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  • Find the centroid of a polygon with weighted vertices

    - by Calle Kabo
    Hi, I know how to find the centroid (center of mass) of a regular polygon. This assumes that every part of the polygon weighs the same. But how do I calculate the centroid of a weightless polygon (made from aerogel perhaps :), where each vertex has a weight? Simplified illustration of what I mean using straight line: 5kg-----------------5kg ^center of gravity 10kg---------------5kg ^center of gravity offset du to weight of vertices Of course, I know how to calculate the center of gravity on a straight line with weighted vertices, but how do I do it on a polygon with weighted vertices? Thanks for your time!

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  • How to correctly pass a float from C# to C++ (dll)

    - by RavelT
    I'm getting huge differences when I pass a float from C# to C++. I'm passing a dynamic float wich changes over time. With a debuger I get this: c++ lonVel -0.036019072 float c# lonVel -0.029392920 float I did set my MSVC++2010 floating point model to /fp:fast wich should be the standard in .NET if I'm not mistaken, but this didnt help. Now I cant give out the code but I can show a fraction of it. From C# side it looks like this: namespace Example { public class Wheel { public bool loging = true; #region Members public IntPtr nativeWheelObject; #endregion Members public Wheel() { this.nativeWheelObject = Sim.Dll_Wheel_Add(); return; } #region Wrapper methods public void SetVelocity(float lonRoadVelocity,float latRoadVelocity){Sim.Dll_Wheel_SetVelocity(this.nativeWheelObject,lonRoadVelocity,latRoadVelocity);} #endregion Wrapper methods } internal class Sim { #region PInvokes [DllImport(pluginName, CallingConvention=CallingConvention.Cdecl)] public static extern void Dll_Wheel_SetVelocity(IntPtr wheel,float lonRoadVelocity,float latRoadVelocity); #endregion PInvokes } } And in C++ side @ exportFunctions.cpp: EXPORT_API void Dll_Wheel_SetVelocity(CarWheel* wheel,float lonRoadVelocity,float latRoadVelocity){ wheel->SetVelocity(lonRoadVelocity,latRoadVelocity);} So any sugestions on what I should do in order to get 1:1 results or atleast 99% correct results.

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  • Given a start and end point, how can I constrain the end point so the resulting line segment is horizontal, vertical, or 45 degrees?

    - by GloryFish
    I have a grid of letters. The player clicks on a letter and drags out a selection. Using Bresenham's Algorithm I can create a line of highlighted letters representing the player's selection. However, what I really want is to have the line segment be constrained to 45 degree angles (as is common for crossword-style games). So, given a start point and an end point, how can I find the line that passes through the start point and is closest to the end point? Bonus: To make things super sweet I'd like to get a list of points in the grid that the line passes through, and for super MEGA bonus points, I'd like to get them in order of selection (i.e. from start point to end point).

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  • About floating point precision and why do we still use it

    - by system_is_b0rken
    Floating point has always been troublesome for precision on large worlds. This article explains behind-the-scenes and offers the obvious alternative - fixed point numbers. Some facts are really impressive, like: "Well 64 bits of precision gets you to the furthest distance of Pluto from the Sun (7.4 billion km) with sub-micrometer precision. " Well sub-micrometer precision is more than any fps needs (for positions and even velocities), and it would enable you to build really big worlds. My question is, why do we still use floating point if fixed point has such advantages? Most rendering APIs and physics libraries use floating point (and suffer it's disadvantages, so developers need to get around them). Are they so much slower? Additionally, how do you think scalable planetary engines like outerra or infinity handle the large scale? Do they use fixed point for positions or do they have some space dividing algorithm?

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  • Polygonal Triangulation - algorithm with O(n log n) complexity

    - by Arthur Wulf White
    I wish to triangulate a polygon I only have the outline of (p0, p1, p2 ... pn) like described in this question: polygon triangulation algorithm and this webpage: http://cgm.cs.mcgill.ca/~godfried/teaching/cg-projects/97/Ian/algorithm2.html I do not wish to learn the subject and have a deep understanding of it at the moment. I only want to see an effective algorithm that can be used out of the box. The one described in the site seems to be of somewhat high complexity O(n) for finding one ear. I heard this could be done in O(n log n) time. Is there any well known easy to use algorithm that I can translate port to use in my engine that runs with somewhat reasonable complexity? The reason I need to triangulate is that I wish to feel out a 2d-outline and render it 3d. Much like we fill out a 2d-outline in paint. I could use sprites. This would not serve cause I am planning to play with the resulting model on the z-axis, giving it different heights in the different areas. I would love to try the books that were mentioned, although I suspect that is not the answer most readers are hoping for when they read this Q & A format. Mostly I like to see a code snippet I can cut and paste with some modifications and start running.

