Raycasted Spheres and Point Sprites vs. Geometry Instancing (OpenGL 3.3)

Source code of sphere_shader examples (raycasted spheres and mesh-based spheres) written in OpenGL 3.3. Code also includes handy camera, shader classes and (GPU) timer functions based on glQueryCounter. Tested on Linux and Windows.

Dependencies: OpenGL, GLEW, and FREEGLUT and glm.

Download source code:
https://github.com/tdd11235813/spheres_shader
(Update: 2016/03/12 - refactoring of code framework)
http://tubafun.bplaced.net/public/sphere_shader.zip
(Update: 2013/05/21 - added: common shaders with VBO and duplicated parameters)

Developing the QPTool I was interested in the performance of the different types of sphere visualizations.

1. Spheres with Geometry Instancing (mesh-based)
2. Sphere Impostors with hardware based Point Sprites (GL_POINT_SPRITE_ARB)
3. Sphere Impostors with Geometry Shaders
4. Sphere Impostors with common Shaders (Parameters via TBO)
5. Sphere Impostors with common Shaders (Parameters duplicated in VBO)

Mesh-based Spheres

The first implementation uses mesh-based spheres by the help of geometry instancing [2]. In the initialization a sphere mesh is sent down to the GPU. By glDrawElementsInstanced(...) the object geometry gets cloned many times. A shader places these objects into the scene. Usually this is a good improvement for visualizing a massive amount of same (complex) geometry. In our case there are two disadvantages:

1. Meshes are a visible inaccurate approximation of the spheres.
2. Polygon data increases with the resolution of the spheres, slows down the application regardless of the screen resolution.

In this picture you see in the bottom part the artifacts by the polygon approximation of the spheres. The upper two parts show raycasted sphere impostors (screenshot from QPTool v0.9.7).

Sphere Impostors by Raycast-Shaders

These two pictures are from the sphere_shader examples. First picture shows the mesh-based spheres and the second one the impostors.

If we want to use shaders to render the spheres in the fragment shader on our own, we need a billboard.

A billboard is a flat object, usually a quad (square), which faces the camera. [3]

This quad must be generated with texture coordinates before we can use it. A good explanation can be found here [4]. There are several ways to achieve this. It is possible to use hardware based point sprites, where for each vertex a billboard is created. In the vertex shader you just have to set the point size, which makes up the billboard size later. There are some issues with compatibility and correct visuals. If you take the particles demo from CUDA SDK, where this rendering method is utilized, you might notice wrong quad sizes, especially when the quads move to the borders of the screen. I tried to fix that, but there are still some glitches.

Another possibility uses geometry shader. Dummy vertices (one per sphere) are sent down to GPU and the geometry shader creates four vertices making the billboard for one sphere.

The last way would be sending four vertices per sphere with predefined texture coordinates down to the GPU. By this either a second data buffer is needed for the parameters OR all the parameters are duplicated (4x) in the vertex buffer object.

One disadvantage of the implemented sphere impostor shaders is, Early depth test is not possible, since z-buffer is computed in fragment shader.

Results

Setting: 800x600
Hardware: GPU Nvidia GTX 470 (Fermi), CPU Intel Quad Core i7 @3.07GHz, 12GB RAM
OS: Windows 7 64bit
Number of spheres: 100,000
Parameters per sphere: Position (4 floats), Color (4 floats), Radius (1 float)
Frames measured after warmup: 200

Benchmark Results of Rendering Methods for Spheres (in ms)
FOV = 60
Method	Avg1	Avg2	Min1	Min2	Max1	Max2
Sphere Geometry Instanced	121.523	0.013	121.173	0.006	122.134	0.023
Sphere Billboard Quads (TBO)	7.080	0.022	6.938	0.017	7.201	0.029
Sphere Billboard Quads (VBO)	1.879	0.002	1.874	0.001	1.886	0.003
Sphere Impostor Geometry Shader	1.915	0.001	1.900	0.001	1.933	0.002
Sphere Point Sprites, with Glitches :(	1.541	0.001	1.501	0.001	1.619	0.002
FOV = 20
Sphere Geometry Instanced	121.606	0.015	121.081	0.003	122.083	0.027
Sphere Billboard Quads (TBO)	8.320	0.023	8.291	0.019	8.359	0.031
Sphere Billboard Quads (VBO)	8.230	0.001	8.227	0.001	8.234	0.001
Sphere Impostor Geometry Shader	8.229	0.001	8.224	0.001	8.243	0.001
Sphere Point Sprites, with Glitches :(	7.756	0.001	7.752	0.001	7.763	0.001
FOV: Field of view. The lower the value, the more sphere-pixels have to be drawn. It is like zoom.
***1: Value for whole frame in ms.
***2: Value for binding and shader config in ms.

Sphere Impostor Geometry Shader: Takes 1 dummy vertex, creates 4 vertices for billboard on the fly.

Sphere Billboard Quads: Takes 4 vertices with preset texture coordinates for billboard. Uses either texture buffer object (TBO) for sphere parameters or parameters which are duplicated (4x) as vertex attributes in the vertex buffer object (VBO).

Sphere Geometry Instanced: Polygon representation of sphere. Using geometry instancing.

Sphere Point Sprites. Hardwarebased Billboard generation (point sprites). Alas, there are glitches and incompatibilities, so I do not recommend to use this.

My current implementation uses the geometry shader method, because it has no glitches so far and it is fast. Nevertheless I would recommend to use predefined billboards with duplicated data, since you will not find geometry shader support "everywhere".

