb, copied the syslinux files, executed syslinux on the drive, and wrote syslinux.cfg, which is improperly formatted in that post. Here is the proper formatting:

DEFAULT linux
TIMEOUT 50
LABEL linux
    kernel install.amd/vmlinuz
    append initrd=install.amd/gtk/initrd.gz preseed/file=/cdrom/default.preseed DEBCONF_DEBUG=developer desktop=steamos auto=true priority=critical video=vesa:ywrap,mtrr vga=788 -- quiet

At this point I had done everything I needed to do from windows. SteamOS cannot be dual-booted, because it obliterates whatever drives are connected to the computer. To work around this, I used a second hard drive and disconnected the first one. To avoid any mistakes, I physically disconnected the drive from the computer to ensure it would be safe. Only after doing that and disabling it in the BIOS did I configure the USB to boot up.

WARNING: DO NOT CONFIGURE YOUR BIOS TO BOOT FROM USB UNTIL ALL OTHER HARD DISKS HAVE BEEN REMOVED FROM YOUR SYSTEM. THERE IS NO CONFIRMATION DIALOG. THERE IS NO BUTTON THAT SAYS "PRESS OK TO IRREVERSIBLY DESTROY ABSOLUTELY EVERYTHING ON THIS SYSTEM." THE MOMENT IT BOOTS OFF THAT USB, ANYTHING CONNECTED TO THE COMPUTER WILL BE DESTROYED.

As expected, the installer failed when installing grub. I dropped into the terminal and typed in the magic instructions, which were a lot easier to do after I realized I could just hit tab and have the terminal complete the filenames for me. Once I got back into the setup, though, the guide didn't mention that you actually have to hit continue 3 or 4 times. However, it did indeed work on the second try, and I soon found myself booting into a GNOME environment.

It's important that, when first logging in, you pick the GNOME shell environment. It isn't the default, and it's not the SteamOS environment. Failure to do so will fuck everything up really bad. However, once I did get into the GNOME shell environment, the steam shortcut didn't work. I don't know why, but I had to go into the terminal and manually execute /usr/bin/steam %U. This is exactly what the shortcut did, but for some reason it only worked when I typed it into the terminal. Once steam was installed, I logged off and logged into the desktop account (password: desktop). I was able to run ~/post_logon.sh without a hitch, and the system rebooted.

Then, the recovery creator failed. It failed spectacularly. It spat out a bunch of errors, tried to continue, then dropped into a terminal. I told it to restart, but it failed. It kept reverting back to the terminal because it somehow couldn't disconnect the terminal session despite me telling it to shut-the-fuck-down-right-now-or-else. So I just held the power button down and did a hard shutdown. Once I rebooted, despite not having a recovery partition, it seemed to work just fine. It booted into steamOS and then...

Nothing. Black screen. Complete failure.

This, however, turns out to simply be what happens when Steam's compositor encounters two monitors. Instead of shutting one off, it just completely dies. After unplugging my second monitor, steam worked like a charm, and I logged in, and then... I was prompted to enter a confirmation code sent to my e-mail. Which I needed a web browser to get to, which required that I logged in. Thankfully, I had a laptop sitting next to me, so this wasn't a problem, but fair warning to anyone else who's logging in.

Now I was in the Steam client, but I had no sound. Dropping back to desktop was easy after checking the "Enable Linux Desktop" option under "Interface", but I had to assign a new password while in desktop mode so I could use sudo (this is done using the passwd command).

The problem is that steam's only default repository is for, well, steam. To get anything else that would make my OS usable, like have sound, I needed to add back in the default debian repositories. SteamOS thankfully ships with nano, so I typed sudo nano /etc/apt/sources.list and added the following:
deb http://http.debian.net/debian wheezy main
deb-src http://http.debian.net/debian wheezy main

Now, after running sudo apt-get update, I had access to the default packages, which crucially included ALSA, the linux audio API and low-level driver package. After installing and restarting ALSA, however, I still didn't have sound. Sometimes when this occurs, it's recommended to open a console and run rm -rf ~/.pulse to delete the .pulse configuration files, which can mess things up. In my case, however, the hardware still wasn't being detected. To solve this, I installed alsa-firmware-loaders. After rebooting, my sound still didn't work, so I ran alsa force-reload. This finally did the trick, and I had sound on the desktop! But, not on the steam client, or the games.

