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POPerformance: Offline surface for hit testing vs Triangle Intersection
 

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  1. POPerformance: Offline surface for hit testing vs Triangle Intersection
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    <p>First, a disclaimer. I'm well aware of the std answer for X vs Y - "it depends". However, I'm working on a very general purpose product, and I'm trying to figure out "it depends on what". I'm also not really able to test the wide variety of hardware, so try-and-see is an imperfect measure at best.</p> <p>I've been doing some googling, and I've found very little reference to using an offline render target/surface for hittesting. I'm not sure of the nomenclature, what I'm talking about though is using very simple shaders to render a geometry ID (for example) to a buffer, then reading the pixel value under the mouse to see what geometry is directly under the mouse pointer. </p> <p>I have however, found 101 different tutorials on doing triangle intersection, a la D3DXIntersect &amp; DirectX sample "Pick".</p> <p>I'm a little curious on this - I would have thought using HW was the standard method. By all rights, it should be many orders of magnitude faster, and should scale far better.</p> <p>I'm relatively new to graphics programming, so here are my assumptions, for you to disabuse.</p> <p>1) A simple shader that does geometry transform &amp; writes a Node + UV value should be nearly free.</p> <p>2) The main cost in the HW pick method would be the buffer fetch, when getting the rendered surface back off the GPU for the CPU to read over. I have no idea how costly this is. us? ms? seconds? minutes?</p> <p>3) This may be obvious, but I am assuming that Triangle Intersection (D3DXIntersect) is only possible on the CPU.</p> <p>4) A possible cost people want to avoid is the cost of the extra render target(s) (zbuffer+surface). I'm a'guessing about 10 megs for 1024x1280 (std screen size?). This is acceptable to me, although if I could render a smaller surface (trade accuracy for memory) I would do so (is that possible?).</p> <p>This all leads to a few thoughts.</p> <p>1) For very simple scenes, triangle intersection may be faster. Quite what is simple/complex is hard to guess at this point. I'm looking at possible 100s of tris to 10000s. Probably not much more than that.</p> <p>2) The HW buffer needs to be rendered regardless of whether or not its used (in my case). However, it can be reused without cost (ie, click-drag, where mouse tracks across a static scene)</p> <p>2a) Possibly, triangle intersection may be preferable if my scene updates every frame, or if I have limited mouse interaction.</p> <p>Now I've finished writing, I see a similar question has been asked: (<a href="https://stackoverflow.com/questions/18270747/3d-graphics-picking-what-is-the-best-approach-for-this-scenario">3D Graphics Picking - What is the best approach for this scenario</a>). My problem with this is (a) why would you need to re-render your picking surface for click-drag as your scene hasn't actually changed, and (b) wouldn't it <em>still</em> be faster than triangle intersection?</p> <p>I welcome thoughts, criticism, and any manner of side-tracking :-)</p>
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    1. USFrozenKiwi
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      1. POPerformance: Offline surface for hit testing vs Triangle Intersection
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    1. COCould you elaborate on this click-drag scenario? In my mind, when you click and drag something the selected object does not change while dragging. So you would only need a single selection test when the drag sequence was initiated.
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      1. USAndon M. Coleman
    2. COBy the way, triangle intersection (or more generally, any sort of proxy geometry intersection test) is _usually_ faster as it does not require CPU/GPU synchronization. There are scenarios where pixel-perfect selection tests are preferred - situations where the GPU does dynamic LOD/visibility independent of the CPU (e.g. tessellated geometry or occlusion culling), etc. If your use-case can tolerate a multi-frame latency, a single pixel read-back is not all that expensive, especially if you are using deferred shading and already pack material/object id into the G-Buffers.
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      1. POPerformance: Offline surface for hit testing vs Triangle Intersection
      1. USAndon M. Coleman
    3. CORegarding your point 4 above. You don't need to use a screen-sized buffer to render into, or trade any accuracy. You can render into a tiny buffer using a narrow projection pointing in the direction of the mouse click.
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      1. USGuyRT
 

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