Do this at your own risk, I've dabbled in downsampling for almost two years and I've never seen, read or heard anyone have "anything break" by using downsampling.  Worst thing that has happened to me while doing it has been a freeze or two during the process.  Hopefully it's easy enough to follow and should work for people with modern nvidia gpus and monitors.  If it does feel free to share the settings that worked ( or those that didn't work for that matter ) along with your own setup.  I won't take any responsibility for the tiny chance of your pc coming to life and strangling you while you sleep poltergeist style. 

What you need

An Nvidia GPU ( use this for downsampling on AMD cards ), Windows 7 ( SOME people have issues with this method on Windows 8 ) and 10 minutes worth of patience.  From my own experience going from 4xx to 5xx and 6xx series the downsampling compatibility has become better.  Everything written here has been tested and has worked for me on the following setup :

Dell u2312hm 1920x1080@60 hz monitor -> Single link DVI-D to HDMI -> Gigabyte GTX 670, 310.64 Gefore Drivers.  Downsampling can be very finicky across different driver versions but apparently any recent set of Geforce drivers should be compatible, version 300.xx and newer. 

What is downsampling

Downsampling, also called OGSSAA : Ordered Grid SuperSampling AntiAliasing, is in this case the concept of rendering the game at a much higher, custom made, resolution than your monitor is capable of displaying and subsequently having the GPU rescale the image ( as in downsample ) to that of your monitor's native resolution to which the end result is a "cleaner", sharper and less aliased image. 

Why

King of all AA methods, affects virtually every kind of aliasing but it's also very taxing which is why we have other less demanding alternatives, SMAA, MLAA, FXAA etc.  Sadly we are in a position where we have pc-games shipping with "AA on/off" options or have AA implementations that leads much to be desired making people use other forms of externally forced AA. 

We're also at a point in this "generation" where the discrepancy between pc and console hardware is bigger than ever giving pc-users a much larger performance overhead.  What I mean by that is that there are a lot of people out there with hardware that can run many/most new games and virtually any older game at higher resolutions than 1920x1080 while still maintaining an acceptable framerate even though 1920x1080 is still the standard resolution of most of our monitors. 

Using downsampling you almost never will have to worry about a game's image quality being ruined by lack of AA since it works in almost any 3D game out there.  Compared to other types of AA solutions like SMAA/FXAA-injectors or "Nvidia Inspector AA" you won't need to fiddle with settings or files at all, when it comes to downsampling via custom resolution once you got it up and running it's just a matter of using that resolution in any game and presto you got yourself some AA brewing.  It is worth pointing at that downsampling still works great with those aforementioned AA methods ( injectors, inspector etc ) and whatever kind of AA solution/s the games you want to play offer, you can mix,match and stack AA to your heart's content. 

How

There are guides out there that go much more in depth in the process, one of them is this thread/guide at Guru3D showing the step by step guide on how to reach your own maximum custom resolution.  The problem is that following those guides myself I've noticed that the end result is almost random at times, giving me different max resolution depending on things like drivers rather than which settings I ended up using.  In the end, the best method I've found is to simply straight up copy other people's settings ( this is by no means a guarantee that it'll work but hopefully it removes a lot of the hassles of trying to stumble across your own max resolution one pixel at a time ) and hope for the best. 

1) Right click your desktop and start "Nvidia Control Panel" -> go to "Adjust desktop size and position" under "Display". 

Use these settings ; Scaling mode : Aspect ratio, Perform Scaling on : GPU, Check "Override the scaling mode set by games and programs"

3)At this point, for the "lower" resolutions, people might not even need to do further tweaking than to simply write their desired resolution in the following window.  Simply replace the "1920" with "2560" and "1080" with "1440" for a desired resolution of 2560x1440.  Leave timings to automatic

Press test and one of the following will happen :

- You get an image, everything works, press save and you got yourself your first downsampling resolution!  Just go in-game and change your resolution settings accordingly and you're done!  Got to step 4. 