Rebooting finally graced me with sound in the steam client, but disabled sound on the desktop. So, now I have sound on Steam, but not on the desktop. This isn't a big problem right now, but hopefully someone can fix it eventually.

Unfortunately, I could not get flash working, and debian comes with a firefox fork, which doesn't support whatever audio API everyone wants to use right now. Luckily, now that we have the default debian packages, we can install things like GNOME tweaker, and other fun things to customize our steamOS experience.

Once I got sound working, I tried out a few games. SpaceChem did not appear to have a fully-working linux client, because it failed to sync my saves from steam cloud, and half it's menu buttons were just gone. Team Fortress 2 worked fine, aside from awkward looking text, but it was running at about 2 FPS until I disabled motion blur. After doing that, it abruptly started running smoothly, with just a few hiccups here and there, so I don't know what that was about.

My computer only has 4 GB of RAM, so TF2 normally has memory issues on windows, which usually cause temporary freezeups. On steamOS, those memory problems still exist, but instead of causing temporary freezeups, the entire game instantly crashes. Given that this is a failure case, it's not really something I'm allowed to complain about, but be warned: don't run out of RAM.

Aside from the occasional whacky interface, playing games on SteamOS feels almost identical to playing them on windows. Performance and load times appear to be identical to windows, controls are responsive, everything usually works. While this is only a beta, I consider this a step in the right direction, and am optimistic about the future of SteamOS.

December 2, 2013

Tile-Based Discrete Wavefront Propagation

I'm currently building a very simplistic first-person shooter in WebGL. An example of this algorithm, with the debug code left in, is available here. The map is represented by a grid - a cell is solid if its 0, and empty if its 1. This is trivial to render by using a standard recursive 4-direction flood fill algorithm. Unfortunately, we can't simply render the entire level if our rooms contain high levels of detail or many objects, because we'll overload the GPU.

Frustum culling is the obvious answer, but I need it to be highly efficient. This means adjusting the frustum to account for corners, so I only render the visible portions of the level. While a lot of the speed concerns I currently have can be alleviated using batch rendering, this will stop working the instant I put in details that can't be batch rendered. Consequently, I want the algorithm to be as close to perfect as possible, only rendering rooms that are visible and no others.

This is where Tile-Based Discrete Wavefront Propagation comes in. The central idea is to represent the viewing frustum as a wave that flows through the level, getting split up when it hits corners. If properly implemented, this will be exact, only rendering rooms that are visible and no others. It turns out that you can implement this in $$O(n)$$ time, but we can't use the naïve recursive flood-fill anymore, because it's depth first. If we want to simulate a wave, we need to render the grid using a breadth-first search, to simulate it slowly spreading outward. Breadth first search is implemented using a queue by simply pushing all neighboring nodes on to the queue and rendering them until the queue is empty.

As the wave propagates through the level, we adjust the left and right angles of each individual wavefront according to the walls that we hit. This requires that we deal with two possible cases: the case where a wall is in front of the wave as it propagates through the level, and the case where it isn't. The only time the wave can actually be split up is when a wall is in front of it - the rest of the time, it is simply clipped on the sides. "Front" and "side" are defined based on what direction the wave came from.

Starting with the case where there isn't a wall in front, we check to see if there is a wall on the right. If there is, we check the angles of it's corners. If either angle is inside the culling frustum, we change the right-angle to the one that is "farthest inside" (done in code by simply checking each angle one after the other). Then we do the same for the left wall, this time changing the left-angle, if appropriate. Then we send the updated angles to the neighboring tiles - the left one gets (left-angle,new-right-angle), the front gets (new-left-angle,new-right-angle), and the right gets (new-left-angle,right-angle).

If there is a wall in front, everything stays the same (note that the front wall has it's own set of corners you must check), but the angles that get sent through the neighboring tiles change. The left wall gets (left-angle,new-left-angle), the front doesn't exist so we ignore it, and the right wall gets (new-right-angle,right-angle). This effectively splits the wave down the middle, but it makes things complicated. Before, if a new left-angle was outside the culling frustum, we would just set it to the old left-angle. This doesn't work anymore, because we're splitting the wave, which means we require a new left-angle that isn't equal to the old left-angle. To solve this, if the new left-angle fails to exist, we set it to the old right-angle instead of the old left-angle, and vice-versa for the new right-angle.

