The aim with soft particles is to remove the ugly artifact that appears when the particle quad intersects the scene. There are a lot of different approaches to solve this, some more complicate than others. The simplest formula for soft particles is to just fade the particle if it’s getting to close to the scene. To do this, the scene without particles has to be rendered first and the depth saved in a texture. When drawing the particles, the depth of the particle will be compared to the scene depth. The alpha should be increased by a smooth fade by this depth difference. The formula below in HLSL is the simplest possible for soft particles, and works very well. Scene_depth is the sampled depth (in viewspace) of the scene in the direction of the current pixel. Particle_depth is the depth(in viewspace) of the current particle pixel. Scale is used to control the “softness” of the intersection between particles and scene:
fade = saturate((scene_depth – particle_depth) * scale);
NVIDIA [1] proposes a method that the following fade should be used instead of the linear one described above, to make the fade even smoother.
float Output = 0.5*pow(saturate(2*(( Input > 0.5) ? 1-Input : Input)), ContrastPower);
Output = ( Input > 0.5) ? 1-Output : Output;
Umenhoffer [2] proposes a method called spherical billboards to deal with these problems. In this method, the volume is approximated by a sphere. This method also deals with the near clipplane problem that particles will instantly disappear if they get to close to the camera.
There is also an idea [3] that the alpha channel can be used to represent the density of the particles. Although this method has the drawback that the textures might need to be redone by the artists.
The method by Microsoft [4] uses a combination of spherical billboards and a texture representation of the volume. But instead of using the alpha channel, they ray march the sphere and sample the density and volume from a 3D noise texture. The result can be seen in the image below.
The video below shows how soft shadows can increase realism in games using large particles. It’s originally an ad for Torque 3D engine.
[1] Soft Particles by NVIDIA
http://developer.download.nvidia.com/whitepapers/2007/SDK10/SoftParticles_hi.pdf
[2] Spherical Billboards and their Application to Rendering Explosions
http://www.iit.bme.hu/~szirmay/firesmoke.pdf
[3] A Gamasutra article about soft particles
http://www.gamasutra.com/view/feature/3680/a_more_accurate_volumetric_.php
[4] A DirectX 10 implementation of soft particles by Microsoft, called Volumetric Particles
http://msdn.microsoft.com/en-us/library/bb172449(VS.85).aspx
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Great article, exactly what I was looking for… Writing news item right now for one game engine that implemented technique №1, description found here is just what I need.
I think something is misleading in this post. The two screenshots are supposed to show hard particles vs. the same particles but softened. However, it is not possible to make particles on the left look like what they are on the right using technique described in this post. The right screenshot shows some fire particles in front of the wooden box. Simply by making the particles’ intersections transparent will not make particles on the left screen look as on the right screen. To do so, it is needed to render sprite particles with depth testing function set to ALWAYS. Otherwise we will not have control over sprites’ pixels that are hidden by the wooden box (depth buffering will discard them).
To sum up, what I have come up with to have better control over soft particles (and not only make them transparent at geometry intersections) is to set depth testing function to ALWAYS and use the following formula:
float fade = 1.0f – saturate(softness * (spriteDepth – sceneDepth));
This works very fine for me.
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