Sunday, November 29, 2009

Where Did All of Those Lights Come From?

Javier posted a video of his CRJ on the dev list today. I have not tried the plane, but there is no question from the video that it looks really good. What makes the video look so nice is the careful management of light. Part of this comes from careful modeling in 3-d, and part of it comes from maxing out all of X-Plane's options for light.

But...what are the options for light on an airplane? I don't know what Javier has done in this case, but I can give you a laundry list of ways to get lighting effects into X-Plane.

Model In 3-D

To really have convincing light, the first thing you have to do is model in 3-d. There is no substitute - for lighting to look convincing, X-Plane needs to know the true shape of the exterior and interior of the plane, so that all light sources are directionally correct. X-Plane has a very large capacity for OBJ triangles, so when working in a tight space like the cockpit, use them wisely and the cockpit will look good under a range of conditions.

You can augment this with normal maps in 940. Normal maps may or may not be useful for bumpiness, but they also allow you to control the shininess on a per-pixel basis. By carefully controlling the shininess of various surfaces in synchronization with the base texture, you can get specular hilights where they are expected.

The 2-D Panel

First, if you want good lighting, you need to use panel regions. When you use a panel texture in a 3-d cockpit with ATTR_cockpit, X-Plane simply provides a texture that exactly matches the 2-d cockpit. Since the lighting on the 2-d cockpit is not directional, this is going to look wrong.

When you use ATTR_cockpit_region, X-Plane uses new next-gen lighting calculations, and builds a daytime panel texture and a separate emissive panel texture. These are combined taking into account all 3-d lighting (the sun and cockpit interior lights - see below). The result will be correct lighting in all cases.

Even if you don't need more than one region and havea simple 1024x1024 or 2048x1024 3-d panel, use ATTR_cockpit_region - you'll need it for high quality lighting.

The 2-d panel provides a shadow map and gray-scale illumination masks. Don't use them for 3-d work! The 2-d "global lighting" masks are designed for the 2-d case only. They are optimized to run on minimal hardware. They don't provide the fidelity for high quality 3-d lighting - they can have artifacts with overlays, there is latency in applying them, and they eat VRAM like you wouldn't believe. I strongly recommend against using them as a source of lighting for a 3-d cockpit.

To put this another way, you really want to have all global illumination effects be applied "in 3-d", so that the relative position of 3-d surfaces is taken into account. You can't do this with the 2d masks.

The 2-d panel lets you specify a lighting model for every overlay of every instrument - either:
  • "Mechanical" or "Swapped" - this basically means the instrument provides no light of its own - it just reflects light from external sources.
  • "Back-Lit" or "Additive" - this means the instrument has two textures. The non-lit texture reflects external light, and the lit texture glows on its own.
  • "Glass" - the instrument is strictly emissive.
You can use 2-d overlays not only for instruments but also to create the lighting effect within instruments, e.g. the back-lighting on a steam gauge's markings, or the back-lighting on traced labels for an overhead panel.

2-d overlays take their lighting levels from one of sixteen "instrument brightness" rheostats. You can carefully allocate these 16 rheostats to provide independent lighting for various parts of the panel.

The 3-d Cockpit

The 3-d cockpit allows you to specify 3 omni or directional lights. These can be placed anywhere in the plane, affect all interior objects, and can be tinted and controlled by any dataref. Use them carefully - what they give you is a real sense of "depth". In particular, the 3-d lights are applied after animation. If a part of the cockpit moves from outside the light to into the light, the moving mesh will correctly change illumination. This is something you cannot do with pre-baked lighting (e.g. a _LIT texture).

Finally, ATTR_light_level is the secret weapon of lighting. ATTR_light_level lets you manually control the brightness of _LIT texture for a given submesh within an OBJ. There are a lot of tricks you can do with this:
  • If you know how to pre-render lighting, you can pre-render the glow from a light onto your object into your _LIT texture, and then tie the brightness of the _LIT texture to a dataref. The result will be the appearance of a glow on your 3-d mesh as the light brightens. Because the lighting effect is pre-calculated, you can render an effect that is very high quality.
  • You can create back-lit instruments in 3-d and link the _LIT texture to an instrument brightness knob.
  • You can create illumination effects on the aircraft fuselage and tie them to the brightness of a beacon or strobe.
There are two limitations of ATTR_light_level to be aware of:
  1. Any given triangle in your mesh can only be part of a single ATTR_light_level group. So you can't have multiple lighting effects on the same part of a mesh. Plan your mesh carefully to avoid conflicts. (Example: you can't have a glow on the tail that is white for strobes and red for beacons - you can only bake one glow into your _LIT texture.)
  2. ATTR_light_level is not available on the panel texture. For the panel texture, use instrument brightness to control the brightness of the various instruments.
I have a sample plane that demonstrate a few of these tricks; I will try to post it on the wiki over the next few days.


Anonymous said...

My eyes glass over and my brain blanks out when reading about lighting, as they do whenever I scan the newfangled prop datarefs. I'll wait for the example. As for jr, I'm not surprised. He's a true artist.

Chris Palmer said...

This is why I respect developers. This is WAY too right brained for me.

Wayne Conrad said...

An outstanding mile-high view of lighting. Ben, you can always get work writing documentation. No, I don't mean that as an insult. Please don't ban me.