I don't know if this will be of use to you, but this topic has remembered
me of this paper:
Ingo Wald, Hans-Peter Seidel, Interactive Ray Tracing of Point-based Models
(2005)
http://www.sci.utah.edu/~wald/Publications/2005/pointbased/pointbased.pdf
On Wed, Mar 25, 2015 at 1:23 PM, Kalpit Thakkar <ceasy...@gmail.com> wrote:
> Hi Sean!
> So here is my progress in terms of understanding how would I apply plate
> mode raytracing to the B-Reps :
>
> 1. To assign an implicit thickness to the surface, I don't need to
> actually sample and find an offset surface - rather I can just assign that
> implicit thickness at a point F(s, t) of the surface whenever required, by
> adding an offset in the direction of (and opposite as well) the normal at
> (s, t).
> 2. Instead of building the BVH (which is highly inefficient right now)
> from the surfaceTree, I will find the bounding boxes of all the faces in
> the faceList of the B-Rep and find the final Bounding Box.This will be the
> prep step for the plate-mode raytracing.
> 3. Once the prep is completed, the shot function will be called, and the
> Newton's method will be used for intersecting the ray with each face in the
> faceList. We now sort the hitList so that we have the nearest hit first and
> farthest hit last.
> 4. To handle the cases of intersection with corners, they will be
> intersected separately (as "in" and "out" won't be obtained in cases of the
> corners).
> 5. Now, there can be duplicate hits in the hitList as we are simply
> iterating the faceList of the B-Rep rather than using a hierarchy which
> avoids overlap. Once we have the histList ready, we call the a function
> that creates segments (I will have to write a new one here!). It evaluates
> the hits, removes the duplicate hits, checks if a ray is grazing the
> surface and if so, uses the FILO approach to evaluate the hits ("in" and
> "out") as there will be multiple entries and exits. Once done, we return
> the seghead filled.
>
> This approach is almost similar to the approach used in BoT besides the
> fact that the intersection was fairly trivial there as it was an
> intersection with a triangle and here it is an intersection with a surface
> and hence it is rather complex than triangles.
>
> The only thing I believe that could be remaining now is thinking about any
> limitations that B-Reps might pose to Plate-mode raytracing. Other than
> that, I believe I have put in my earnest efforts to find a reasonable
> enough a solution to propose for the project.
>
> Waiting to listen from you. Eagerly.
>
> With Regards,
> Kalpit Thakkar
>
> PS : I am having lots of fun in learning this stuff and I'm really
> excited. Please reply when you get time, I'm waiting. :)
>
> On Tue, Mar 24, 2015 at 7:49 PM, Kalpit Thakkar <ceasy...@gmail.com>
> wrote:
>
>> Hello Sean!
>> So I tried to understand plate-mode implementation for triangle meshes
>> through BoT primitive as mentioned in the project description page. The
>> file "bot.c" includes "g_bot_include.c" (Is it a good practice to include a
>> .c file?), which has all the required functions to raytrace a BoT according
>> to the precision required (float or double). Now, I observed the following
>> things after studying it :
>>
>> Function rt_bot_prep_pieces_ :
>> Routine to find the bounding RPPs of the pieces of the BoT. It starts
>> with assigning a thickness value to a variable "los", by performing some
>> sort of bit test using (BU_BITTEST(bot->bot_facemode, surfno)), which makes
>> it assign either full thickness on both sides of the plate, or a thickness
>> centered about the plate (1/2 on both sides). After that an "offset" is
>> found using this thickness value assigned to "los" in the direction of the
>> unit normal at each vertex and the face is offset by that amount, to
>> provide it a thickness (on both sides). Finally, this is assigned to the
>> "min" and "max" vectors of structure representing the solid (struct soltab).
>> Return type : void
>>
>> Function rt_bot_prep_ :
>> As usual, it checks the validity of the database record being the
>> required primitive (here, BoT) and then finds the bounding RPPs required
>> for raytracing if the validity test passes (Uses the rt_bot_pieces_prep_
>> function). In this case, it also finds the approximate sphere and a
>> bounding sphere for the BoT.
