# Radiosity C++ Directx 11 implementation

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Hello! Can you advice me learning resources how to develop (practically) Real-time Radiosity system using c++ and directx 11/OpenGL hybrid approach like this one
?
Thanks!

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Posted (edited)
17 minutes ago, Michael Davidog said:

directx 11/OpenGL

The visualization/rendering part (special purpose GPU programming) is IMHO the easiest part.

Iteratively solving your system of equations efficiently in parallel (general purpose GPU programming) is a harder part. You can try to use the compute pipelines (e.g. DirectCompute, etc.) or some higher level GPGPU programming languages like CUDA which you can use in combination with OpenGL and Direct3D (though for the latter I only find info from NVidia itself instead of other tutorials). The benefit of CUDA are the many and large math APIs available.

On the other hand, I have not really an idea about the "real-time" aspect of radiosity (how much trade offs does this impose on the indirect illumination?).

Edited by matt77hias

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Posted (edited)

The algorithm described in the linked paper requires that all geometry (and local light emitters) are static in order for the light transfer approximation to be pre-computable. If the geometry changes, then so does each transfer function of the probes that "see" the changed geometry either directly or indirectly. The environment light map (infinitely far skybox, essentially) does not need to be static, as the spherical harmonic functions are evaluated against its contribution at runtime.

The algorithm allows for dynamic meshes (as in moving characters and objects) be lit with the existing radiosity data by taking n nearest probes of the mesh and evaluating the lighting of the object from them, but the dynamic objects cannot easily contribute to the radiosity solution so shadows and light bleeding of said dynamic objects would not work without further processing.

Edited by Nik02

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Posted (edited)
7 hours ago, matt77hias said:

The visualization/rendering part (special purpose GPU programming) is IMHO the easiest part.

I don't have practical expirience and I don't know how to make all this things work in a code, so maybe I need to start from graphics programming (using directx 11/OpenGL)?

7 hours ago, matt77hias said:

Iteratively solving your system of equations efficiently in parallel (general purpose GPU programming) is a harder part.

Using hybrid approach (cpu + gpu) will give most benefits than pure cpu or pure gpu solution. So I want to make architecture like Enlighten.

7 hours ago, matt77hias said:

You can try to use the compute pipelines (e.g. DirectCompute, etc.) or some higher level GPGPU programming languages like CUDA which you can use in combination with OpenGL and Direct3D (though for the latter I only find info from NVidia itself instead of other tutorials). The benefit of CUDA are the many and large math APIs available.

I want to make hardware- and API-agnostic system

6 hours ago, Nik02 said:

If the geometry changes, then so does each transfer function of the probes that "see" the changed geometry either directly or indirectly.

This is indirect lighting cache that interpolate indirect light on dynamic (e.g. animated)/small/complex objects using light probes?
So this is spherical harmonic function (irradiance volumes)?

6 hours ago, Nik02 said:

If the geometry changes, then so does each transfer function of the probes that "see" the changed geometry either directly or indirectly.

but how ray-tracing used to make new lightmaps?
Or lightmaps get information (normal and shadow mapping) from SH - one light probe per pixel?
"The radiosity technique uses scalar form factors to describe the mutual influence of patches. In our solution, these form factors are replaced. For every light probe, each surface group as seen from the position of the probe is projected to SH coefficients and stored. This directional information allows to shade surfaces with normal mapping and dynamic objects whose normals are unknown during precomputation. For the shading of dynamic objects, light probes are placed in a regular 3D lightgrid. For static geometry, it is preferable to use lightmaps in order to avoid light bleeding, i.e. light being interpolated through solid objects. A lightmap is a texture that stores some kind of lighting information, in our case one light probe per pixel. To use lightmaps, a special set of UV coordinate is created for the scene. The surfaces are unwrapped in range [0, 1]^2 of the UV space without overlaps. The result is that every surface point references a unique position on the texture. Traditionally, lightmaps have been used to precalculate high-frequency static lighting and save it into an RGB texture. In our approach, a set of three textures for the tristimulus values are used to store spherical SH information about low-frequency indirect lighting only. The resolution of these textures is thus considerably lower than the resolution of commonly used lightmaps."

6 hours ago, Nik02 said:

but the dynamic objects cannot easily contribute to the lighting so shadows and light bleeding of said dynamic objects would not work

Yes, this approach and Enlighten approach don't allow that.

