Lets make Lightmass EPIC (and understandable)

[rafareis123]

I did some quick tests

* I used "Static Lighting Level" 0.07 and "Indirect Lighting Smoothness 1.3"
* It's possible to achieve the same lighting quality from vray
* Kola's work it's pretty much the same as Alex Roman's vray works. The secret is in the
shaders and the contrast. In the lighting pass you will definitely spot a lot of lighting errors. Even
in Alex Roman's work, you can see lighting erros from Irrmap + LC solution. Darkness is our best friend
in this case.
* Here is a nice article for understanding better the photomapping process : http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

Have a nice study!

[Robbie222]

My two cents

99% of the time I notice that if you go over a value of 2-3 on the "Indirect light intensity" ( on the main sun light I mean ) you'll have those light leaks all over.
I usually use, to boost up the intensity of the interiors, a skylight with a limited amount of intensity ( 0.2/0.5 ) and 1-2 of Indirect lighting.

Then bounce cards all over the place.

@DanielW: +1 Raghu, to be able to build the lighting only for certain object will be a godsend, since if you're working on different environments and you just want to test part of the scene it'll be great to process only a portion of that or if you move some objects just reuild the lighting for those ( and the affected surfaces of course )

A workaround for that, which is what I am doing in my current project is organize the scene outline on different parts. I have exterior yard, ground level and level 1. As now I am working only on the level 1 I simply select all actors which are grouped in a folder from exterior yard ( easier if you arrange by type so you can only select static meshes) and simply set to movable and all the lights to not affect world. They will be ignored in lightmass. Might not be as easy as having an option like you mentioned ( which I also hope will be implemented at some point ) but does the job of lowering the building time.

[RI3DVIZ]

Heh, maybe I got too technical.



Short answer - not without some significant work on our part. Pure HDRI quality is lacking. But I hear you guys loud and clear, that's how you want to light (as opposed to directional or spot lights).

Okay, thank you. At least I know not to try and transfer my techniques over to Unreal and just work with what it gives me. Sure would be nice to be able to light scenes this way though.

[Enter Reality]

My two cents

99% of the time I notice that if you go over a value of 2-3 on the "Indirect light intensity" ( on the main sun light I mean ) you'll have those light leaks all over.
I usually use, to boost up the intensity of the interiors, a skylight with a limited amount of intensity ( 0.2/0.5 ) and 1-2 of Indirect lighting.

Then bounce cards all over the place.

@DanielW: +1 Raghu, to be able to build the lighting only for certain object will be a godsend, since if you're working on different environments and you just want to test part of the scene it'll be great to process only a portion of that or if you move some objects just reuild the lighting for those ( and the affected surfaces of course )

[DanielW]

Thanks Raghu, I'll take a look at that scene when I get a chance, hopefully monday.

is there a way (or is it planned, even possible) to calculate lighting for just selected objects?

It's possible, although a fair amount of lightmass processing (editor export, lightmass import, photon mapping) have to be done no matter what you want to build. Only the blue part in the Swarm visualizer (double click the swarm icon hiding in the taskbar when building lighting). It's not something we have planned just due to not enough time though.

[Raghu]

In the meantime, here's an overview of the methods used along with their pitfalls. I tried to inline the relevant BaseLightmass.ini settings. First, there are two completely separate paths for punctual lights (directional, spot, point) than for sky lights.

Lightmass is heavily optimized around solving indirect lighting from punctual lights with high quality. In particular, the case where you have bright sunlight coming in a small window and hitting the wall/floor, lighting up the whole room, is what I would consider the primary use case. Lighting with sky lights / HDRI is a much more recent implementation and honestly it's not very high quality.

Punctual light method:

The short summary is that Final gathering is used to solve the first lighting bounce, and photons are used for bounces after that along with guiding the final gather.

1) Many photons are emitted from the light and deposited on the scene as directly visible (direct photons). We bounce these once and record any direct photon paths that resulted in a bounced photon being deposited. This identifies small (but critical) light entrances like windows. DirectPhotonDensity controls how many photons are emitted in this stage, and IndirectPhotonPathDensity controls how many paths are recorded (how accurately we find small windows).
2) Many more photons are emitted from the light, along the direct photon paths. These are bounced for as many times as you requested NumIndirectLightingBounces. Each bounce has a much smaller number of photons successfully bouncing so more bounces after 2 has only a small impact on build times. IndirectPhotonDensity + IndirectIrradiancePhotonDensity control how many photons are emitted to represent multi-bounce lighting.

Now that the photons are ready for the entire scene we can start operating on each lightmap texel.