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  • understanding floating point variables

    - by Syom
    There is some problem, i can't understand anyway. look at this code please <script type="text/javascript"> function math(x) { var y; y = x*10; alert(y); } </script> <input type="button" onclick="math(0.011)"> What must be alerted after i click on button? i think 0.11, but no, it alerts 0.10999999999999999 explain please this behavior. thanks in advance

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  • Decimal point issue on cocoa app

    - by Manuel Rocha
    I there, I'm trying making my first cocoa app, but I'm having problems with float numbers because of the regional settings. If I write on the TextBox the float number 1.2 I only can get the number 1, but If I write on the same TextBox the same float number but this time with the ',' sign instead (1,2) I can get the right float value. How can I bypass the regional settings? Kind Regards, Manuel Rocha

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  • C++ floating point precision

    - by Davinel
    double a = 0.3; std::cout.precision(20); std::cout << a << std::endl; result: 0.2999999999999999889 double a, b; a = 0.3; b = 0; for (char i = 1; i <= 50; i++) { b = b + a; }; std::cout.precision(20); std::cout << b << std::endl; result: 15.000000000000014211 So.. 'a' is smaller than it should be. But if we take 'a' 50 times - result will be bigger than it should be. Why is this? And how to get correct result in this case?

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  • how IEEE-754 floating point numbers work

    - by hatorade
    Let's say I have this: float i = 1.5 in binary, this float is represented as: 0 01111111 10000000000000000000000 I broke up the binary to represent the 'signed', 'exponent' and 'fraction' chunks. What I don't understand is how this represents 1.5. The exponent is 0 once you subtract the bias (127 - 127), and the fraction part with the implicit leading one is 1.1. How does 1.1 scaled by nothing = 1.5???

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  • floating point precision in ruby on rails model validations

    - by Chris Allison
    Hello I am trying to validate a dollar amount using a regex: ^[0-9]+\.[0-9]{2}$ This works fine, but whenever a user submits the form and the dollar amount ends in 0(zero), ruby(or rails?) chops the 0 off. So 500.00 turns into 500.0 thus failing the regex validation. Is there any way to make ruby/rails keep the format entered by the user, regardless of trailing zeros?

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  • how floating point numbers work in C

    - by hatorade
    Let's say I have this: float i = 1.5 in binary, this float is represented as: 0 01111111 10000000000000000000000 I broke up the binary to represent the 'signed', 'exponent' and 'fraction' chunks. What I don't understand is how this represents 1.5. The exponent is 0 once you subtract the bias (127 - 127), and the fraction part with the implicit leading one is 1.1. How does 1.1 scaled by nothing = 1.5???

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  • Floating Point Arithmetic - Modulo Operator on Double Type

    - by CrimsonX
    So I'm trying to figure out why the modulo operator is returning such a large unusual value. If I have the code: double result = 1.0d % 0.1d; it will give a result of 0.09999999999999995. I would expect a value of 0 Note this problem doesn't exist using the dividing operator - double result = 1.0d / 0.1d; will give a result of 10.0, meaning that the remainder should be 0. Let me be clear: I'm not surprised that an error exists, I'm surprised that the error is so darn large compared to the numbers at play. 0.0999 ~= 0.1 and 0.1 is on the same order of magnitude as 0.1d and only one order of magnitude away from 1.0d. Its not like you can compare it to a double.epsilon, or say "its equal if its < 0.00001 difference". I've read up on this topic on StackOverflow, in the following posts one two three, amongst others. Can anyone suggest explain why this error is so large? Any any suggestions to avoid running into the problems in the future (I know I could use decimal instead but I'm concerned about the performance of that).

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  • Rounding floating-point numbers to 4 decimal points

    - by Himadri
    I have two decimal numbers. I want those number to be same upto 4 decimal points without rounding. If numbers are different I want 2nd number to be replaced by 1st. What if condition should I write? Eg, 1. num1 = 0.94618976 num2 = 0.94620239 If we round these numbers upto 4 decimal then we get 0.9462 same number, but I don't want to round these numbers. 2. num1 = 0.94620239 num2 = 0.94639125 The one way I found is take absolute difference of both numbers say diff and then check the value. My problem is of checking the range of diff. Thank You.

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  • Floating point precision and physics calculations

    - by Vee
    The gravity Vector2 in my physics world is (0; 0.1). The number 0.1 is known to be problematic, since "it cannot be represented exactly, but is approximately 1.10011001100110011001101 × 2-4". Having this value for the gravity gives me problems with collisions and creates quite nasty bugs. Changing the value to 0.11 solves these problems. Is there a more elegant solution that doesn't require changing the value at all?