Conceptually, this isn't too complicated, but the actual implementation gets complicated very fast due to angles being a giant pain in the ass to work with. We have to seed the initial queue with the neighboring tiles, and deal with getting proper frustum culling working, which is far more difficult than it seems. This will require a sizable number of utility functions, so we'll look at those first:

function realmod(i,m) { i=(i%m); return i+((i<0)*m); } // Mathematically correct modulo
// Queue implementation using a circular array. Resize is very expensive, but almost never happens, because it remembers its size after each frame.
function Queue(sz) {
  this.array = new Array(!sz?1:sz);
  this.cur=-1;
  this.length=0;
  this.push = function() {
    for (var i = 0; i < arguments.length; i++) {
      if(this.length>=this.array.length) {
        this.resize(this.array.length*2);
      }
      this.cur=((this.cur+1)%this.array.length);
      this.array[this.cur] = arguments[i];
      this.length += 1;
    }
  }
  this.pop = function() { return this.get(--this.length); }
  this.peek = function() { return this.get(this.length-1); }
  this.get = function(i) { return this.array[this.modindex(i)]; }
  this.clear = function() { this.cur=-1; this.length=0; }
  this.resize = function(nsize) { 
    var sz=this.array.length;
    var c = this.cur+1;
    for(var i=sz-c; (i--)>0;) {
      this.array[c+nsize-sz+i]=this.array[c+i];
    }
  }
  this.modindex = function(i) { return realmod(this.cur-i,this.array.length); }
}
function realfmod(x,m) { return x - Math.floor(x/m)*m } // Implements mathematically correct fmod
// Gets absolute distance between two angles
function getAngleDist(u,v) { return Math.PI - Math.abs((Math.abs(u - v)%(Math.PI*2)) - Math.PI); } 
// Gets the signed distance between two angles
function getAngleDistSign(u,v) { return ((realfmod(v - u,Math.PI*2) + Math.PI)%(Math.PI*2)) - Math.PI; } 

  function getdx(dir) { dir=((dir+4)%4); return (dir==0)-(dir==2); }
  function getdy(dir) { dir=((dir+4)%4); return (dir==1)-(dir==3); }
  function exists(x,y,m,w) { var i = x + (y*w); return x>=0 && x<w && i<m.length && (m[i]&1)!=0; }
  function exists_dir(x,y,m,w,dir) { return exists(x+getdx(dir),y+getdy(dir),m,w); }

  // Gets the angle of a point from the player's origin (which is the camera's origin)
  var getangle = function(x,y) { return Math.atan2(y-player[0].elements[2],x-player[0].elements[0]); }
  // Gets the angle of a corner, offset from the given point by dir and diri.
  var getangle_dir = function(x,y,dir,diri) { return getangle(x+getdx(dir)*5 + getdx(diri)*5,y+getdy(dir)*5 + getdy(diri)*5); }

First, the modulo operators in most programming languages are actually remainder operators. They do not perform mathematically correct modulo, and their behavior when you feed them negative numbers is not what you'd expect. The first thing we do, then, is define a modulo function that actually behaves like the modulo operator. We then use this to build a circular queue that resizes itself when necessary. When resizing, the queue must move all items to the right of the index over, which is a costly operation, so we let it keep it's size across frames. This makes the number of resize operations essentially zero.

The next few functions deal with angles. Angles are inherently circular, and this causes all sorts of problems. If our left-angle is 355°, and our right angle is 5°, the distance between these two angles is 10°, not 350°. There is a standard method to getting the absolute distance between two angles using the floating point modulo operator, which is implemented in getAngleDist(). This is all well and good, but we have defined left and right angles, which means we need to know if something is on the left hand side, or the right hand side. This requires a signed angular distance function, which makes things much more complicated because, once again, the floating point modulo operator is not actually modulo, it's a remainder.