- You get a black ( sometimes flashing red ) screen OR your monitor will tell you the current resolution is not supported : press ESC once or twice and wait for the screen to revert back to normal.  If even 2560x1440 didn't work for you then your setup might now jive well with downsampling.  Still go to Step 5 and give that a try. 

- You get a black screen, monitor goes into power save mode : your shit crashed you gotta manually reboot your pc and the settings will revert back to normal once you're in windows again.  If even 2560x1440 didn't work for you then your setup might now jive well with downsampling.  Still go to Step 5 and give that a try. 

4) If everything worked, you can go higher and try the following two resolutions that are relatively common and tend to work for people who downsample, again without having to resort to manual timings. [ All of the mentioned resolutions between
2560x1440 and 3840x2160 are all abitrary 16:9 resolutions but try to use the ones I've tried and know work for myself and a handful of others on GAF.] 

2880 x 1620
3200 x 1800

If the above two resolutions won't work at 60 Hz try 59 Hz. 

OK.  Following the different guides that exist out there, my own max resolution was 3200x1800 for a long stretch of time.  That was until I started copying other people's settings which showed me how arbitrary the results could be.  Using person A's settings didn't work but somehow using person B's which used an even higher resolution did without changing anything other than the resolution.  But you want an even higher resolution so let's see what we can reach! 

5) 3600 x 2025@60 Hz

Use these settings.  Set timings to manual, copy every setting and pray to your deity of preference.  Press test and see if it works.  This is generally the highest resolution I use when playing games as it is the one where I downsample yet still have it running
at 60 Hz.  It's a good compromise between having a very high resolution and still being able to achieve 60 fps in the games that your pc has enough juice to do so, whereas if you try a lower refresh rate your framerate will obviously be capped accordingly. 

6) Are you still with me?  Using the timing settings from the previous resolution ( 3600 x 2025 ) you can go for broke and try 3840x2160.  Most people will probably get somekind of black screen at 60 Hz unless you are on some very specific setup.  What you can do here is that you lower your refresh rate, the refresh rate variable that is at the top of the window not the bottom one.  Some have had luck with 30 Hz even if 59 Hz,58 Hz,57 Hz etc didn't work.  This resolution is outside the realm of my setup regardless of the settings I use so I have yet to be able to have a 3840x2160 custom resolution. 

Performance

What kind of performance to expect?  Roughly speaking your performance will be 1/N, where N is supersample value or "how many times more pixels you're rendering". 

Example : When downsampling from 3840x2160 to 1920x1080, which is technically 4XSSAA, you're rendering 4 times as many pixels at 3840x2160 as you are at 1920x1080, so you'll be getting 1/4th performace.  E.g, if you have 120 fps at 1920x1080 you'll get 30 fps at 3840x2160 in game X ( barring vram limitations, other bottle necks and everything in between ). 

Issues

Doesn't appear to be working with laptops and/or "m" line laptop gpus. 

For some people using refresh rates that aren't 30 Hz or 60 Hz exact, the custom resolution won't show up ingame OR selecting the resolution in game will make it crash. 

Bandwidth issues - Depending on the way you've hooked up your monitor/gpu you have different maximum bandwidth that can come into play.  Interestingly enough, using my monitors dvi output with a single-link DVI cable gave me better results compared to using the displayport ouput which has a higher bandwidth. 

Depending on your general preferences you might notice an increase in input lag as you downsample from a higher resolution.  Some people claim they don't notice any kind of increase in inputlag whereas me and my irl buddy and fellow gaffer Tyrantguardian ( both fighting game nerds ) experience a noticeable increase in inputlag in certain games where such a thing is easily noticed. 

You can't downsample in games where you use windowed mode unless your desktop is set to the high resolution you want to downsample FROM. 

120 Hz and 3D-gaming : due to current bandwidth limitations it isn't feasible to downsample down to a 1920x1080 resolution @ 120 Hz. 

