Tag Archives: Rendering

Shadows using a voxel grid

Dynamic shadow casting point lights for tiled deferred rendering

A while ago, I started to experiment working with voxels. More precisely, my idea was to test what could be possible if we had our scene fully voxelized. Dynamic shadows is one of those tests.

For my tests I implemented a tiled deferred rendering engine, and one of the difficulties with tiled deferred is shadows. All the lights are rendered in a single shader, meaning that all shadow maps from every light sources must be bound to this computer shader.

The last years have seen a lot of techniques increasing the number of simultaneous dynamic light sources (deferred, clustered, tiled deferred, forward+), but always ignoring shadows. Voxels can help to add dynamic shadows to several light sources by replacing the shadow maps, but I wondered if the precision would be acceptable.

 

I described in a previous blog post the technique I used to dynamically voxelize a scene. I think there might be some ways to optimize this process, but that will be for an other blog post !

All the following screenshots and timmings are from a GTX 780, and the resolution is 1280×720. There is 32 point lights in the scene.

First of all, here what the voxelized scene looks like with a 256x256x256 grid:

Voxelized scene

And the scene without shadows:

SceneWithoutShadows

The main idea is really simple. In the tiled deferred shader when computing the light, with the voxel structuring I am able to check if the current pixel is hidden from the light by something.

My main interest in a first time is to see how it could look, so I started with a straightforward raymarching, starting from the current pixel position to the point light. At each step I transform the world position into voxel grid position and check if the voxel is full or empty.

Here are the first result:

Shadow with voxels first result

The result depends on the number of steps, and of course the more steps you use, the slower it will be. In this screenshot there is 25 steps per raymarching.

Some lights are leaking, and the shadows are very harsh, the soft attenuation is removed because of the voxel size.

Let’s try with more steps, 75 :

Shadows70Steps

No more light leaking, but it’s more costly, and the shadows are still very sharp, etiher present or not, it would be great to have some much subtle values.

During the last GDC, Michal Valient from Guerrilla Games gave a talk (the slides are available here: http://www.guerrilla-games.com/publications.html ) where he showed how to use a dither offset followed by a Gaussian blur to improve raymarching visual quality and reduce the number of steps needed. This is detailed in the chapter “Volumetric Light Effects in Killzone:Shadow Fall” in GPU Pro 5.

So here is the result when I added the dithered offset, with 25 steps:

WithShadowsWithoutBlur

With less steps, the result are far better. With the pseudo random offset the raymarching captures more details. But without applying any blur the noise induced by the dither pattern is very noticeable.

This was a little bit more complicated. It would be too expensive to raymarch the shadows for every samples needed for the blur, so this value must be stored somehow.

Few weeks ago I implemented SSAO with temporal sampling, as described by Bart Wronski here, soI tough I could do something in the same way, use the results of the previous frame to improve the current one.

The previous frame  informations must be stored in order to be able to know for a given pixel which point light is occluded or not, in order to apply the proper lighting. The results of the raymarching being either 0 or 1 ( the light is occluded or not) it takes only one bit to store the shadow information of a single light. A uint32 texture can store shadow informations for 32 lights per pixels. there could be more thant 32 lights, using an other data structure, but it’s a good start !

Once the raymarching is done, the result is stored in the texture like that:


//

uint currentLightFlag = 1 << CurrentLightIndex;

if (result)

g_PointLightShadows[CurrentCoord] |= currentLightFlag;

//

 

And to get the datas from a given light:

 


//

int previousShadow = (g_PreviousPointLightShadows[sampleCoords + motionOffset] & currentLightFlag) == currentLightFlag;

//

And the result:

Shadows using a voxel grid

I think it’s far better, the noise in the raymarching help to capture more details and smooth some of the voxels imprecisions, while the blur gives a nice soft shadow look.

 

I didn’t talk much about performances, because in this first test I was mainly interested by the image quality, and the implementation is really straightforward. For example the current blur implementation is very unoptimal, I sample directly in the “point light shadow map” for each lights. I could pre load the needed values only once per tile, store them in shared memory, for a quicker access.