6 hours ago, Nik02 said:

further processing.

Is it possible somehow?

Edited by Michael Davidog

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Here is a similar technique: http://copypastepixel.blogspot.co.at/2017/04/real-time-global-illumination.html

1 hour ago, Michael Davidog said:

Is it possible somehow?

Some techniques designed for dynamic scenes:

Light Propagation Volumes (very approximating)

Voxel Cone Tracing (more accurate, but still very limited as voxels can not represent scenes at meaningful resolution)

Many LoDs (very accurate, but micro frame buffer too low resolution to cover direct lighting)

Reflective shadow maps in combination with SDF occlusion (actual CryEngine approach - limited accuracy but very practicable performance)

... to name just a few.

Paper comparing many of them: https://people.mpi-inf.mpg.de/~ritschel/Papers/GISTAR.pdf

Personally i work on an algorithm to overcome all those limitations for ten years. (So i would be disappointed if you get it to work in short time  )

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26 minutes ago, JoeJ said:

Personally i work on an algorithm to overcome all those limitations for ten years. (So i would be disappointed if you get it to work in short time

I just want to start making things by mysefl=) I need practical expirience, there so much papers around but I can't find any "paper to code" guide. So I don't know how to make existing solutions to work (in a code that I can use futher in an engine).

29 minutes ago, JoeJ said:

Some techniques designed for dynamic scenes:

Light Propagation Volumes (very approximating)

Voxel Cone Tracing (more accurate, but still very limited as voxels can not represent scenes at meaningful resolution)

Many LoDs (very accurate, but micro frame buffer too low resolution to cover direct lighting)

Reflective shadow maps in combination with SDF occlusion (actual CryEngine approach - limited accuracy but very practicable performance)

... to name just a few.

Paper comparing many of them: https://people.mpi-inf.mpg.de/~ritschel/Papers/GISTAR.pdf

I know there a lot of techniques but I can't implement any of them. Personally I would like to realize some Enlighten/Kuri ray-tracing+radiosity hybrid method.
Here something similar
This guy also figured out how to implement it somehow

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I found it a lot easier to do all on CPU for research. Basically i tried pretty any idea that was around and GPU is too cumbersome to get things going quickly. You also need some experience with GPU to know if your algorithm can utilize it properly, but the first thing to do is to learn how the math works.

The methods you are interested in work by calculating a form factor between two surfaces, and the form factor tells how much light interacts between them. I suggest you begin with this and get a simulation to work that converges to ground truth. That's easy - radiosity math is very easy - the only hard part is performance.

Personally i divide any surface into small disks and interreflect light as said. Enlighten does the same using polygons - they are fast because they precalculate form factor times visibilty.

The simulation works like this:

while (1)

foreach (surfaceSample emitter)

Note that due the visibility term runtime becomes O(n^3), but for a very small scene (cornell box with 500 samples) this is fast enough to learn the math. After that you can focus on optimizations and precalculating things.

I can dig up the exact math for you if you want, but first let's see if you want to go this way...

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21 hours ago, JoeJ said:

I found it a lot easier to do all on CPU for research. Basically i tried pretty any idea that was around and GPU is too cumbersome to get things going quickly.

Enlighten split direct lights to GPU and indirect to CPU (Enlighten is for indirect lights only on CPU):
1. Point sampling geometry surface - done via Final gathering (without any irradiance caching) with denoising - VPL/Instant radiosity method - I guess there no surfels/disks (GPU)
2. Project on low detail environment mesh -
"The framebuffer of the previous frame is sampled sparsely on the GPU and subsequently those samples are transferred to CPU memory as input for the radiosity algorithm. The samples are projected on the low resolution proxy mesh which initializes the radiosity algorithm. Only one iteration of radiosity propagation is executed per frame. Multiple light bounces are simulated by using the previous frame as light input for the computation."
"The radiosity algorithm itself is executed on the CPU using a low resolution proxy mesh of the original mesh which resides in GPU memory."
4. Transfer radiosity textures to the GPU -
"After the radiosity solution has been computed, the solution is transferred back to the GPU in a lightmap format." - I guess using wavelet compression to make data flow faster.
5. Sample radiosity on high detail mesh -
"The lightmap is sampled on the high resolution mesh with the use of a smart upsampling technique." - I guess using UV unwrapper.

