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Having problems getting soft particles to work

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#1 Telanor   Members   -  Reputation: 1359


Posted 10 June 2013 - 06:22 PM

I've been trying to implement soft particles as described in the nvidia paper. I'm having a problem with this part: "If we want to compare consistent depth values, the fetched value Zbuf needs to be transformed into projection space". I'm using log-depth, so I can't use the formula they're using. They also never explain what any of the variables in the equation are or why the comparison needs to be done in projection-space and not "Normalized device coordinate" space.

I know how to reconstruct a world-space position from a log-depth value from a previous topic, so I tried several different ways to get it to work for the soft particles.

For reference, this is the equation for the reconstruction:
float depthVal = DepthMap.Sample(depthSampler, texCoord).r; 
depthVal = (pow(0.001 * FarPlane + 1, depthVal) - 1) / 0.001;
depthVal /= (1 * FarPlane) / (FarPlane - 1); //The 1 is the near plane value

float2 invProjPos = mul(input.ScreenPosition.xy * depthVal, InverseProjection);
float4 position = mul(float4(invProjPos, -depthVal, 1), InverseView);

position /= position.w;
So I figured to get the projection-space value, the equation should be just the first 3 lines. Next, I figured the particle's Z value should be calculated like this (from the vertex shader):
float4 worldPosition = mul(float4(input.Position, 1), instanceTransform);
float4 viewPosition = mul(worldPosition, View);
output.Position = mul(viewPosition, Projection);

output.Position.z = log(0.001 * output.Position.z + 1) / log(0.001 * NearFar.y + 1) * output.Position.w;
output.Z = output.Position.z;
Now I know the value output to the depth buffer will be divided by the W normally, so I also tried changing the last line to:

output.Z = output.Position.z / output.Position.w;

In either case, the result is the same: no fading what-so-ever. The particles still have their hard edges. I'm not really sure what I'm doing wrong here and was hoping someone could point it out.

Full shader code:


#2 Ashaman73   Crossbones+   -  Reputation: 8001


Posted 11 June 2013 - 01:06 AM

As long as you use some kind of linear z distribution it should not matter in which space you are. Best to use either camera/eye space or projection space, which ever is easier in your engine. In this case world space is not really useful, in general it is not really useful for billboard/particle/projection handling. I think that you will already align your particles to the camera, therefor you particle will most likely use a constant depth. Use this depth (make it lineare if neccessary) to compare it with the (linear) camera depth of your gbuffer.

#3 Telanor   Members   -  Reputation: 1359


Posted 11 June 2013 - 01:44 AM

Well, as far as I can tell, that is what I'm doing. The gbuffer value I'm reading in should be converted to projection-space and compared with the projection-space particle depth. I must have done something wrong though, because the particles look exactly the same as before; there's no fade-out near the intersection area.

Edited by Telanor, 11 June 2013 - 01:45 AM.

#4 Styves   Members   -  Reputation: 1077


Posted 11 June 2013 - 05:29 AM

Try flipping the order you do the difference or use abs(), your value might just be negative.

#5 phil_t   Crossbones+   -  Reputation: 4109


Posted 11 June 2013 - 01:25 PM

Have you stepped through the shader to see where your values are going wrong?

#6 Telanor   Members   -  Reputation: 1359


Posted 11 June 2013 - 03:42 PM

Ok so I ran through the shader a few times. It seems that input.Z is in the range of 0-1 (which is what the nvidia contrast function seems to expect) and depthVal wasn't, so I removed these two lines:
depthVal = (pow(0.001 * NearFar.y + 1, depthVal) - 1) / 0.001;
depthVal /= NearFar.z / NearFar.w;
Now they're both in the 0-1 range... but all the particles are invisible. Stepping through one of the pixels came out with the following values:

depthVal = 0.210246200
input.Z = 0.184789200
zdiff = 0.025457070
c = 0.001296125

My only guess at this issue is that maybe my Z values aren't linear, but I don't know how to fix that if that's the case.

#7 Ashaman73   Crossbones+   -  Reputation: 8001


Posted 11 June 2013 - 11:31 PM

What is your blending code ?

Here's an example:

depthVal = normalized and linear depth value of g-buffer, 1 is farest
z = current depth of the pixel, normalized and linear

zdiff = depthVal-z;
alpha_blend_factor = smoothstep(0.0, 0.1, zdiff); //<-- play around with the 0.1 value

final_alpha_blend_factor = alpha_blend_factor * particle_color.a;

#8 Telanor   Members   -  Reputation: 1359


Posted 18 June 2013 - 08:35 PM

Sorry for not replying sooner, somehow I didn't get a notification email. I'm using the same blending function from the nvidia sample:

float Contrast(float Input, float ContrastPower)
	//piecewise contrast function
	bool IsAboveHalf = Input > 0.5 ;
	float ToRaise = saturate(2*(IsAboveHalf ? 1-Input : Input));
	float Output = 0.5*pow(ToRaise, ContrastPower); 

	Output = IsAboveHalf ? 1-Output : Output;

	return Output;
The spoiler tag in the OP has the full shader code

#9 Ashaman73   Crossbones+   -  Reputation: 8001


Posted 19 June 2013 - 02:29 AM

Double check your zdiff and depth reconstruction implementation. Btw. why are you using a log-depth buffer, why don't use a linear one ? Nevertheless, the blending seems ok, even if your example pixel will be rendered, the very low c = 0.001296125 value will make it more or less invisible. Use testcode first, until you see the particle squares (like the uncommented output.Color = float4(depthVal * 100, 0, 0, 1); ), then try to add some smooth blending.

#10 Telanor   Members   -  Reputation: 1359


Posted 20 June 2013 - 07:29 PM

Well the reconstruction code looks fine to me but I'm particularly bad at this kind of math, so even if I stare at it for hours it's going to look fine to me with my basic understanding.

We're using a log depth because a) we didn't want all the artifacts associated with the standard z/w buffer and b) it doesn't require disabling early-z. While I probably could switch, it'd be a bit of a pain, and I think it could work with log depth with the right math.

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