Still, the timmings are not that bad. You can see that by looking at the “lighting” section, this is where the lighting and shadow casting happens. The shadows plus offset and blur add 4.7 ms. As it’s done in the tiled lighting pass, performances are linked to the number of lights seen on screen, and to the number of lights in a single tile.

That’s not that bad considering it’s 32 dynamic shadow casting point lights but of course it needs to have a voxel structure.

 

Here is an other comparison from a different point of view.

WithoutShadow2 WithShadow2

In this example the artifact due to the coarse voxel structure are noticeable.

I was able to reduce them using an offset at the begining of the raymarching, to remove some self shadowing pixels (I just noticed that it’s not exactly the same point of view) :

RaymarchingWithOffset

 

It’s a little better, but still something I’ll need to investigate.

I also tried to change the number of steps for the raymarching, to see which values would be the best compromise between performance and quality.

5 steps:

 

5steps

10 steps:10steps

15 steps:15steps

50 steps:50steps

 

5 steps are not enough, the shadows are missing where none of the steps hit a voxel. This is because with a grid size of 256x256x256 the voxels are quite small, and only the surface of the mesh is voxelized, and not the inside of the mesh.

With 10 steps there are still some issues. I think that  maybe a better blur could hide those imperfections, while smoothing the squared edges..

When there is too much steps, there is no holes in the shadows, but it give a very sharp result, making the voxels more noticeables. This values needs to be tweaked according to the scene, the grid size and the lights radius.

Here is a last test, using smaller grid sizes:

128x128x128:

128VoxelGrid

10 steps raymarching:128VoxelsGrid-10Steps

64x64x64:64VoxelGrid

5 steps: 64VoxelGrid-5steps 

The smaller the grid is, the less steps it needs to have a correct result, because of the bigger voxels. But obviously the shadows are far less precise.

As I said this is a quick test, and there is room for improvments but I find the results quite encouraging. It can’t replace a shadow map for important lights or complicated shadows, but for it’s good enough for secondary lights.

Now the next step is to try other techniques and compare the results. With the proper mipmap, the voxel structure can be used as an octree, and I should be able to cast ray efficiently. It may be quicker and more precise thant raymarching. I also want to try shadows with voxel cone tracing.

I would also like to try to do the raymarching/raycansting/voxel cone tracing later in the frame. In the current implementation the number of raymarching for a pixel depend on the number of lights hitting that pixel. Some pixels will need 5 raycasts while others won’t need a single one. It would be better if instead of doing the raymarching it creates some sort of “GPU raymarching job”, and those jobs would be done later, equally distributed within the threads.

So there will be more blog post on this subject ! I will also try to do a video, because it looks better in movement.

marble2

Tweaking the Cook Torrance BRDF

I’m still learning things about physically based shading using my PBRViewer, and this time, I wanted  to be able to experiment the variations of the Cook Torrance BRDF.

The Cook Torrance BRDF looks like this:

Cook Torrance BRDF

 

This equation is composed of three distinct terms:

  • F: The fresnel, represents how the reflectivity change at grazing angles.
  • G: The Geometry term, represents the probability that a microfacet will be visible from the light and view directions.
  • D The normal distribution term, defines the distribution of the orientation of the microfacets.

For more infomations you can read the very interesting “Physics and math of shading” by Naty Hoffman. For each term there is more than one possibility, and you can choose according to your need, and your budget the terms of your BRDF. Even if GGX is becoming the new standard, I wanted to experiment the other possibilities.

Brian Karis, while he was doing research on physically based shading for the Unreal Engine 4, listed all lot of variation for the different terms. This wonderful blog post can be found here. I used this references to implement each term in my viewer, so I can directly see the impact of each functions on the lightning, the shader being recompiled automatically when a term is changed.

I also added some other modifications, like beeing able to change the background color, light position, intensity, ambient light and reflection intensity, etc.

If you want to try it, you can download it here.

As always, if you see an error or if you have any feedback, please contact me, as I’m doing this to learn I would be happy to hear from you.

I also made my first step with substance designer, trying to do a marble texture.

Marble

 

diffspec

Physically Based Shading, Metallic and Specular workflows

Physically based shading is more and more adopted and even if the core mechanism is pretty much always the same, the workflow may differ from an engine to another.

