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Sampling the Render target while rendering to it


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#1 Naruto-kun   Members   -  Reputation: 339

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Posted 25 August 2014 - 11:46 AM

Hi guys

 

I am working on a project where I need to sample the pixel on the render target before the pixel shader returns so I can determine what blend operation I want to use on it (I have several that are color based). Is there any way to sample the render target like this?

 

Thanks

JB



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#2 Yourself   Crossbones+   -  Reputation: 1189

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Posted 25 August 2014 - 12:20 PM

I have done it in the past using an UAV and in-place (un)packing.

See this msdn article for more info



#3 HappyCoder   Members   -  Reputation: 2879

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Posted 25 August 2014 - 12:44 PM

Another option would be to use two seperate render targets. You render everything to one target that doesn't require any special treatment, then you copy the contents of that render target to another render target. You then use one as the input and one as the output.



#4 Buckeye   Crossbones+   -  Reputation: 6298

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Posted 25 August 2014 - 12:45 PM

Although it can be done as Yourself mentions, note that it's for a single format - DXGI_FORMAT_R32_UINT - which may not be compatible for color sampling.


Please don't PM me with questions. Post them in the forums for everyone's benefit, and I can embarrass myself publicly.


#5 MJP   Moderators   -  Reputation: 11761

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Posted 25 August 2014 - 03:30 PM

Although it can be done as Yourself mentions, note that it's for a single format - DXGI_FORMAT_R32_UINT - which may not be compatible for color sampling.

 

Indeed, which is why he mentioned having to do manual packing and unpacking. Another possible workaround is to use a StructuredBuffer instead of a texture.



#6 SeanMiddleditch   Members   -  Reputation: 7170

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Posted 25 August 2014 - 04:16 PM

I am working on a project where I need to sample the pixel on the render target before the pixel shader returns so I can determine what blend operation I want to use on it (I have several that are color based). Is there any way to sample the render target like this?


I don't quite understand the use case, but generally you don't need to sample the render target itself. Since you're in the process of overwriting the value in the render target in your pixel shader, you should already have the (pre-blending) color you're about to write out.

If you need the value after the blending operation is done, this isn't possible. The pixel shader runs before blending (some hardware may interleave the operations, but this isn't guaranteed to happen by the spec), so there's no possible way to have the result.

The best way to achieve this effect is to use multiple passes. In your first pass, write out the new color values. In the second pass, read in the generated texture bound to the first pass's render target and use that to write out to a second render target.

You can also consider making use of the stencil buffer instead of color, as this may be faster for many hardware implementations. Write out a different number to the stencil buffer for each desired blend operation, then do a pass over the input to the output for each blend predicated on the corresponding stencil value. This both lets the hardware do early outs for pixel shader evaluation as well as make it easier to use the built-in blending hardware (I'm not sure what you're trying to do so I don't know if that matters or not).

For a lot of hardware, multiple passes with simple shaders can be more efficient than trying to make a single complex shader that does everything in one pass. It depends on target resolution and color accuracy and the hardware's shader execution cores vs its memory bandwidth.

#7 vstrakh   Members   -  Reputation: 503

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Posted 26 August 2014 - 02:50 AM

+1 for HappyCoder solution.

With one "fix" - don't copy content of render target. Just swap them between passes.



#8 Naruto-kun   Members   -  Reputation: 339

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Posted 27 August 2014 - 06:58 AM

Ok what im trying to do is control the color of a bunch of billboard sprites so that for a specific number of sprites overlapping, will give me a specific color that I use in a post processing pixel shader for a type of map. Setting the blend state to SRC_COLOR/INV_SRC_COLOR with a ADD operation doesn't add the color of the sprite in front to the one behind. I actually end up with a color that is darker than the original. Any ideas?



#9 vstrakh   Members   -  Reputation: 503

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Posted 28 August 2014 - 02:44 AM

Please clarify.

You want to know how many sprites overlapped after some drawing in total, or you want to have some specific color (depending on its order) on every single sprite?



#10 Naruto-kun   Members   -  Reputation: 339

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Posted 28 August 2014 - 04:43 AM

the former.



#11 Yourself   Crossbones+   -  Reputation: 1189

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Posted 28 August 2014 - 09:38 AM

If you want a overdraw 'debug' shader, you should simple render the scene using additive blending with a static color (for example float4(0.1, 0.1 ,0.1 ,1) ).

places with little overdraw will be quite dark while places with a lot of overdraw will be white.

 

In case everthing is black or white (the minimum/maximum), simple adjust the fixed color. for example change it to float4(0.25, 0.25 ,0.25 ,25) if it is to dark, float4(0.02, 0.02 ,0.02 ,1) if it is to bright. Even better is to have a user-controlled slider ;)

 

You could do it dynamically (determine the blend color based on the minimum/maximum overdraw count) but that will be quite a lot of work for a debug feature... If you chose to calculate it dynamically, I recommend something like this



#12 Naruto-kun   Members   -  Reputation: 339

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Posted 28 August 2014 - 12:26 PM

Sorted. It was a lack of understanding how the blend state works. Changing from SRC_COLOR/INV_SRC_COLOR to ONE for both the source and dest blends gave me what I wanted.



#13 Jason Z   Crossbones+   -  Reputation: 5336

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Posted 30 August 2014 - 10:59 AM

I'm not sure if it applies to your situation, but it is pretty common to use the stencil buffer for counting the number of times that a pixel is touched during rendering.  Perhaps that could be suitable in your situation?



#14 Naruto-kun   Members   -  Reputation: 339

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Posted 31 August 2014 - 09:54 AM

I am using it for other info. Its a weather radar simulation where I determine the area of the cloud with the highest density in order to create the precipitation zone and then render the return.






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