On Linux with a Kepler GPU I obtained following interesting results:

Setting: 800x600
Hardware: GPU Nvidia GTX 670 (Kepler), CPU Intel Quad Core i7 @3.4GHz, 8GB RAM
OS: Linux Kubuntu 12.04
Number of spheres: 100,000
Parameters per sphere: Position (4 floats), Color (4 floats), Radius (1 float)
Frames measured after warmup: 200

Benchmark Results of Rendering Methods for Spheres (in ms)
FOV = 60
Method	Avg1	Avg2	Min1	Min2	Max1	Max2
Sphere Geometry Instanced	50.943	0.003	50.370	0.001	53.117	0.013
Sphere Billboard Quads (TBO)	1.087	0.003	1.069	0.001	1.159	0.010
Sphere Billboard Quads (VBO)	1.061	0.001	1.059	0.001	1.064	0.002
Sphere Impostor Geometry Shader	1.066	0.001	1.063	0.001	1.069	0.003
Sphere Point Sprites, with Glitches :(	0.897	0.001	0.895	0.001	0.902	0.003
FOV = 20
Sphere Geometry Instanced	50.798	0.004	50.356	0.001	53.487	0.015
Sphere Billboard Quads (TBO)	6.191	0.001	6.137	0.001	6.747	0.008
Sphere Billboard Quads (VBO)	6.133	0.001	6.130	0.001	6.137	0.003
Sphere Impostor Geometry Shader	6.121	0.001	6.116	0.001	6.127	0.002
Sphere Point Sprites, with Glitches :(	5.442	0.001	5.440	0.001	5.446	0.002
FOV: Field of view. The lower the value, the more sphere-pixels have to be drawn. It is like zoom.
***1: Value for whole frame in ms.
***2: Value for binding and shader config in ms.

Please leave a comment, if you encounter bugs or know more or if you have other ideas.

[1] - http://www.arcsynthesis.org/gltut/Illumination/Tutorial%2013.html
https://web.archive.org/web/20150215085719/http://www.arcsynthesis.org/gltut/illumination/tutorial%2013.html
[2] - http://wiki.delphigl.com/index.php/shader_Instancing
[3] - http://nehe.gamedev.net/article/billboarding_how_to/18011/
[4] - http://www.sunsetlakesoftware.com/2011/05/08/enhancing-molecules-using-opengl-es-20

QPTool - A swelling process simulator running on GPU (CUDA, OpenGL)

This post is about my recent work. A swelling process simulator (german: Quellprozess) with glass fibers. The project is running at the IKGB Department of the University of Freiberg (East Germany). I have been working mostly on my own, doing this besides of my master study for one year and it makes a lot of fun. The simulator is open source, licensed under GPL v3, so you can grab the source from git (qptool). At the moment third party libraries are not provided, but they can be obtained from the web. I will post a running application here, when v1.0.0 is released.

The swelling process is driven by 'expanding' hydrogen gas bubbles within the fluid concrete. It is a part of the production process of the AAC, a really versatile, lightweight, cheap precast building material. That hydrogen gas is triggered by a really small amount of aluminium powder in the raw mix. The gas can escape later and in the end the pores contains just air.

To improve the properties of the AAC, (alkali-resistant) glass fibers will be added to the raw mix. The swelling process will distribute the fibers. The simulation helps to understand the movements and directions of the fibers from beginning to the end of the whole swelling process.

QPTool is written in C++, Qt 4.7.4, CUDA 5.0, OpenGL 3. The simulation and visualization is completely done on the (Nvidia) GPU. This gives a good framerate even for medium sized simulations (131k Spheres, 8k Fibers at ~16 FPS). The pores are just expanding spheres, following the Cherrypit Model. The fibers are just lines in the simulation colliding only with spheres (no fiber-fiber interaction), since the glass fibers are very thin.

Renderer:

• Simple Raycast-Shader (e.g. Spheres are computed in Shader)
• Multipass Raycast-Shader with Shadows and Edge Filter
• Geometry Renderer (using instancing technique, but it is slower and uglier than simple raycast-shader)

Simulation

• CUDA driven physics engine
• Using uniform space subdivision (based on SDK example 'particles' from Nvidia)
• Slice-cutting computation (simulates cutting of the concrete block)
• User can create fiber clusters with preferred directions
• Real-data based and generic distribution of end radii of pores

Analysis

• Distribution of pores and fibers
• Radii of pores
• Fiber directions (phi/theta diagram)
• ...

Tools

• Recording long-term simulations into video alongside with statistics
• Benchmark tool to compare algorithms
• Export simulation data to csv for further processing
• Import simulation data
• Session and simulation setting management
• ...

Just some pictures:

Comparison Shader vs. Geometry

Fiber Clusters

Simulation

QPTool v0.9.7

Radius Distribution

Slice cutting the block

Credits:
- S. Matthes: My boss :) He does the higher mathematics and experimental investigations for this simulation. Thanks for all the great discussions and for giving me this opportunity.
- Prof. B. Steinbach, allowing me to use an office room for free (including Desktop PC with Nvidia GPU of course :) )
- Nvidia, CUDA, Simon Green, SDK Demo 'particles'.
- Qt (used Qt 4.7.4), Qwt, Qxt / libqxt
- CAviFile Class, Xvid Encoder used for recording tool (video rendering)
- ...

Further sources which I used or adapted or helped me to understand the things better
- FastDelegate
- PixelBufferObjects Class
- Camera Control Class
- several tutorials over the web about OpenGL buffer objects, renderbuffer, framebuffer, shader instancing, shadows shader, ... (take the appropriate buzzword and google it)
- Christian Sigg (GPU-Based Ray-Casting of Quadratic Surfaces)
- Article on Molecules: Enhancing Molecules using OpenGL ES 2.0
- ... and many more ... :)


v0.9.7 still has some bugs and cylinder impostor shader is not finished yet.