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  • Rejigging a floating point equation ...

    - by Jamie
    I'd like to know if there is a way to improve the accuracy of calculating a slope. (This came up a few months back here). It seems by changing: float get_slope(float dXa, float dXb, float dYa, float dYb) { return (dXa - dXb)/(dYa - dYb); } to float get_slope(float dXa, float dXb, float dYa, float dYb) { return dXa/(dYa - dYb) - dXb/(dYa - dYb); } might be an improvement. Suggestions? Edit: It's precision I'm after, not efficiency.

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  • Floating point comparison in STL, BOOST

    - by Paul
    Is there in the STL or in Boost a set of generic simple comparison functions? The one I found are always requiring template parameters, and/or instantiation of a struct template. I'm looking for something with a syntax like : if ( is_greater(x,y) ) { ... } Which could be implemented as : template <typename T> bool is_greater(const T& x, const T& y) { return x > y + Precision<T>::eps; }

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  • Floating point precision nuances.

    - by user247077
    Hi, I found this code in NVIDIA's cuda SDK samples. void computeGold( float* reference, float* idata, const unsigned int len) { reference[0] = 0; double total_sum = 0; unsigned int i; for( i = 1; i < len; ++i) { total_sum += idata[i-1]; reference[i] = idata[i-1] + reference[i-1]; } // Here it should be okay to use != because we have integer values // in a range where float can be exactly represented if (total_sum != reference[i-1]) printf("Warning: exceeding single-precision accuracy. Scan will be inaccurate.\n"); } (C) Nvidia Corp Can somebody please tell me a case where the warning would be printed, and most importantly, why. Thank you very much.

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  • Time difference in seconds (as a floating point)

    - by pocoa
    >>> from datetime import datetime >>> t1 = datetime.now() >>> t2 = datetime.now() >>> delta = t2 - t1 >>> delta.seconds 7 >>> delta.microseconds 631000 Is there any way to get that as 7.631000 ? I can use time module, but I also need that t1 and t2 variables as DateTime objects. So if there is a way to do it with datettime, that would be great.

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  • What is a Bias Value of Floating Point Numbers?

    - by mudge
    In learning how floating point numbers are represented in computers I have come across the term "bias value" that I do not quite understand. The bias value in floating point numbers has to do with the negative and positiveness of the exponent part of a floating point number. The bias value of a floating point number is 127 which means that 127 is always added to the exponent part of a floating point number. How does doing this help determine if the exponent is negative or positive or not?

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  • Dlink DWA-556 Access point fails to start on 2.6.35-25 while 2.6.35-24 works. How can I do this with >2.6.35-24?

