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resle

OpenGL [DX10] pixel-precise picking (reading Z values) [Yes, again!]

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Hi, I've been coming back and forth from the forums through the years, since the inception of DX10, which marked my final choice to move on from OpenGL. I've discussed the matters in several threads but still I couldn't get a straightforward technique about this... simply put: I am trying to pick a full x,y,z point from a rendered scene, using DX10. It's something that was fairly easy in OpenGL by unprojecting the x,y mouse pickpoint and then reading the final z value. Nonetheless, I am still failing to accomplish the same apparently simple task in DX10. Is there anyone perhaps that worked on this previously? Any hints? Thanks in advance andrea

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Quote:
Original post by majek
There's a picking example included the DirectX SDK that uses D3D10, it uses the ID3DX10Mesh::Intersect function.


Indeed... but that has nothing to do with my request :)

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Quote:
Original post by MJP
Quote:
Original post by resle
It's something that was fairly easy in OpenGL by unprojecting the x,y mouse pickpoint and then reading the final z value.


By that, do you mean sampling a value from the z-buffer?


Yes exactly: sampling a value from the z-buffer at given x,y coords, just after the whole frame has been rendered.

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http://www.gamedev.net/community/forums/topic.asp?topic_id=568139

There's a brief discussion of it here. I believe it's a lot more complex than it used to be with OpenGL :(.

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I haven't done this myself, but I believe the best way to do this with D3D10 is to create a Texture2D resource with D3D10_USAGE_STAGING that has the same dimensions and format as your depth-stencil buffer. Then you use ID3D10Device::CopyResource to copy the data from your actual depth-stencil buffer to your staging resource, and you can then use ITexture2D::Map() to read the data.

You'll want to be careful though if performance is a concern, since doing a CopyResource to CPU memory will cause a stall to ensure that the GPU processes all pending commands before the data is copied. You can avoid this by double or triple-buffering your buffer so that you're not copying from a resource that is currently being used by the GPU.

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It would be better to use CopySubresourceRegion to copy just the relevant pixel. Moves a lot less data to the PC side. However, copying depth this way is only supported on DX10.1 and higher.

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Thanks Matt, thanks ET3D, and... yes Burnhard, it's a lot more complex than OpenGL's way. I wonder if doing that stalled GL drivers too or the architecture is completely different.

I will try with copying the... 1-pixel subregion I need, although I remember trying the whole staging/copy/subcopy one year or so ago without succeeding. Maybe I should try it under DX 10.1 rather than 10

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Original post by resle
Thanks Matt, thanks ET3D, and... yes Burnhard, it's a lot more complex than OpenGL's way. I wonder if doing that stalled GL drivers too or the architecture is completely different.

I will try with copying the... 1-pixel subregion I need, although I remember trying the whole staging/copy/subcopy one year or so ago without succeeding. Maybe I should try it under DX 10.1 rather than 10


I'd imagine that it did cause a stall...the problem is a consequence of the way the CPU and GPU work (asynchronously), and isn't caused by anything specific to D3D.

If you don't need to be very precise about your depth value, what you could do is shoot off a series of occlusion queries at increasing depths. That would only give you a range of possible depth values, but I'm not sure if that would be sufficient for your purposes.

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Assuming you plan to always have a decent frame rate then you could use the previous frames depth texture, I'd imagine that would cause less of a slowdown if you do it before starting your render loop for the current frame.

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Quote:
Original post by Rompa
The other guys have covered things pretty well - may I ask *why* you need to pick an exact depth value?


I need to attach some pixel-precise "pins" over geometry that's created procedurally by a geometry shader, so I don't have any cpu-held 3d mesh to perform intersections.

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Original post by MJP
Is it possible then that you could position the pin entirely in the shader by sampling your depth buffer? That would simply things quite a bit for you, and would probably be the best performance scenario.


It would be possible but extremely, extremely complicated too because of the general architecture of the rendering sequence in this software. Performance is not an issue anyway, the software in question wasn't even suppose to be "realtime" initially (biomedical stuff)

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Quote:
Original post by ET3D
It would be better to use CopySubresourceRegion to copy just the relevant pixel. Moves a lot less data to the PC side. However, copying depth this way is only supported on DX10.1 and higher.