21 hours ago, JoeJ said:

You also need some experience with GPU to know if your algorithm can utilize it properly, but the first thing to do is to learn how the math works.

21 hours ago, JoeJ said:

The methods you are interested in work by calculating a form factor between two surfaces, and the form factor tells how much light interacts between them

This is the same principle as "patches" but clusters are hierachical and bigger?

21 hours ago, JoeJ said:

I suggest you begin with this and get a simulation to work that converges to ground truth. That's easy - radiosity math is very easy - the only hard part is performance.

How can I to begin?

21 hours ago, JoeJ said:

Personally i divide any surface into small disks and interreflect light as said. Enlighten does the same using polygons - they are fast because they precalculate form factor times visibilty.

So it is possible to divide surface into surfels and polygons? What is better? And how I can make it with polygons?

21 hours ago, JoeJ said:

The simulation works like this:

while (1)

foreach (surfaceSample emitter)

Thank! But I still don't understand it.
That's will looks like this:

#include <iostream>
Using namespace std;
vector outgoingLight;
vector color;
};

Struct emitter {
vector outgoingLight;
};

vector formFactor;
vector visibility;
}

void surfaceSample emitter {
vector formFactor;
vector visibility;
}

Int main (){
emitter * e;
e = new emitter;

while (1) // What means "1" - one bounce?

foreach (surfaceSample emitter)

delete r;
delete e;
}

?

21 hours ago, JoeJ said:

I can dig up the exact math for you if you want, but first let's see if you want to go this way...

Yes, I want! I still don't understand all this code.
I found some implementation:
https://github.com/ands/lightmapper
http://dudka.cz/rrv

but I don't understand all this code - I want to understand=)

Edited by Michael Davidog

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5 minutes ago, Michael Davidog said:

Just found about that reference myself in the "Further Reading and Resources" section of the "Global Illumination" chapter of the "Real-Time Rendering" book. The authors of the latter state: "Implementing radiosity algorithms is not for the faint of heart. A good practical introduction is Ashdown's book, sadly now out of print.

8 minutes ago, Michael Davidog said:

Seems to me for offline (i.e. not real-time) rendering.

9 minutes ago, Michael Davidog said:

Are you sure this uses the radiosity algorithm?

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1 hour ago, matt77hias said:

Just found about that reference myself in the "Further Reading and Resources" section of the "Global Illumination" chapter of the "Real-Time Rendering" book. The authors of the latter state: "Implementing radiosity algorithms is not for the faint of heart. A good practical introduction is Ashdown's book, sadly now out of print.

http://www.helios32.com/resources.htm
But if I will follow this book I will get modern knowledge how to implement things like Enlighten and plug into an engine?
Enlighten using radiosity with Final Gathering (i.e. photon mapping?)?

Edited by Michael Davidog

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31 minutes ago, Michael Davidog said:

But if I will follow this book I will get modern knowledge how to implement things like Enlighten and plug into an engine?

It is a book from 1994 so you will get basic and practical knowledge, not modern knowledge (though radiosity is already quite old; it predates ray tracing).

33 minutes ago, Michael Davidog said:

Enlighten

Enlighten has an SDK perhaps that contains some useful insights?

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2 minutes ago, matt77hias said:

basic and practical knowledge

that's what I need! Thanks! Did you read this book?

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2 hours ago, matt77hias said:

Seems to me for offline (i.e. not real-time) rendering.

To make that in real time, we can use the Instant Radiosity (from Keller (in 1997))
And Final gathering - that much faster than photon-based GI

2 hours ago, matt77hias said:

Are you sure this uses the radiosity algorithm?

It based on site of Hugo Elias who documented his radiosity light mapper

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2 hours ago, Michael Davidog said:

The chapter in OpenGL Super Bible about compute shaders - it's great and covers all basics, trains the kind of thinking you need to have for massive parallelization. To me this was enlightening and the rest then is mostly technical details you get while doing and from experience. So even if you want to use another language, read it. (OpenCL 1.x, Compute shader in OGL/VK / DX11or12 - it's all the same)

I recommend using OpenCL to start. It's by far the easiest way on the API side. VK/DX12 is too complicated to get going, but you can port your OpenCL 1.x kernels to them later. (I use both OpenCL and VK code paths, both using the same shader code but preprocessed to adapt syntax, VK is almost two times faster.)

