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Volumetric Objects in Unity

Posted by Matt Carr, 29 June 2012 · 4,588 views

unity unity3d volumetric volume shader shader advanced surface ray casting tracing marching
For something a little different than just posting hours of video of me making an unnamed game, I thought it would be good to write something and talk about something different here for a change Posted Image.

First, I recently left my lead programming job to start doing indie development full time. The job was slowing and a combination of events created what felt like the perfect moment to pull the trigger on this. I'll post more about my experiences trying to generate in income to live on from the comfort of my home as I have more to write about, for the moment I thankfully have some great contract work that still keeps things in the black week to week at the moment.

For this entry however, I want to talk a little bit about volumetric objects, specifically the volumetric objects I've created and added to my Advanced Surface Shaders package for the Unity engine.

Volumetric Objects

http://www.youtube.com/watch?v=HPo2gfy675o&hd=1


The volumetric effects I've created so far are 3 basic shapes: box, sphere and cone. Rather than create and modify actual geometry for these shapes, I simply render a basic mesh just to have something render and I extend it's size/bounding volume on the CPU when the object's attributes change to encapsulate the size of the object. At this time I also pass information about the shape to the shader.

The shader is where the actual visual shape of the object is determined. Raycasting is used per pixel to determine the near and far intersection points (if any) of the volume and from that information the opacity of the object can be determined. The further the ray travels through the volume, the higher the opacity. I provide a visibility property for the shaders to determine how far you can see through an object before it becomes opaque. The effect also support the camera being inside the volume.

Posted Image
Click for higher resolution image


One of the key features of the volumes though is the support for texture sampling. Currently I support only world space sampling which is useful for the typical use cases of dust and fog within the volumes, especially when they are moving or there are multiple volumes near each other or intersecting. This way the textures shown within them will remain "in place" in the world as the volumes move and the textures can appropriately "move" from volume to volume like pieces of dust moving from within range of one spotlight to another. In future updates I will support local space texture sampling with control over the direction of the sampling for effects like light enclosures casting shadows down the length of a spotlight cone shaft.

To achieve the texture effects within the volumes I ray march along the intersecting section of the ray and sample the texture at that sampled position.

Posted Image
Image from the Volume ray casting page on Wikipedia


I really love what can be done with these sorts of effects and just throwing an arbitrary texture onto a volume and seeing the results gives new ideas about possible uses that would provide effects I've never seen before.

Over the coming weeks I'll add all the additional functionality I can think to add to the shaders for the package and provide variations on the shaders that are optimised by not doing what that isn't needed. For example if there is no texture used then there is no ray marching required and a far simpler path can be used in the shader to find the opacity per pixel. Or for cone volumes if there is no top cap required then calculations within the cone intersection test can be ignored.

I'll post more about work I do in Unity and for the Unity Asset Store here when there's something I think is cool enough to share. I'm working on a new system at the moment that could be released in the next couple of weeks that I'll be sure to detail here Posted Image.

If you want to grab the Advanced Surface Shaders package for these volumetric effects and a host of other shaders (e.g. Triplanar texturing, cone stepped relief mapping, parallax occlusion mapping, skin shading, etc) then you can check it out on the Asset Store from here or go to the Unity forum thread for more videos and info.




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