Hi ET3D, I hope you're "around" to see this :)

I did what follows with DX 10.0:
A) Created a 1x1 2D Texture with the same format of my depth/stencil buffer, and mae it a staging resource
B) Performed CopySubresourceRegion so that I copy exactly the pixel I need

I get no errors, and the same code works if use that to copy from, say, the backbuffer (despite the format is different!). Only, the returned pixel is always = 0

So I wonder what you mean that copying depth this way is only supported on DX10.1: by initializing the whole system for DX10.1 features rather than 10 would possibly fix things?
Thanks for the clarification(s),

andrea


edit: I noticed this just now

Quote:
CopyResourceSubRegion(...)
Differences between Direct3D 10 and Direct3D 10.1:

Direct3D 10 has the following limitation:
You may not use a depth-stencil resource as a destination.

Direct3D 10.1 has added support for the following:
You may use a depth-stencil buffer or a multisampled resource as a source or a destination.


The docs talk about depth-stencil as a destination anyway, when making a difference between 10.0 and 10.1

[Edited by - resle on May 19, 2010 4:18:40 AM]

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Sorry I missed the PM. :)

I was basing my previous reply on that SDK comment, but reading it again, I think it would be possible to copy from a D24 to an R24 format.

Do you have debugging output enabled (D3D10_CREATE_DEVICE_DEBUG)? Is there any message printed for the copy?

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Quote:
Original post by ET3D
Sorry I missed the PM. :)

I was basing my previous reply on that SDK comment, but reading it again, I think it would be possible to copy from a D24 to an R24 format.

Do you have debugging output enabled (D3D10_CREATE_DEVICE_DEBUG)? Is there any message printed for the copy?


Thanks a lot for coming back to the thread, your help has been invaluable to me in the past :)

IT's indeed possible to copy from D24 to R24 in general, I did that for other purposes. I've enabled DEBUG (and using the 10d.dll), but there's no error message.

The last thing I can do is stripping the whole zbuffer reading thing out of the engine and writing a simple app around it, something that does that and nothing else, and see if it works...

Just out of curiosity, as far as you know, does reading back the zbuffer stalls the pipeline in OpenGL too?

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It's hard to see how it wouldn't, given that both D3D and GL are layers on top of the same piece of hardware.

I'm back looking at this question again, except with D3D9. I'm veering towards a good old ray/mesh intersection test (optimised for the particular cases I have - i.e. a height-field). Depending on your data and any acceleration structure you're using, it might be the more flexible and overall the faster approach. Note that while your CPU is executing the code to do the intersection test, the graphics card can be busy rendering your scene - so you may win on performance there in any case, depending on the specific scenario you have.

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I'm veering towards a good old ray/mesh intersection test (optimised for the particular cases I have - i.e. a height-field). Depending on your data and any acceleration structure you're using, it might be the more flexible and overall the faster approach.


Only, I can't intersect cause I have no geometry data to use. It's a complicated scenario, and I don't even really have to "pick" something, I actually need the precise pixel in x,y,z space - that's it.

Anyway, in the loooong loooong loooong ago when Opengl 1.1 was the only choice out there, I've actually used that to perform geometry-independent picking. Think about it: you get the precise position of a point in x,y,z space. Good. Then you check this point againist the bounding box of an object, which may be a cube or a 10,000 triangles super complex shape. And that's all, you don't even have to transform geometry, interpolation or bone animation or whatever is automatically taken into account without further calculations.

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When you get wrong answers the usual suspects are a driver bug or a bug in your app.

If you get the correct result when you use the reference device, it could be a driver bug. Many years ago (early D3D9 days) both NVIDIA and ATI had a bug where GetRenderTargetData didn't work correctly for small render targets (NVIDIA drivers could even freeze Windows in some cases). If it's a similar bug, you can do what I did then, which is report the bug and use a larger surface (such as 16x16) as a workaround.

If the reference device returns the same result, try checking PIX, to see where things go wrong. (You can probably just use PIX in the first place, if the reference device is too slow.)

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