Hello! In this guide, you will learn how to heighten the graphical quality to the maximum without mods or Reshade. I will explain how to do this trick, then I will explain what things do, with images and beginner-friendly non-techie explanations to hopefully understand what some settings do, and possibly allow you to mess with some settings yourself. Enjoy!
(Note: the game already looks great on max settings, but these settings improve the finer details)

First, you will need a text editor. If you are on Windows, Notepad is all you really need. You can use whatever text editor you prefer though. I don't know what text editor comes with Mac or Linux, but use what comes with the OS, but again you can use what you like, as long you can paste and edit large amounts of text.
Now, This guide isn't for the weakest of PC's, but it is lighter than you might expect. A good indicator that you can use this mod is if you (most often or always) reach the Source Engine frame limit, which is 300 FPS on maxed settings (shown below), which doesn't take too much power, but the more often you reach 300, the better. You don't have to turn the AA to max when you're done, but to test if you can run the super high settings, I recommend using 16x Q CSAA. When you finish the guide, you can change the AA to 8x MSAA with little to no quality loss.



To show how little power it takes to reach 300 often, below is my rig.
CPU: Intel Core i7-3770K @ 3.5GHz
Graphics Card: NVIDIA GeForce 660 2GB
RAM: 16GB DDR3
HDD
With the highest quality graphics on (the one that I'm going to show in this guide), I average around 200, with some peaks at 300, and in some areas I dip below 60, but it's rare (one place I usually dip is the giant underground vault door that leads to old Aperture (mid-game spoiler)).

If you have both of the requirements, then I can move onto the actual contents of the guide.

So first, you need to go to Portal 2's Steam library page. Then you need to press the gear button and click properties. (shown below)



Now, you should see a new window appear. In this new window, click the Local Files tab, then click the Browse Local Files button. (shown below)



This should open File Explorer. In File Explorer, go to portal2 > cfg. Look at the top of File Explorer and see if the end of the list looks like below.



Now, you should see a bunch of files for things like chapters, GPU levels, CPU levels, memory levels and joystick configuration files. Right click any empty area (biggest empty area is to the right), the hover over new, and click anything other than a folder (image, text file, etc.). The name doesn't matter right now, so just click off it. Click on the new file, and press F2 on the keyboard to rename the file. This file needs to be called autoexec.cfg, otherwise this won't work.

Now open this file, and paste this in.

Press ctrl+s to save and you're done! In the next section, I'll give more context to what these console commands do.

Okay, now I'll explain what some of these console commands do.

FYI, the double-slashes (//) are comments. In programming, comments are areas where the text written is not executed, thus allowing anything to be written. There are two types of comments, single line (//) and multi-line (/* */). Single line affects ahead of it in the same line. Multi-line comments affect whatever is enclosed by it. Most programming languages have comments like this, but some like HTML have weird comments. In layman's terms, comments allow human speak.

Also, if you want to mess around with the values in-game, you'll need to be able to access the developer console, which you can enable in controls in the normal in-game settings menu. Since the autoexec.cfg file executes when the game starts, if you change it and save it mid-game it won't do anything until the game is closed and opened again.

sv_cheats 1
This allows some commands not normally available. It's what it sounds like.

---

mat_software_aa_blur_one_pixel_lines, mat_software_aa_edge_threshold)
mat_software_aa_blur_one_pixel_lines is how much a one-pixel line is blurred. I don't notice a difference changing the value, but I commonly see it set to 0.5 so I follow suit. mat_software_aa_edge_threshold is how big a line/edge has to be in pixels (including less than a pixel) to be counted as a line/edge. The smaller this value is, the softer the image is. I found 0.6 to be a good value.

---

mat_fastspecular 1 & mat_specular 1
This makes the specularity[en.wikipedia.org] look better with a trade-off of a performance hit of about 10 FPS.

mat_forceaniso 16
Anisotropic Filtering has to do with textures. In most cases, it is best to have it on because what Anisotropic Filtering does is that it makes textures look less blurry at steeper angles. It doesn't fix all blurring, but it helps a lot. Below is an example scene showing Anisotropic Filtering off and at 16x.