So, we need to implement a proper floating point modulo operator. The standard fmod() function is implemented using the following formula:\[ \DeclareMathOperator{\fmod}{fmod} \DeclareMathOperator{\trunc}{trunc} \fmod(n,d) = n- \trunc\left(\frac{n}{d}\right) d\]It's trivial to change this formula into one that gives us the correct behavior (remember, truncation is not flooring!):
\[ \fmod(n,d) = n- \left\lfloor\frac{n}{d}\right\rfloor d\]We can use this to define a function such that, as long as $$b-a$$, when forced into the range $$\left[0,2\pi\right)$$, is less than $$\pi$$ (or 180°), we get a positive distance. If it goes past $$\pi$$, it becomes negative and heads back towards 0, just like in the absolute value distance function, but now with the proper sign. We will use this later to determine if a tile crosses over our angular culling frustum. getdx(),getdy(),exists(), and exists_dir() are all used to either bump x/y coordinates according to a direction, or determine if a specific tile exists. Now we get to the real function:

var roomq = new Queue(10); // queue holding nodes
var drawmap = function(x,y,m,w,langle,rangle) { // map drawing function
  var i = x + (y*w);
  m[i]=(m[i]|2);
  // draw floor
     
  for(var j = 0; j < 4; ++j) { // Push initial neighbors on to the queue
    var nx=x+getdx(j);
    var ny=y+getdy(j);
    if(!exists(nx,ny,m,w)) { // If it doesn't exist, we ran off the edge of the map or hit a wall
      // draw wall
    } else {
      roomq.push(nx,ny,langle,rangle,j);
    }
  }
  
  var dir=0; // Stores the direction the wave is going. 0: (+) x-axis, 1: (+) y-axis, 2: (-) x-axis, 3: (-) y-axis
  while(roomq.length>0) {
    x = roomq.pop();
    y = roomq.pop();
    langle = roomq.pop();
    rangle = roomq.pop();
    dir = roomq.pop();
    var i = x + (y*w);
    var room=m[i];
    if((room&2)==2) continue; // If true, we already visited this node
    var mid = [langle,getAngleDistSign(langle,rangle)/2];
    if(mid[1]<0) continue; // If this is less than 0, langle has cross over rangle, so there's nothing to render.
    mid[0]=langle+mid[1];
    var angles=[getAngleDistSign(getangle(x*10+5,y*10+5),mid[0]),
                getAngleDistSign(getangle(x*10-5,y*10-5),mid[0]),
                getAngleDistSign(getangle(x*10-5,y*10+5),mid[0]),
                getAngleDistSign(getangle(x*10+5,y*10-5),mid[0])];
    if(angles[0]>mid[1] && angles[1]>mid[1] && angles[2]>mid[1] && angles[3]>mid[1]) continue;
    if(angles[0]<-mid[1] && angles[1]<-mid[1] && angles[2]<-mid[1] && angles[3]<-mid[1]) continue;
    if(Math.abs(angles[0])>Math.PI/2 && Math.abs(angles[1])>Math.PI/2 && Math.abs(angles[2])>Math.PI/2 && Math.abs(angles[3])>Math.PI/2) continue;
    m[i]=(m[i]|2); // Only mark as done if we actually render it. This let's us recover from inconsistencies.
    // Draw floor
    var nlangle=langle;
    var nrangle=rangle;
    var front = exists_dir(x,y,m,w,dir); // Is there a wall in front of us?
    var lwall = exists_dir(x,y,m,w,dir-1); // To the left?
    var rwall = exists_dir(x,y,m,w,dir+1); // To the right?
    
    if(!front || !lwall) { //left wall or front wall check
      nlangle=getangle_dir(x*10,y*10,dir,dir-1);
      if(getAngleDist(mid[0],nlangle)>mid[1]) nlangle=(!front)?rangle:langle; 
    }
    if(!lwall) { // This corner is only checked for left walls
      nlangle=getangle_dir(x*10,y*10,dir-2,dir-1);
      if(getAngleDist(mid[0],nlangle)>mid[1]) nlangle=langle; 
    }
    if(!front || !rwall) { //right wall or front wall check
      nrangle=getangle_dir(x*10,y*10,dir,dir+1);
      if(getAngleDist(mid[0],nrangle)>mid[1]) nrangle=(!front)?langle:rangle;
    }
    if(!rwall) { // Only right wall
      nrangle=getangle_dir(x*10,y*10,dir-2,dir+1);
      if(getAngleDist(mid[0],nrangle)>mid[1]) nrangle=rangle;
    }
    