For example let’s compare two common ones, often called Metallic and Specular.

The metallic workflow uses a color input, the base color, and two scalar parameters, rouhghness and metallic. On a specular workflow there is two color inputs, an albedo and a specular, and a scalar, the roughness.

In my PBRViewer, I first implemented a metallic workflow, I now added a specular workflow. Here is a brief overview of the differences between those two.

First of all, it’s important to understand the kinds of materials we want to represent in games. They can be divided in two groups, dielectrics (plastics, wood, concrete, etc) and metals. Their properties are very well summarized in the wonderful chart made by Sebastien Lagarde for Dont nod. Here are some interesting facts:

  • Dielectrics material have a monochromatic specular, in a range going from 0.017 to 0.067
  • Metals have a black diffuse, except when they are not pure, they can have a little diffuse
  • Metals have a colored specular

 

Now let’s get back to our workflows. The specular one is pretty straightforward, each map is directly used,  artist create their own specular and diffuse map. You need to make sure that your artists have a chart and know the propreties of each kind of materials to have a coherent result. It’s a lot of control, but it’s easy to break.

Specular workflow

Specular workflow. As you can see on the sliders, the diffuse is set to 0, and the color of the material is given by the specular tint.

On the data side, it’s 7 channels (diffuse rgb + specular rgb + roughness) to store in your GBuffer (for deferred rendering). It’s not awfull, but it’s pretty high, especially if you look closer. For dielectric you only  have a greyscale specular, which still takes three channels, and for metals the diffuse is mainly black. That’s a lot of space wasted. The metallic workflow allow you to avoid that.

Disney introduced in their siggraph talk in 2012 their “principled” BRDF which is based on the following rules:

  1. Use intuitive rather than physical parameters.
  2. Use as few parameters as possible.
  3. Paramters should be zero to one, remapped over their plausible range.
  4. Parameters should be allowed to be pushed beyond their plausible range where it makes sense.
  5. All combinations of parameters should be as robust and plausible as possible.

The metallic workflow follow those rules, by introducing a metallic parameter and by removing the specular texture. The metallic parameter is really intuitive. 0 represent a dielectric material, 1 is a metal one. The values beetween 0 and 1 should not be used, except in some special cases, like a transition beween two materials.

Workflow Metallic

The metallic slider is set to one, so the material is a metal

This parameter is in fact a blend between the dielectric and metallic models. For the dielectric model the diffuse is the base color, and the specular is a constant value we defined. For the metallic materials the diffuse is set to black, and the baseColor is used as specular.

// Lerp with metallic value to find the good diffuse and specular.
float3 realAlbedo = albedoColor - albedoColor * metallic;

// 0.03 default specular value for dielectric.
float3 realSpecularColor = lerp(0.03f, albedoColor, metallic);

As you can see, in the end, it’s transformed into the same inputs, but much simpler to use and more error prone. And it’s only using 5 channels.

Using only these inputs you can’t change the specular value of your dielectric materials, but you can add another one, in the range 0.017 – 0.063, remapped to 0 – 1 to control this value.

Some effects can’t be obtained in a metallic workflow, but as they don’t really have a physical reality you may not want to use them anyway.

 

A material with a colored specular and a colored diffuse.

A material with a colored specular and a colored diffuse.

 

This is just an overview of two ways of feeding a physically based renderer, and I think that each engine/studio/project as his own specific workflow. As often it’s all about knowing what you want, what your artists want, the possibilities offered by your engine (deferred/forward). The Disney paper is a very good place to find what kind of inputs can be implemented, but as the Disney BRDF is the next feature I’ll add to my viewer, I’ll talk a bit more about it in an other article.

 

Physically Based Rendering

Physically based rendering viewer

Physically based rendering is becoming the new standard for materials. It was already used a lot in AAA productions, and it’s now in Unreal Engine, Cry Engine and Unity.

As a graphic programmer I’ve read a lot of papers and seen lots of presentation on that topic, but I never had the chance to try it. That’s why I made a small software, to be able to experiment both on a code and data point of view.

You can try it here: PBRViewer .

