Image-Based Lighting Workflows

This article is currently a work in process

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!

[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 our 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.