Screenshots

I won't bother with posting a sea of screenshots, for that I'll redirect you to the 2013 High-Res PC Screenshot Thread where you can see how good games can look when using downsampling.  I will however post a couple of cropped comparison pics from Dota 2.  It's no technical showcase game by any means BUT I do think one thing comes across and that is the visual difference between downsampling and "just" running the game at max settings.  This can be quite striking considering you already had the game running at what you thought was the highest visual fidelity.  Again these aren't meant to WOW anyone but you will notice that everything that is dependent on resolution will look tangibly better ( certain lighting effects, texture detail, shadow detail, model detail and of course a sharper, less blurry, less aliased image ). 

Downsampling, also called OGSSAA : Ordered Grid SuperSampling AntiAliasing, is in this case the concept of rendering the game at a much higher, custom made, resolution than your monitor is capable of displaying and subsequently having the GPU rescale the image ( as in downsample ) to that of your monitor's native resolution to which the end result is a "cleaner", sharper and less aliased image. 

nice, all i can add is that sometimes a game's own AA wont work if you downsample from high resolutions for whatever reason

but it shouldnt matter much since you're already ds

Downsampling is basically "stupid" AA.  You just choose to render everything at a higher resolution, not just the edges of polygons, like with Multisampling.  The method described in the OP is equivalent to FSAA or full scene AA.  Great OP btw. 

Downsampling is basically "stupid" AA.  You just choose to render everything at a higher resolution. 

Yup this is correct, you're just bruteforcing everything which is why it's the least efficient form of AA, although as mentioned nowadays feels like you'd be lucky if multiplatform games offer anything other than postprocess AA. 

Things don't get real until you downsample from 4k while using 4-8x SGSSAA. 

I guess Corky didn't think the world was ready. 

Binary Domain was one of the few games that I could achieve 8xSGSSAA while keeping it at 60fps locked.  What other games would you recommend to try it out that doesn't get framerate drops? 

OGSSAA is fantastic.  It basically works with almost any game and you don't need to change AA settings in inspector anymore.  I used Decyx's guide back then where it taught me how to create a custom driver for my monitor.  Cool stuff. 

Cheer for this, nice and easy to follow - too often PC tweak guides go into tons of detail

Question for you - would you consider dropping detail from max to eg 'high' if it meant the difference between downsamplimg at a playable framerate or not? 

Seems like this should be considered as a specific image quality improvement in its own right, and could perhaps offer a bigger improvement to your eye than some of the more esoteric effects like SSAO etc. 

SGSSAA is something that's a step below, it's less taxing but gives a slightly fuzzier/softer picture compared to OGSSAA, has to be enabled and tweaked on a game by game basis as far as I know and isn't compatible with all games like OGSSAA is. 

Maybe TXAA will be our AA-savior. 

Originally Posted by mrklaw

Question for you - would you consider dropping detail from max to eg 'high' if it meant the difference between downsamplimg at a playable framerate or not? 

Seems like this should be considered as a specific image quality improvement in its own right, and could perhaps offer a bigger improvement to your eye than some of the more esoteric effects like SSAO etc. 

Oh definitely, I always tend to do this if I'm straddling the 60 fps line too much and OCing the GPU won't help.  Incidentally SSAO is actually the first thing I bump down a notch if I'm downsampling but can't keep 60.  For what it's worth things like SSAO also look better when downsampling considering they correlate to the resolution you use, notice the AO in those Dirt 3 pics, the picture with downsampling has the "most clean/accurate" AO. 

Cheer for this, nice and easy to follow - too often PC tweak guides go into tons of detail

Question for you - would you consider dropping detail from max to eg 'high' if it meant the difference between downsamplimg at a playable framerate or not? 

Seems like this should be considered as a specific image quality improvement in its own right, and could perhaps offer a bigger improvement to your eye than some of the more esoteric effects like SSAO etc. 

I sometimes drop effects that don't impact the image quality too much, like AO or something, as usually the difference between High and Normal is usually how dark it is and the size of the shitty halo it produces around your character.