The viewer is easy to use, you move the camera with the mouse and the keyboard (using ZQSDAE or WASDQE) and the mouse. SHIFT allows you to move faster.

I can use data form textures, or use the sliders to set my own values. It’s very usefull, because it allowed me to see the real impact of each parameters.

Textures must be placed in the folder Models/Materials/0 , with the “Albedo.tga”, “Normal.tga” etc, and will be updated in the viewer automatically. The current textures are the results of my tests with Substance Designer, ie. add node at random and export. Results will be better with real textures.

I didn’t test it on a non programmer pc, so it may require some redistribuables, such as visual studio redistribuable or directX redistribuable.

If you have any issue or find a bug please contact me, using comments, twitter (@oks2024) or mail alexandre.pestana (at) supinfo.com. Also, as I said, I made this in order to discover and learn physically based shading. So if you see something strange or wrong I’d be happy to hear from you.

I used informations I gathered on internet, mainly:

  • Sébastien Lagarde’s blog:

http://seblagarde.wordpress.com/

Sebastien Lagarde shared a lot of informations on how they implemented physically based rendering in Remember Me. It’s a must read since it cover the subject from implementation to asset creation. http://seblagarde.wordpress.com/

  • Brian Karis’s blog :

http://graphicrants.blogspot.ca/2013/08/specular-brdf-reference.html

While implementing PBR in UE4 he tried many options for the specular BRDF and shared them. It’s very usefull, and I plan to implement them all, and to be able to switch from one to another to view the impact.

  • Stephen Hill’s blog:

http://blog.selfshadow.com/publications/s2012-shading-course/

http://blog.selfshadow.com/publications/s2013-shading-course/

The pages on the siggraph PBR courses are full of informations, if you want more informations on PBR, go read them all !

I also used informations in books such as “Real Time Rendering “or, of course “Physically Based Rendering”.

The cube map comes from Emil Persson (aka Humus) wondefull texture library : http://www.humus.name/index.php?page=Textures&&start=0

And now here some screenshots of different presets:

Physically Based Rendering

Glossy dielectric

Physically Based Rendering

A semi glossy copper like metal

Physically Based Rendering

Yes, my textures are ugly.

Physically Based Rendering

Physically Based Rendering

Physically Based Rendering 7

Black rough plastic

 

GPU Particles

English version is coming soon !

Une première vidéo pour montrer et expliquer le fonctionnement de base de mon moteur de particules.

Tous les calculs de mise à jour, physique et collisions s’exécutent sur le GPU, ce qui permet d’avoir de bonnes performances pour un grand nombre de particules (ici 1 000 000 de particules, locké à 30 fps pour les besoin de l’enregistrement).

Toutes les informations dynamiques des particules (position X et Y dans les canaux RG et velocité X et Y dans les canaux BA) sont stockées dans une texture (ici 1024×1024) Chaque particule est identifié par un ensemble de trois vertices. A la place de leur position est stocké une coordonnée de texture, qui permet de retrouvé les informations dans la texture contenant les données.

La mise à jour se déroule en deux temps. Tout d’abord il y a une phase de mise à jour de la physique. En dessinant un quad fullscreen, pour chaque pixel de la texture de données on extrait les informations de la frame précédente afin d’en déduire celles de la frame courante, en fonction de la gravité, des collisions, des forces externes, etc. Ensuite vient la phase d’affichage. On envoie à la carte les vertices représentant chaque particule, et dans le vertex shader, grâce au Vertex Texture Fetching et aux UVs, on retrouve la position réelle ce qui permet d’afficher un triangle au bon endroit.

On peut voir dans la vidéo l’influence d’une force d’attraction contrôlée par la souris et celle de la gravité. Il n’y a de collisions qu’avec le bord de l’écran. La couleur des particules peut être soit fixe, soit influencée par leur vélocité. On voit aussi un post process qui dessine une couleur en fonction de la densité des particules, donnant un aspect “fluide”.

Dans la prochaine vidéo je montrerai les collisions avec des objets dynamiques, ainsi que l’utilisation de flowmaps pour influencer le mouvement de toutes les particules.

Le code source est disponible sur github.

Le setup du projet est téléchargeable ici.