Anisotropic Filtering Off


Anisotropic Filtering 16x


Anisotropic Filtering is one of the options in the advanced video settings, it is just here to verify that it is set to 16x. You can remove it for less clutter if you like.

mat_compressedtextures 1
This enables texture compression. This is for optimization purposes, and it doesn't affect texture quality.

---

r_shadowmaxrendered 256
This sets the maximum amount of drawn shadows. Lower this if your performance is really bad, but my setup runs the game with this fine.

r_shadowrendertotexture 1
This toggles RTT shadows. By default it should be set to 1.

r_shadows_on_renderables_enable 1
This enables RTT shadows on renderables. I can't find what a renderable is, but on Wikitionairy it says a renderable is "any object that can be rendered to the display". In scenes I test, it doesn't seem to do anything, but it might add shadows to things that normally wouldn't, so I just keep it on.

r_portal_use_dlights 1, portal_viewmodel_use_dlight 1, portal_transmit_light 1
These make the portals have a nice glow to them. Sadly, if you put one portal in a bright area and one in a dark area, the light from the bright portal doesn't carry over. The portals do glow a lot though! When you first shoot the portal and the opening/growing animation plays, you can see the dynamic light and it doesn't fade in. It is only visible for a brief moment though, so it should be fine. Below, I have an example scene with it on and off.

r_portal_use_dlights 0 (default)


r_portal_use_dlights 1


When changing this value mid-game, you need to make the game load again for it to take affect. To do this, either die, go into a loading zone, or type reload into the developer console and press enter.

r_maxdlights 256
This sets the maximum amount of dynamic lights.

r_avglight 3
I'm not too sure what this one does, but I saw someone say it "puts more lighting on objects". I tested it, and for me it doesn't do anything. If you find any differences, tell me in the comments. If you find any differences and this setting isn't the best options, tell me in the comments.

mat_reducefillrate 0
Oddly enough makes lighting look better.

---

mat_postprocess_x 8, mat_postprocess_y 8
I found online that people say these improve the look of the post-processing, but I don't see any differences. Let me know in the comments if you find what it does and if there are values that make the game look even better.

mat_hdr_level 2
This doesn't have anything to do with HDR, it is just the quality of "HDR emulation effects". What I mean by that is pretty much just bloom.

mat_motion_blur_strength 2
This should be self explanatory. Change it how you like.

mat_bloomscale 2
mat_bloomscale is self-explanatory, it is the amount of bloom shown on screen. Turn it up for more bloom, turn it down for less.

---

r_portal_stencil_depth 3
This changes how many portal recursions can happen. For normal maps, I'd say 3 (the default) is fine, but for community maps that use world portals, turning this up will make the portals more realistic (keep in mind this won't fix the glitches of too many portals being on screen, the game was designed for a maximum of 4 portals (because co-op mode)).

---

r_waterdrawreflection 1, r_waterdrawrefraction 1
These should be self explanatory. These should be on by default, so you can remove them if you want less clutter.

r_waterforcereflectentities 1
This makes water reflect everything.

---

r_rainradius 4096
This sets how far rain is rendered.

r_rainsplashpercentage 100
This sets how often (in percent) a rain drop makes a splash.

r_3dsky 1
This enables usage of 3D skyboxes. 3D skyboxes is just a camera's view blown up and projected as a skybox. 3D skyboxes are used for distant, inaccessible scenery This is on by default, so you can remove it if you want less clutter.

jpeg_quality 100
This makes it so that screenshots made from pressing F4 are at maximum quality, with no compression artifacts (but might not look great due to being a jpeg).

That is all that I have in this guide!

If you find any console commands that improve the look of the game further, or a certain setting looks better, tell me and other people in the comments!