3) Direct shadowing is computed for static and stationary lights. For static lights which use area shadows, NumShadowRays and NumPenumbraShadowRays control the quality of the penumbra, more samples are needed to support very large, smooth penumbras.
4) We check to see if this texel is covered by other irradiance computations (steps 5-7). If there is, we reuse these nearby irradiance samples through interpolation. If not, we continue to steps 5-7. This is called Irradiance caching. Too much interpolation causes indirect shadows to be lost. The amount of reuse is controlled by RecordRadiusScale, PointBehindRecordMaxAngle, InterpolationMaxAngle. During the interpolation, we can gather even more nearby irradiance samples to further smooth the lighting.
5) Nearby indirect photons are gathered around the lightmap texel. These tell us where most of the bright lighting is coming from, eg the small bright spot that lights up the entire room.
6) We begin the final gather. Rays are traced out in all directions for the hemisphere of the texel. At the end of the ray we either hit some geometry, where we gather the lighting from photons at that point, or we miss geometry and hit the sky, where we evaluate the sky lighting. NumHemisphereSamples controls how many rays there are in this first refinement step.
7) We do multiple iterations of refining the final gather. Each sector of the hemisphere is subdivided and we trace more rays to discover incoming lighting. We subdivide deeply around incoming indirect photons to make sure we resolve the small bright spot on the ground lighting the whole room. This is a form of importance sampling, also called importance driven final gathering. We also subdivide anytime there are significant differences between neighbors to reduce noise, this is called adaptive sampling. NumAdaptiveRefinementLevels controls how many refinement levels there are, AdaptiveBrightnessThreshold controls how big of a neighbor difference there must be to refine a sector.

As to the Lightmass WorldSettings options, IndirectLightingSmoothness operates on the settings in step 4), the amount of irradiance cache smoothing.
IndirectLightingQuality increases the number of samples and depth of refinement in steps 5-7, the final gathering.

Sky light method (assuming static skylight, stationary is a bit different):

1) The sky light HDR texture is exported to Lightmass as an 3 band spherical harmonic, which you can think of as an extremely low resolution cubemap (2x2 on each face approx). So if you have a very bright spot in the sky cubemap it will be spread out over a large region. This is basically prefiltering to avoid noise.
2) We do a mini final gather at each lightmap texel and compute sky lighting with a very low number of rays. This is cached on the surface and will become the first bounce of skylight GI once the final gather accesses it.
3) During the final gather, any rays that hit the scene look up the cached sky direct lighting from step 2), which now becomes first bounce sky lighting. Any rays that miss the scene and hit the background lookup from the sky SH, this is sky direct lighting. We refine the final gather based on brightness differences between sectors of the hemisphere, but no importance sampling is done because we don't have any photons from the sky.

So now you can see why static sky lights have limited quality and only support one indirect lighting bounce. This is something we could improve, but it's a lot of work and complexity. Specifically, we should actually export a cubemap to Lightmass, and do importance sampling of the cubemap's contents (trace more rays to the brighter texels).

These explanations are spot on (Made so many things easy and this is the minimum information required to understand lightmass). Till you guys made HDRI Lighting awesome in unreal engine, kindly let us know how to resolve some common issues which come when we simulate HDRI lighting with added support of Punctual lights.

Link to download above scene file - https://www.dropbox.com/s/kjp8sow7do...tmass.rar?dl=0

[heraSK]

while we're talking about lightmass...is there a way (or is it planned, even possible) to calculate lighting for just selected objects? for example, if we add single object/move or add a light, the engine tells us how many objects need their lightmaps calculated. if the engine can tell that, why not enable lighmaps calculations only for affected geometry? it would speed up things even further instead of waiting another hour and a half for light calculation to finish for the whole level when only one object was added...

[DanielW]

Heh, maybe I got too technical.

This is where I would like to get to in Unreal - not sure it's possible? It's just an HDRI and a Sun in Blender/Cycles render engine.

Short answer - not without some significant work on our part. Pure HDRI quality is lacking. But I hear you guys loud and clear, that's how you want to light (as opposed to directional or spot lights).

[RI3DVIZ]

What kind of method cycles use for G.I? path tracing?

http://wiki.blender.org/index.php/De...es/2.61/Cycles

[heartlessphil]

What kind of method cycles use for G.I? path tracing?

[RI3DVIZ]

Daniel:

This is where I would like to get to in Unreal - not sure it's possible? It's just an HDRI and a Sun in Blender/Cycles render engine. There is some post processing in Photoshop.

[heartlessphil]

This is pretty much Chinese to my hear but I'm happy to see devs interested in improving lightmass for arch-viz users!

You may have a potentially HUGE market in sight because everybody in arch-viz communities are talking about real-time/Unreal. I think eventually we all want to go real-time!

[DanielW]

In the meantime, here's an overview of the methods used along with their pitfalls. I tried to inline the relevant BaseLightmass.ini settings. First, there are two completely separate paths for punctual lights (directional, spot, point) than for sky lights.

