Hello,

I'm trying to generate deep ocean waves according to the Tessendorf model (Phillips spectrum + FFT). So I've got two compute shaders, one that runs at the start of my program and generates the base heightfield in frequency space "h0", and another which runs every frame and generates the frequency heightfield according to the current time ("ht"), and performs a two-dimensional FFT over this field to produce the final heightfield containing the wave heights at a certain time t.

My heightfield is 512x512 in size, so my compute shaders run on 512 threads, and each loops over the row/column that it's currently processing; so the first compute shader, each thread processes one row, and in the second compute shader, at the first pass each thread processes a row, and then processes a column.

But it doesn't work. This is the output I'm getting at the various stages of the program (mapped onto a vertex grid by displacing the vertex heights):

- Generation of "h0"



The first FFT pass (on the rows of "ht"):



And the final heightfield after both FFT passes is.. well.. apparently it's just a bunch of NaN's. I don't know what I'm doing wrong, I'm getting no debug warnings from DX11 so I suppose it's an algorithm problem. But really this is difficult, because I have no reference at all to work with except a paper of which I don't understand half the math and a piece of CUDA code for generating "h0" and "ht" (CUDA has an inbuilt FFT so I can't see how they do it). And using CUDA is not an option because I have an ATI card (this also means I can't run the reference code and compare outputs at various stages). I tried several different FFT implementations and they all produce the same result... I've been spending days on this and I'm getting desperate. Could anyone help me? Here is the shader code:

Generation of h0:http://pastebin.com/N3SXHeNj

FFT with ht calculation: http://pastebin.com/Br2qjjB3

My global.hlsl file (contains utility functions and constants): http://pastebin.com/tt5qmvp5

And the "complex.fxh" file is just complex arithmetic (which works perfectly because I use it in my fractal renderer too).


The FFT stage works as follows: set the h0 heightfield as a shader resource (texture2D), then perform the "ht" calculations along with the FFT row pass, and output the result to the output buffer. Then, copy the output buffer to the input buffer, and perform the FFT column pass on the input (for each pass the FFTPASS constant buffer member is updated).

Does anyone know where the problem is?? Please tell me if you need more information.. I'm already not that good in higher-level mathematics so all these complex equations are really confusing. I really tried to understand them and implement them the best I could but I'm at my wit's end here.

Thanks!!

I'm also getting artifacts where certain rows/columns of the heightfield will seem to change wildly from frame to frame (but only very select ones, and always the same ones), as if there was uninitialized memory somewhere (if that makes sense).

Debugging compute shaders can be painful, but it's possible. For AMD GPU's you can use GPUPerfStudio, although I'll warn you that it's very clunky. Another low-tech way of debugging/verifying results is to simply copy your compute shader resolutions to a staging buffer/texture so that you can read it back on the CPU. It will kill your performance, but it works.

Right, well I just found out something... I managed to get the FFT more or less working (it now looks somewhat like water, just with some high frequency components at the edges, dunno why, will debug, probably a texture addressing mistake - I'll try porting the FFT to the CPU and see what the result is), and so I'm left with this weird vertex stuttering issue I described in my last post. However I discovered something: when I run my code I get the stuttering, however if prior to starting the exact same code I launch some GPU-intensive program like a benchmark (furmark, etc..), then the stuttering disappears!!

Could this be a driver bug, or is my card fried or something? It does tend to fail on some particular applications and games but overall this is the first time I see it artifact like this. What's going on? No wonder I can't produce working stuff if I can't trust that my hardware respects logic, lol.




I tried the reference rasterizer and it seems to not artifact in REF mode but it's so damn slow I can't really be sure.




EDIT: thoroughly tested RAM/VRAM with memtest86 and memtestCL... no issues. I'm going with driver bug of some sorts

Ok I ported the code to the CPU and it turns out I needed to apply sign correction to the real parts of the resulting FFT (for some reason). Going to port it back to the GPU and see if it works now. Once I get that working I'll be able to focus on calculating the normal and "chopiness" FFT's.

CPU heightmap result (the rasterization code is kind of rushed but at least it shows the FFT is working):



Although that doesn't explain the GPU glitch I was seeing before - I guess I can live with it but it's bugging me.

Ok, it worked and I now get some nice FFT waves. But I'm completely stumped (once again ) on how to get the analytic normal vectors. I don't quite understand the formula given in the PDF (something with multiplying the fourier amplitudes of each point by "ik" where k is the frequency vector for that point). I have no idea how it's possible to get a 3D normal vector out of a single 1D value obtained from the normal FFT... all the implementations I've found on the net just resort to finite differences but I want to use the analytical normal because it should look better right? Really the PDF doesn't elaborate at all on how to obtain the normal, it just goes "the slope vector can be obtained like this [insert large formula]", I guess it must be obvious or something but I really don't get it. Could someone shed some light on how to do this please? I would be very grateful!




PS: the paper I'm talking about is this one: graphics.ucsd.edu/courses/rendering/2005/jdewall/tessendorf.pdf

Update: found this online after a last ditch effort for finding something on google:

http://www.keithlantz.net/2011/10/ocean-simulation-part-one-using-the-discrete-fourier-transform/

(there is a second part too which is important as well)

It covers pretty much everything I was confused about and actually contains useable information for people like me, perhaps others will find it useful.