    for(var j = dir-1; j <= dir+1; ++j) {
      var k = (j+4)%4; // get proper direction
      var nx=x+getdx(k);
      var ny=y+getdy(k);
      if(!exists(nx,ny,m,w)) { // We ran off the edge of the map or hit a nonexistent block
        // Draw wall
      } else {
        if(j==dir-1) { roomq.push(nx,ny,langle,(!front)?nlangle:nrangle,k); }
        else if(j==dir) { roomq.push(nx,ny,nlangle,nrangle,k); }
        else { roomq.push(nx,ny,(!front)?nrangle:nlangle,rangle,k); }
      }
    }      
  }
}
var reversefill = function(x,y,m,w) {
  var i = x + (y*w);
  if(x<0 || x>=w || i>=m.length) return;
  if((m[i]&2)==2)
  {
    m[i]=(m[i]&(~2));
    reversefill(x+1,y,m,w);
    reversefill(x,y+1,m,w);
    reversefill(x-1,y,m,w);
    reversefill(x,y-1,m,w);
  }
}

Straddle Error
First, we push our 4 neighboring tiles, provided they exist, on to the queue, seeding them with appropriate directions that radiate outward from our starting point. Then, we start running through the queue, and do an initial check to see if we already dealt with this tile. If not, we check to make sure that the left-angle is really less than the right-angle, and then go into standard frustum culling. In order to figure out if a tile is within our view, we need to ensure that the entire tile is either to the right or to the left of our angular viewing frustum. This means that all 4 corners must have angles outside of our frustum, and they must all be on the same side. If they are not on the same side, then the cone could have passed through the center.

This is what the first two if statements do. The problem is that a tile directly behind us will also be considered straddling the angular range, because our distance function wraps at 180°. To solve this, we have a third if statement that throws away everything that is more than 90° (or $$\frac{\pi}{2}$$) from the center of our frustum. This means the maximum horizontal field-of-view is 180°. Note that this culling is independent of the algorithm, you'd be using this same logic if you were only doing simple frustum culling.


Split Error
If the tile survived the culling function, we mark it as visited. It's important that we only mark a node as visited after we have decided to render it because of a very nasty edge-case that can happen during the breadth-first traversal. It's possible for a tile that is rendered on one side of a split frustum to reach a visible tile on the other side of the split, and erronously mark it as invisible, using it's own frustum. The tile actually is visible, but only to the frustum on the other side of the split.

Then, we check the appropriate corners and assign langle and rangle appropriately. The neighbors are iterated and new values passed as needed. Once this phase of the algorithm is completed, we call reversefill(), starting on the same tile we called drawmap() with. This will use the standard fill algorithm to reset the "visited" marks on the map. By doing this, we avoid having to set all 2500 nodes of a 50x50 map to 0. The algorithm is now complete.

Because this algorithm runs in $$O(n)$$ time and is exact, it is asymptotically optimal. The problem is that it is only optimal in a single-threaded sense. The breadth-first iteration technique allows us to run each individual level concurrently, but this will only reduce the worst-case complexity from $$O(mn)$$ to $$O(\max(m,n))$$. 50 is a lot better than 2500, but this is only for a very, very large room. If we're dealing with a hallway, the concurrent performance would be identical to the single-threaded performance!

Consequently, while this is useful for tight corridors, the algorithm itself isn't really that useful outside of the game's narrow domain. Once the rooms start getting large enough, you're probably better off brute-forcing most of it and using approximations. However, the algorithm is exact, which is very important for calculating Enemy Line-of-Sight. So if you're dealing with a grid-based game and need to figure out if an enemy has a direct line of sight, this technique allows you to make a precise calculation, since it only hits tiles that are visible, and no others. This gives it an advantage over naïve raytracing, which requires many rays and is prone to giving false-negatives when dealing with narrow hallways.

Unfortunately, it still breaks when you try to make it work with FoV's greater than 180°, so unless you split them up into 90° chunks, it's pretty useless for things like lighting. I'm probably not the first to come up with the algorithm, either; someone probably invented this in their garage in 1982. Oh well.