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# JoeJ

Member Since 30 Aug 2006
Offline Last Active Nov 24 2015 10:33 AM

### equations of motion for position dependent acceleration

18 October 2015 - 03:16 PM

Hi,

i'd like to calculate position, velocity and acceleration at a given time
when acceleration depends linearily on position.

Here is some code using integration to show what i mean:

```	float start = 0.8f;
float accFactor = -5.0f;
float timestep = 0.008f;
float p = start;
float v = 0;
float a;
for (float t = 0; t <= 2.0f; t += timestep) // position dependent acc
{
a = (1.0f - p) * accFactor;
v += a * timestep;
p += v * timestep;

float aA = how to calculate acceleration without integration?
float vA = ?
float pA = ?

RenderPoint (2, sVec3 (t, 5+aA, 0), 1,1,1);
}
```

Here is another snippet just for reference with constant acceleration.
No problem there to claculate position and velocity.

```p = start;
v = 0;
a = -5.0f;
for (float t = 0; t <= 2; t += timestep) // constant acc
{
v += a * timestep;
p += v * timestep;

float pA = start + 0.5f * a * t*t;
float vA = a * t;
}
```
The jpg shows plots of this code, pos = red, vel = green, acc = blue (constant acc right).

Any help appreciated!

### Slow down when using OpenMP with stl vector per thread

14 June 2015 - 12:05 PM

Hi,

I'm new to OpenMP and for the second time i notice a slow down in code like this:

const Graph::GraphData &graph = graphPerPart[part];

#pragma omp parallel for
for (int borderIndex = 0; borderIndex < newBorders.size(); borderIndex++)

{
int source = borders[borderIndex].FirstVertex(*mesh);
int target = borders[borderIndex].LastVertex(*mesh);

std::vector<float> min_distance;
std::vector<int> previous;

Graph::DijkstraComputePaths (source, graph.vertex_adjacency_list, min_distance, previous); // this grows both vectors

newBorders[borderIndex].DoSomethingWithTheClosestPath (previous, target, ...); // this also grows some std::vectors
}

OMP code runs only 0.75 times as fasat as single threaded (VS 2013, quad core).

I guess it is because each thread has to manage memory for the stl::vectors - can this be true and is there a clue to fix this?

This is for a preprocessing tool and avoiding stl surely isn't worth the time,

but usually i get the expected speed ups with very little affort so i'm curious.

Also i'd like to know if OpenMP can get closely as fast as other multithreading techniques for runtime code, because it's so easy to use.

### VC Debugging too slow

05 May 2015 - 08:33 AM

Hi,

Debug mode became very slow for me somewhen the last months, on both VS 2013 Community and VS 2012 Express.

Sometimes it takes a few seconds to step over a very simple code line, but using STL it becomes a nightmare.

Example:

mesh->BlurVertexMap (minVal, maxVal, temp, vertexHeight);

-> This function takes just the time i need to press the key,

note the function uses a lot stl::vectors (see below, marked in red)

for (int i=0; i<mesh->mVertices.size(); i++) vertexHeight[i] += temp[i];

-> this simple addition of two stl::vector<float> (size:8000) takes 2 MINUTES

I've tried _ITERATOR_DEBUG_LEVEL=0, but no effect.

Is this a common problem?

The project is about 10 years old and links to old libs. Maybe something is messed up.

void BlurVertexMap (float &minVal, float &maxVal, std::vector<float> &out, const std::vector<float> &in) const
{
minVal = FLT_MAX;
maxVal = -FLT_MAX;

for (int i=0; i<mVertices.size(); i++)
{
const Vertex &vert = mVertices[i];
float area = mVertexArea[i];
float val = in[i] * area;
for (int j=0; j<vert.edgeIndices.size(); j++)
{
int nI = mEdges[vert.edgeIndices[j]].OppositeVertex(i);
area += mVertexArea[nI];
val += in[nI] * mVertexArea[nI];
}
val /= area;
out[i] = val;
if (val < minVal) minVal = val;
if (val > maxVal) maxVal = val;
}
}

### Can i do this using the fourier transform?

15 April 2015 - 07:06 AM

Hi,

i'm working on a mesh preprocessing tool and have the idea to calculate neighbourhood information for each vertex to detect things like edge flow, convex / concave regions etc.

E. g. to sum up neighbouring convexity of polygons i'd project their center to the vertex normal plane to get 3 values:

Angle (representing position to vertex), polygon area / distance relation (=weight) and polygon convexity (=value).

To sum this up using a 1D FT, and looking at the attached picture, the black dot represents a single polygon, one times with large area (red) or with small area (blue).

Is there a way to add such a sample to a FT for each polygon, one after another?

Actually my solution would be: Don't use FT at all and do this using a simple 1D array without any cool math.

But if this is a common problem with an elegant solution please let me know - my math background is so weak i don't know what search terms to use

### Compression questions

19 November 2014 - 09:27 AM

Hi there,

i've some questions about compression tricks in mind.

Because trying those all is a lot of work i'd like to hear anything from people who already have some experience.

I'm using OpenCL on GPU, but API should not matter here.

1. E.g i have a normal in xyz and a radius in w component of a vec4, but now i want a cone angle too. I see those options:

Either convert the normal to spherical coords so needing only two values.

Or pack two half floats in the last component.

Or create another buffer or tex to store the additional stuff.

Is there an answer like 'mostly option 2 is the fastest way, because trig is slow',

Or is it more like 'it totally depends on usecase'?

2. I do GI and cover the geometry with samples at 10x10cm resolution.

Can i expect using half floats for everything (position, direction, color...)

should be accurate enough for a 1km game world?

Is this usually done only for memory savings or can it help / hurt performance too?

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