Lightmass is heavily optimized around solving indirect lighting from punctual lights with high quality. In particular, the case where you have bright sunlight coming in a small window and hitting the wall/floor, lighting up the whole room, is what I would consider the primary use case. Lighting with sky lights / HDRI is a much more recent implementation and honestly it's not very high quality.

Punctual light method:

The short summary is that Final gathering is used to solve the first lighting bounce, and photons are used for bounces after that along with guiding the final gather.

1) Many photons are emitted from the light and deposited on the scene as directly visible (direct photons). We bounce these once and record any direct photon paths that resulted in a bounced photon being deposited. This identifies small (but critical) light entrances like windows. DirectPhotonDensity controls how many photons are emitted in this stage, and IndirectPhotonPathDensity controls how many paths are recorded (how accurately we find small windows).
2) Many more photons are emitted from the light, along the direct photon paths. These are bounced for as many times as you requested NumIndirectLightingBounces. Each bounce has a much smaller number of photons successfully bouncing so more bounces after 2 has only a small impact on build times. IndirectPhotonDensity + IndirectIrradiancePhotonDensity control how many photons are emitted to represent multi-bounce lighting.

Now that the photons are ready for the entire scene we can start operating on each lightmap texel.

3) Direct shadowing is computed for static and stationary lights. For static lights which use area shadows, NumShadowRays and NumPenumbraShadowRays control the quality of the penumbra, more samples are needed to support very large, smooth penumbras.
4) We check to see if this texel is covered by other irradiance computations (steps 5-7). If there is, we reuse these nearby irradiance samples through interpolation. If not, we continue to steps 5-7. This is called Irradiance caching. Too much interpolation causes indirect shadows to be lost. The amount of reuse is controlled by RecordRadiusScale, PointBehindRecordMaxAngle, InterpolationMaxAngle. During the interpolation, we can gather even more nearby irradiance samples to further smooth the lighting.
5) Nearby indirect photons are gathered around the lightmap texel. These tell us where most of the bright lighting is coming from, eg the small bright spot that lights up the entire room.
6) We begin the final gather. Rays are traced out in all directions for the hemisphere of the texel. At the end of the ray we either hit some geometry, where we gather the lighting from photons at that point, or we miss geometry and hit the sky, where we evaluate the sky lighting. NumHemisphereSamples controls how many rays there are in this first refinement step.
7) We do multiple iterations of refining the final gather. Each sector of the hemisphere is subdivided and we trace more rays to discover incoming lighting. We subdivide deeply around incoming indirect photons to make sure we resolve the small bright spot on the ground lighting the whole room. This is a form of importance sampling, also called importance driven final gathering. We also subdivide anytime there are significant differences between neighbors to reduce noise, this is called adaptive sampling. NumAdaptiveRefinementLevels controls how many refinement levels there are, AdaptiveBrightnessThreshold controls how big of a neighbor difference there must be to refine a sector.

As to the Lightmass WorldSettings options, IndirectLightingSmoothness operates on the settings in step 4), the amount of irradiance cache smoothing.
IndirectLightingQuality increases the number of samples and depth of refinement in steps 5-7, the final gathering.

Sky light method (assuming static skylight, stationary is a bit different):

1) The sky light HDR texture is exported to Lightmass as an 3 band spherical harmonic, which you can think of as an extremely low resolution cubemap (2x2 on each face approx). So if you have a very bright spot in the sky cubemap it will be spread out over a large region. This is basically prefiltering to avoid noise.
2) We do a mini final gather at each lightmap texel and compute sky lighting with a very low number of rays. This is cached on the surface and will become the first bounce of skylight GI once the final gather accesses it.
3) During the final gather, any rays that hit the scene look up the cached sky direct lighting from step 2), which now becomes first bounce sky lighting. Any rays that miss the scene and hit the background lookup from the sky SH, this is sky direct lighting. We refine the final gather based on brightness differences between sectors of the hemisphere, but no importance sampling is done because we don't have any photons from the sky.

So now you can see why static sky lights have limited quality and only support one indirect lighting bounce. This is something we could improve, but it's a lot of work and complexity. Specifically, we should actually export a cubemap to Lightmass, and do importance sampling of the cubemap's contents (trace more rays to the brighter texels).

[DanielW]

Great specific breakdown Raghu, that helps me identify issues tremendously. There are clearly some bugs causing splotchiness under those quality settings. Hook me up with the download for the scene, it will take some detailed investigation.

[darthviper107]

You shouldn't change the lighting scale. Some of the settings you want are in the Lightmass.ini file, that allows you to adjust photons and stuff like that--that's what Koola was adjusting for his stuff, though he said it wasn't really necessary.

What can help, is increasing the lighting quality while decreasing the smoothing. It increases build times though.