I'm hoping to write a 2D fluid dynamics simulation using HLSL code, but can't really figure out a good way to access multiple textures (velocity, color and pressure). I'm able to access the colors being drawn each frame using "tex2D(ScreenSample, texCoord.xy)", which accesses the ScreenSample sample2D. Is there any way to add more samples so that I can access each frame's velocity and pressure? I've searched for quite a while and have only found remotely similar solutions in GLSL, and most access static textures. I hope there is a fairly straightforward solution!

Thanks,
Doug

Unless you're doing something I'm unfamiliar with, you just have to add more texture samplers:

Check out the Sprite Effects sample (on this site) for some code to look at.

One thing to watch out for.

IF you are using spritebatch with your shaders, it will grab texture channel 0, so either make sure the texture you pass to spritebatch matches what is in your shader, or leave channel 0 alone.

You specify shader channels by adding :register(s1) to the sampler definition.

sampler ColorMapSampler : register (s1) = sampler_state

Just one thing that has caught me out in the past.

Thanks a lot guys. I was traveling this weekend and then the forum was down last night, so I have only just now figured it all out. I haven't incorporated it yet, but judging by the sprite effects tutorial it should work fine for what I'm looking at doing.

Thanks a ton,
Doug

I've spent quite a bit of time looking into coding for the diffusion in a shader, but have been hung up on saving the texture using floating point variables for colors. I believe that the floating point values are truncated to the closest 1/256th when the texture is saved, so I can't incrementally change the value by anything less than 1/256th of the maximum pressure. This makes the diffusion look clunky and doesn't allow for it to work with small diffusion rates or subtle diffusion between relatively similar values.

Any insight or proposed solutions would be much appreciated.

If you would like to look over my coder, here is the C# code used to incrementally change the pressure texture:


And the HLSL code that is called:

The precision of texture data depends what format you store that texture in. By default you are most likely using SurfaceFormat.Color, which has 8 bits per channel.

If the values stored in this texture are all very small, or have a limited range, you could scale and/or bias the values in the texture to spread them over the full 0-1 range (which will obviously increase precision) then reverse that scaling before using the value in your shader.

If you really need the full 0-1 range with >8 bit precision, you will have to use a texture format with more bits per channel. 16 bits per channel and floating point texture formats are supported on some of the more modern GPU's, but not universally on all hardware, so this really depends what cards you aim to run this program on.

Thanks Shawn. I actually just solved the problem earlier today by encoding a 24bit floating point value into the R, G and B color values of the texture. Because this is a texture for the pressure at different points in the fluid I only need it to represent one value. For velocity, however, I'm going to probably have to use R and G for x velocity and B and A for y velocity. The problem with scaling over a 0-1 range was that, if I wanted to incrementally change the value by something less than 1/256, then the value would be reverted back to it's original value after I saved the texture again.

Thanks for all the great advice!
Doug