Image-Based Lighting Workflows
Originally written by Spooks on the TDM forum:
https://forums.thedarkmod.com/index.php?/topic/19803-tutorial-image-based-lighting-workflows-for-tdm/
I (datiswous) copied over the original formatting, but it might change somewhat because of (possibly better) organizational features of the wiki.
Hello, everyone! In this multi-part, comprehensive tutorial I will introduce you to a new light type that has been available in The Dark Mod since version 2.06, what it does, why you would want to use it and how to implement it in your Fan Missions.
This tutorial is aimed at the intermediate mapper. Explanations of how to use DarkRadiant, write material files, etc. are outside of its scope. I will, however, aim to be thorough and explain the relevant concepts comprehensively. Let us begin by delineating the sections of the tutorial:
Part 1 will walk you through four, distinct ways to add ambient light to a scene, the last way using irradiance environment maps (or IEMs). Lighting a scene with an IEM is considered image-based lighting. Explaining this concept is not in the scope of this tutorial; rather, we will compare and contrast our currently available methods with this new one. If you already understand the benefits IBL confers, you may consider this introductory section superfluous.
Part 2 will review the current state of cubemap lights in TDM, brief you on capturing an environment cubemap inside TDM and note limitations you may run into. Three cubemap filtering applications will be introduced and reviewed.
Part 3 will go into further detail of the types of inputs and outputs required by each program and give a walkthrough of the simplest way to get an irradiance map working in-game.
Part 4 will guide you through two additional, different workflows of how to convert your cubemap to an irradiance map and unstitch it back to the six separate image files that the engine needs.
Part 5 will conclude the tutorial with some considerations as to the scalability of the methods hitherto explained and will enumerate some good practices in creating IEMs. Typical scenes will be considered. Essential links and resources will be posted here and a succinct list of the steps and tools needed for each workflow will be summarized, for quick reference.
Without further ado, let us begin.
Part 1
Imagine the scene. You’ve just made a great environment for your map, you’ve got your geometry exactly how you want it… but there’s a problem. Nobody can appreciate your efforts if they can’t see anything!
(click on the image for a bigger view)
[Fig. 1] This will be the test scene for the rest of our tutorial — I would tell you to “get acquainted with it” but it’s rather hard to, at the moment.
The Dark Mod is a game where the interplay between light and shadow is of great importance. Placing lights is designing gameplay. In this example scene, a corridor with two windows, I have decided to place 3 lights for the player to stealth his way around. Two lights from the windows streak down across the floor and a third, placeholder light for a fixture later to be added, is shining behind me, at one end of the corridor.
Strictly speaking, this is sufficient for gameplay in my case. It is plainly obvious, however, that the scene looks bad, incomplete. “Gameplay” lights aside, the rest of the environment is pitch black. This is undesirable for two reasons.
1. It looks wrong
In real life, lights bounce off surfaces and diffuse in all directions. This diffused, omni-directional lighting is called ambient lighting and its emitment can be termed irradiance. You may contrast this with directional lighting radiating from a point, which is called point lighting and its emitment — radiance. One can argue that ambient lighting sells the realism of a scene. Be that as it may, suppose we disregard scary, real-life optics and set concerns of “realism” aside…
2. It’s bad gameplay
Being in darkness is a positive for the player avatar, but looking at darkness is a negative for the player, themselves. They need to differentiate obstacles and objects in the environment to move their avatar. Our current light level makes the scene illegible. The eye strain involved in reading the environment in these light conditions may well give your player a headache, figurative and literal, and greatly distract them from enjoying your level.
This tutorial assumes you use DarkRadiant or are at least aware of idtech4’s light types. From my earlier explanation, you can see the parallels between the real life point/ambient light dichotomy and the aptly named “point” and “ambient” light types that you can use in the editor. For further review, you can consult other places in this wiki.
Seeing as how there is a danger in confusing the terms here, I will hereafter refer to real life ambient light as “irradiant light”, to differentiate it from the TDM ambient lights, which are our engine’s practical implementation of the optical phenomenon. A similar distinction between “radiant light” and point lights will be made for the same reason.
Back to our problem. Knowing, now, that most all your scenes should have irradiant light in addition to radiant light, let’s try (and fail, instructionally) to fix up our gloomy corridor.
Atdm:ambient_world
[Fig. 2] The easiest and ugliest solution: ambient lights.