>> Return type : int (-1 on failure, 0 on success)
>>
>> Function rt_bot_plate_segs_ :
>> Given an array of hits, it provides us the segments of the BoT is the bot
>> mode is RT_BOT_PLATE / RT_BOT_PLATE_NOCOS. Now, each segment has it's "hit
>> structure", which tells what is the surface number that the ray has hit and
>> what is the "in" and "out" distances of the ray for this purticular
>> segment. So, this hit distance for "in" and "out" points depends on that
>> bit test we performed in rt_bot_prep_pieces_, ofcourse it's performed here
>> again.
>> Return type : int (returns the numberOfHits * 2)
>>
>> Function rt_bot_unoriented_segs_ :
>> Same functionality as function above, for an unoriented BoT Solid. Makes
>> the final number of hits even.
>> Return type : int (return final numberOfHits)
>>
>> Funciton rt_bot_makesegs_ :
>> Make segments of the BoT given the array of hits. This function is called
>> in rt_bot_shot_ and rt_bot_pieces_shot_. Gives us the segments for the hits
>> array given. The interesting thing in this function is the FILO algorithm
>> used for evaluating the "in" and "out" hit points for a ray that grazes a
>> surface and it's comparison with LIFO. The visual diagram provided is just
>> awesome for understanding.
>> Return type : int (return final numberOfHits)
>>
>> Function rt_bot_shot_ :
>> Intersects a ray on the BoT and finds the hits. Sorts the hits from near
>> to far and then finds the segment using rt_bot_makesegs_.
>> Return type : int (return final numberOfHits)
>>
>> Function rt_bot_pieces_shot_ :
>> Intersects a ray with a list of pieces of BoT. Same as above in the
>> intersecting algorithm.
>> Return type : int (return final numberOfHits)
>>
>> I have understood what is plate mode through these functions. If there is
>> anything that these functions might have missed explaining me, please tell
>> me where should I learn it from. I would now proceed to understanding NURBS
>> and how to apply this method to them. I hope to finish understanding things
>> by tonight so that I can start the proposal tomorrow and submit it for
>> review.
>>
>> With Regards,
>> Kalpit Thakkar
>>
>> PS : Can you explain to me what does BITTEST do?
>>
>> PPS : Can you please review my proposal on Object Oriented C++ Geometry
>> API? I have submitted it on Melange :)
>>
>>
>> On Tue, Mar 24, 2015 at 12:25 AM, Christopher Sean Morrison <
>> brl...@mac.com> wrote:
>>
>>>
>>> On Mar 23, 2015, at 2:05 PM, Kalpit Thakkar <ceasy...@gmail.com> wrote:
>>>
>>> > Hello Sean!
>>> > I was thinking of proposing another project and I would like to work
>>> on this project. I checked past 3-4 years' projects, but there hasn't been
>>> any efforts in this direction.
>>>
>>> Indeed. It’s not a hard project, algorithmically speaking, but nobody
>>> has yet put forth a quality proposal to work this area.
>>>
>>> > So, according to the description there, BoT has a raytracing routine
>>> that, when finds an "in" point for a ray, due to extremely thin surface,
>>> assigns the "out" point as the "in" point + the implicit thickness added to
>>> each coordinate of "in" point and I would have to implement the same
>>> methodology for NURBS surfaces now.
>>> >
>>> > To analyze the problems that I would face in implementing the plate
>>> mode raytracing for NURBS, I would have to understand exactly how
>>> raytracing NURBS works. So, is there any specific resource that might help
>>> me understand this aspect quickly?
>>>
>>> Nope. Understanding how NURBS ray tracing works is not something that
>>> can be achieved quickly.
>>>
>>> But you don’t really need to understand it. You need to understand what
>>> plate-mode means — and more specifically you need to understand BRL-CAD’s
>>> existing plate-mode implementation for triangle meshes. If you understand
>>> that, you’ll know exactly what is needed for NURBS plate mode, even without
>>> understanding NURBS ray tracing. This requires a lot of reading of code
>>> and playing with sample geometry.
>>>
>>> Cheers!
>>> Sean
>>>
>>>
>>>
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>>
>>
>
>
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--
Vasco Alexandre da Silva Costa
PhD Student at Department of Information Systems and Computer Science
Instituto Superior Técnico/University of Lisbon, Portugal
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