2 hours ago, Michael Davidog said:

This is the same principle as "patches" but clusters are hierachical and bigger?

Yes, you can divide your surfaces into surfels, disks, quads, polygons... what ever you want. For form factor calculation you typically use only their area and ignore the shape. Error becomes noticeable only if surfaces come very close together, but at this point you subdivide before this becomes a problem. Hierarchies are the main idea to make things faster. Distant objects can be approximated by a single piece of area, as anywhere else where LOD is used.

I'll write a simple simulator in some hours in another post. This should answer most of your questions...

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1 hour ago, Michael Davidog said:

No, that is solely based on the citation. The only book I read about radiosity is "Advanced Global Illumination" which explains among other topics pure and hybrid radiosity methods (though less practical).

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Here is the code. It shows a cylinder with grey / sides and a small area of emitting light.

The scene looks boring and ugly but indirect lighting / color bleeding shows up - assuming i've done nothing wrong it's ground truth. Because there can't happen any occlusion, i simply set visibility to 1. You could model a cornell box without the boxes in the middle (so just the walls and the light) to get a nicer image. Adding brute force raytracing to support occlusion should be a matter of minutes.

The main problem here is that form factor calculation is optimized and it's impossible to understand / explain the math. I'll do this tomorrow...

But you already can see how simple it is.

struct Radiosity
{
typedef sVec3 vec3;
inline vec3 cmul (const vec3 &a, const vec3 &b)
{
return vec3 (a[0]*b[0], a[1]*b[1], a[2]*b[2]);
}

struct AreaSample
{
vec3 pos;
vec3 dir;
float area;

vec3 color;
float emission; // using just color * emission to save memory
};

AreaSample *samples;
int sampleCount;

void InitScene ()
{
// simple cylinder

int nU = 144;
int nV = int( float(nU) / float(PI) );
float scale = 2.0f;

float area = (2 * scale / float(nU) * float(PI)) * (scale / float(nV) * 2);

sampleCount = nU*nV;
samples = new AreaSample[sampleCount];

AreaSample *sample = samples;
for (int v=0; v<nV; v++)
{
float tV = float(v) / float(nV);

for (int u=0; u<nU; u++)
{
float tU = float(u) / float(nU);
float angle = tU * 2.0f*float(PI);
vec3 d (sin(angle), 0, cos(angle));
vec3 p = (vec3(0,tV*2,0) + d) * scale;

sample->pos = p;
sample->dir = -d;
sample->area = area;

sample->color = ( d[0] < 0 ? vec3(0.7f, 0.7f, 0.7f) : vec3(0.0f, 1.0f, 0.0f) );
sample->emission = ( (d[0] < -0.97f && tV > 0.87f) ? 35.0f : 0 );

sample++;
}
}
}

void SimulateOneBounce ()
{
for (int rI=0; rI<sampleCount; rI++)
{
vec3 rP = samples[rI].pos;
vec3 rD = samples[rI].dir;
vec3 accum (0,0,0);

for (int eI=0; eI<sampleCount; eI++)
{
vec3 diff = samples[eI].pos - rP;

float cosR = rD.Dot(diff);
if (cosR > FP_EPSILON)
{
float cosE = -samples[eI].dir.Dot(diff);
if (cosE > FP_EPSILON)
{
float visibility = 1.0f; // todo: trace a ray from receiver to emitter and set to zero if any hit (or use multiple rays for accuracy)

if (visibility > 0)
{
float area = samples[eI].area;
float d2 = diff.Dot(diff) + FP_TINY;
float formFactor = (cosR * cosE) / (d2 * (float(PI) * d2 + area)) * area;

vec3 reflect = cmul (samples[eI].color, samples[eI].received);
vec3 emit = samples[eI].color * samples[eI].emission;

accum += (reflect + emit) * visibility * formFactor;
}
}
}
}

}
}

void Visualize ()
{
for (int i=0; i<sampleCount; i++)
{
vec3 reflect = cmul (samples[i].color, samples[i].received);
vec3 emit = samples[i].color * samples[i].emission;

vec3 color = reflect + emit;

//float radius = sqrt (samples[i].area / float(PI));

float radius = sqrt(samples[i].area * 0.52f);
}
}

};

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