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A new implementation of the Array Modifier, without BMesh, gives 100 fold performance improvement
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Description

Patch D443 is a proposition for a new implementation of the Array Modifier, that does not use BMesh.
Modifiers using BMesh have very poor performance.

This patch is a proposed new implementation. It is identical in its features and results. It gives a performance improvement of more than 100 fold and more when merge option is not selected, and of around 10-20 with merge option checked. These improvements are measured without OpenMP, another /2 or /4 factor could be gained through multi-threading, which is much easier to implemement on the simple loops of direct derived mesh processing.

In this patch is a proposed implementation of doubles detection that is inspired from the algorithm used in the "Remove Doubles" operator, but with a few differences, using separate sorted arrays for target and source, and I think a slightly improved performance. This map_doubles() function could be added to cdderivedmesh.c, and made available to other modifiers.

The new implementation of the array modifier also uses CDDM_merge_verts() from cdderivedmesh.c, which up until now was only called by the mirror modifier.

I understand there is some risk in overhauling such rock-solid old work horse as the array modifier, but a 100 times gain is a lot, I believe it's definitely worth it, it can be the difference between a 10 seconds wait and a 0.1 second result.

Although the implementation is fully functionnal and has been tested in various conditions, it is meant for evaluation purposes only at this time. Depending on the "count" value of modifier, it will call the former implementation (for odd count values), or the new implementation (for even count values). It will also print the delay to console so that one can compare both implementations.

I have another refinement in mind, where the mapping of doubles performed at rank n of the array would be cached and re-applied with an offset at rank n+1. This would approximately cut time by half when merge is on.

One last word: the doubles mapping algorithm, very much inspired from the remove doubles operator and slightly improved, uses a sorted array of vertices according to the sum of x+y+z. Then candidate doubles are tested from within -3d to +3d of a given vertex. Actually 3 could be replaced by sqrt(3), or ~1.74, which is a (admittedly minor) optimization that could be put into remove doubles operator as well.

Event Timeline

Patrice Bertrand (PatB) claimed this task.
Patrice Bertrand (PatB) raised the priority of this task from to 90.
Patrice Bertrand (PatB) updated the task description. (Show Details)
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Campbell Barton (campbellbarton) lowered the priority of this task from 90 to Normal.Apr 9 2014, 12:48 PM

Hi,
For those who may be interrested, I'd like to say a few words on the merge doubles algorithm based on the sum of x, y, z coordinates. I picked it up from the remove doubles operator, and tried to improve it very slightly. But this algorithm is actually very poor. When I first saw it, I thought "How clever, the way it manages to handle all 3 coordinates into a single dimention array, which will be sorted, then processed in order". But this transformation of a 3 dimension complex problem into a 1 dimension simple problem is really a complete illusion: it brings very little gain as compared to an algorithm that would simply process vertices based on just their X coordinate, and for each x, would scan all vertices with X within [x-d; x+d]. Think of it this way, in 2 dimensions for a start : imagine all vertices are random points within a square of size DxD. Algorithm A1 would sort by x coordinate only, and compare all vertices within [x-d; x+d], scanning x for all source vertices. Algorithm A2 sorts by (x+y) and compares all vertices such that s=x+y is within [s-2d; s+2d], or best [x-sqrt(2)d; x+sqrt(2)d]. The average number of vertices to be scanned is, in both cases N x d x D. The only difference is that instead of scanning vertical strips, we are scanning diagonal strips around the line of equation y= x0 - x .

This algorithm is an illusion. For a given d (merge distance), the amount of processing is still in N^2 in 2 dimensions, and in N^3 in 3 dimensions. The only benefit maybe, is that in many mesh objects, there is some kind of alignment of vertices with one or more of the X, Y, Z axis, so scanning in diagonals might have a tiny bit of benefit.

The algorithm which I propose in the latest diff is totally different. It implies some setup overhead, but can be hugely better when d/D (merge distance over size of object) gets big, and thus the number of candidates to be processed explodes.

Bastien Montagne (mont29) changed the task status from Unknown Status to Unknown Status.Aug 11 2014, 3:51 PM

Closing that task, no need to keep both open. :)