Atdm:ambient_world is a game entity that is basically an ambient light with no falloff, modifiable by the location system. One of the first things we all do when starting a new map is putting an ambient_world in it. In the above image, the darkness problem is solved by raising the ambient light level using ambient_world (or via an info_location entity).
Practically every Dark Mod mission solves its darkness problem1 like this. Entirely relying on the global ambient light, however, is far from ideal and I argue that it solves neither of our two, aforementioned problems. Ambient_world provides irradiant light and you may further modulate its color and brightness per location. However, said color and brightness are constant across the entire scene. This is neither realistic, nor does it reduce eye strain. It only makes the scene marginally more legible. Let’s abandon this uniform lighting approach and try a different solution that’s more scene-specific.
Non uniform
[Fig. 3] Non-uniform, but has unintended consequences.
Our global ambient now down to a negligible level, the next logical approach would be hand-placed ambient lights with falloff, like ambient_biground. Two are placed here, supplementing our window point lights. Combining ambient and point lights may not be standard TDM practice, but multiple idtech4 tutorials extol the virtues of this method. I, myself, have used it in King of Diamonds. For instance, in the Parkins residence, the red room with the fireplace has ambient lights coupled to both the electric light and the fire flame. They color the shadows and enrich the scene, and they get toggled alongside their parent (point) lights, whenever they change state (extinguished/relit).
This is markedly better than before, but to be honest anything is, and you may notice some unintended side-effects. The AI I’ve placed in the middle of the ambient light’s volume gets omnidirectionally illuminated far more than any of the walls, by virtue of how light projection in the engine works. Moving the ambient lights’ centers closer to the windows would alleviate this, but would introduce another issue — the wall would get lit on the other side as well. Ambient lights don’t cast shadows, meaning they go through walls. You could solve this by creating custom ambient light projection textures, but at this point we are three ad hocs in and this is getting needlessly complicated. I concede that this method has limited use cases but illuminating big spaces that AI can move through, like our corridor, isn’t one of them. Let’s move on.
More directional
[Fig. 4] More directional, but looks off.
I have personally been using this method in my WIP maps a lot. For development (vs. release), I even recommend it. A point light instead of an ambient light is used here. The texture is either “biground1” or “defaultpointlight” (the latter here). The light does not cast shadows, and its light origin is set at one side of the corridor, illuminating it at an angle.
This solves the problem of omnidirectional illumination for props or AI in the middle of the light volume, you can now see that the AI is lit from the back rather than from all sides. In addition, the point light provides that which the ambient one cannot, namely specular and normal interaction, two very important features that help our players read the environment better. This is about as good as you can get but there are still some niggling problems.
The scene still looks too monochromatic and dark. From experience, I can tell you that this method looks good in certain scenes, but this is clearly not one of them. Sure, we can use two, non-shadowcasting point lights instead of one, aligned to our windows like in the previous example, we can even artfully combine local and global ambient lights to furnish the scene further, but by this point we will have multiple light entities placed, which is unwieldy to work with and possibly detrimental to performance.
Another problem is that a point light’s movable light origin helps combat ambient omnidirectionality, but its projection texture still illuminates things the strongest in the middle of its volume. I have made multiple experiments with editing the Z-projection falloff texture of these lights and the results have all left me unsatisfied. It just does not look right.
A final, more intellectual criticism against this method is that this does not, in a technical sense, supply irradiant light. Nothing here is diffuse, this is just radiant light pretending the best it can.
Irradiance map
[Fig. 5] The irradiance map method provides the best looking solution to imbuing your scene with an ambient glow.
This is the corridor lit with irradiance map lights, a lighting method introduced in The Dark Mod 2.06. Note the subtle gradients on the left wall and the bounced, orange light on the right column. Note the agreeable light on the AI. Comparing the previous methods and this, it is plainly obvious that an irradiance environment map looks the most realistic and defines the environment far better than any of the other solutions.
Why exactly does this image look better than the others? You can inform yourself on image-based lighting and the nature of diffuse irradiance, but images speak louder than words. As you can see, the fact of the matter is that the effect, subtle as it may be, substantially improves the realism of the scene, at least compared to the methods previously available to us.
Procuring irradiance environment maps for use in lighting your level will hereafter be the chief subject of this tutorial. The next part will review environment cubemap capture in TDM, the makeIrradiance keyword and three external applications that you can use to convert a TDM cubemap into an irradiance map.
1 “ Note that the color buffer is cleared to black: Doom3 world is naturally pitch black since there is no "ambient" light: In order to be visible a surface/polygon must interact. with a light. This explains why Doom3 was so dark ! “ source