    - by Azendale
    I'm using hostapd to run an access point with a Dlink DWA-556 wireless N card. However, I can no longer get it to start when I use kernels greater than 2.6.35-24. Here's a log where I ran the uname -a&&hostapd -c <configfile> on the different kernel versions. Linux erikbandersen 2.6.35-24-generic #42-Ubuntu SMP Thu Dec 2 02:41:37 UTC 2010 x86_64 GNU/Linux Configuration file: hostapd.conf ctrl_interface_group=0 Opening raw packet socket for ifindex 248 BSS count 1, BSSID mask ff:ff:ff:ff:ff:ff (0 bits) SIOCGIWRANGE: WE(compiled)=22 WE(source)=21 enc_capa=0xf nl80211: Added 802.11b mode based on 802.11g information HT40: control channel: 2 secondary channel: 6 RATE[0] rate=10 flags=0x2 RATE[1] rate=20 flags=0x6 RATE[2] rate=55 flags=0x6 RATE[3] rate=110 flags=0x6 RATE[4] rate=60 flags=0x0 RATE[5] rate=90 flags=0x0 RATE[6] rate=120 flags=0x0 RATE[7] rate=180 flags=0x0 RATE[8] rate=240 flags=0x0 RATE[9] rate=360 flags=0x0 RATE[10] rate=480 flags=0x0 RATE[11] rate=540 flags=0x0 Passive scanning not supported Mode: IEEE 802.11g Channel: 2 Frequency: 2417 MHz Flushing old station entries Deauthenticate all stations Using interface wlan1 with hwaddr 1c:bd:b9:d5:e8:3c and ssid 'erikbandersen.com/freewifi' wlan1: Setup of interface done. MGMT (TX callback) ACK Malformed netlink message: len=436 left=256 plen=420 256 extra bytes in the end of netlink message MGMT (TX callback) ACK mgmt::proberesp cb MGMT (TX callback) ACK mgmt::proberesp cb MGMT (TX callback) ACK mgmt::proberesp cb mgmt::auth authentication: STA=3c:4a:92:0e:41:2f auth_alg=0 auth_transaction=1 status_code=0 wep=0 New STA wlan1: STA 3c:4a:92:0e:41:2f IEEE 802.11: authentication OK (open system) wlan1: STA 3c:4a:92:0e:41:2f MLME: MLME-AUTHENTICATE.indication(3c:4a:92:0e:41:2f, OPEN_SYSTEM) wlan1: STA 3c:4a:92:0e:41:2f MLME: MLME-DELETEKEYS.request(3c:4a:92:0e:41:2f) authentication reply: STA=3c:4a:92:0e:41:2f auth_alg=0 auth_transaction=2 resp=0 (IE len=0) MGMT (TX callback) ACK mgmt::auth cb wlan1: STA 3c:4a:92:0e:41:2f IEEE 802.11: authenticated mgmt::assoc_req association request: STA=3c:4a:92:0e:41:2f capab_info=0x421 listen_interval=10 Validating WMM IE: OUI 00:50:f2 OUI type 2 OUI sub-type 0 version 1 QoS info 0x0 HT: STA 3c:4a:92:0e:41:2f HT Capabilities Info: 0x102c handle_assoc STA 3c:4a:92:0e:41:2f - no greenfield, num of non-gf stations 1 handle_assoc STA 3c:4a:92:0e:41:2f - 20 MHz HT, num of 20MHz HT STAs 1 hostapd_ht_operation_update current operation mode=0x0 hostapd_ht_operation_update new operation mode=0x7 changes=2 new AID 1 wlan1: STA 3c:4a:92:0e:41:2f IEEE 802.11: association OK (aid 1) MGMT (TX callback) ACK mgmt::assoc_resp cb wlan1: STA 3c:4a:92:0e:41:2f IEEE 802.11: associated (aid 1) wlan1: STA 3c:4a:92:0e:41:2f MLME: MLME-ASSOCIATE.indication(3c:4a:92:0e:41:2f) wlan1: STA 3c:4a:92:0e:41:2f MLME: MLME-DELETEKEYS.request(3c:4a:92:0e:41:2f) wlan1: STA 3c:4a:92:0e:41:2f RADIUS: starting accounting session 4DAC8224-00000000 MGMT (TX callback) ACK mgmt::action cb MGMT (TX callback) ACK mgmt::proberesp cb MGMT (TX callback) ACK mgmt::proberesp cb MGMT (TX callback) ACK mgmt::proberesp cb MGMT (TX callback) ACK mgmt::proberesp cb MGMT (TX callback) ACK mgmt::proberesp cb Signal 2 received - terminating wlan1: STA 3c:4a:92:0e:41:2f MLME: MLME-DEAUTHENTICATE.indication(3c:4a:92:0e:41:2f, 1) wlan1: STA 3c:4a:92:0e:41:2f MLME: MLME-DELETEKEYS.request(3c:4a:92:0e:41:2f) Removing station 3c:4a:92:0e:41:2f hostapd_ht_operation_update current operation mode=0x7 hostapd_ht_operation_update new operation mode=0x0 changes=2 Flushing old station entries Deauthenticate all stations . Linux erikbandersen 2.6.35-25-generic #44-Ubuntu SMP Fri Jan 21 17:40:44 UTC 2011 x86_64 GNU/Linux Configuration file: hostapd.conf ctrl_interface_group=0 Opening raw packet socket for ifindex 248 BSS count 1, BSSID mask ff:ff:ff:ff:ff:ff (0 bits) SIOCGIWRANGE: WE(compiled)=22 WE(source)=21 enc_capa=0xf nl80211: Added 802.11b mode based on 802.11g information Allowed channel: mode=1 chan=1 freq=2412 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=2 freq=2417 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=3 freq=2422 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=4 freq=2427 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=5 freq=2432 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=6 freq=2437 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=7 freq=2442 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=8 freq=2447 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=9 freq=2452 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=10 freq=2457 MHz max_tx_power=27 dBm Allowed channel: mode=1 chan=11 freq=2462 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=1 freq=2412 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=2 freq=2417 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=3 freq=2422 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=4 freq=2427 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=5 freq=2432 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=6 freq=2437 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=7 freq=2442 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=8 freq=2447 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=9 freq=2452 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=10 freq=2457 MHz max_tx_power=27 dBm Allowed channel: mode=0 chan=11 freq=2462 MHz max_tx_power=27 dBm HT40: control channel: 2 secondary channel: 6 RATE[0] rate=10 flags=0x2 RATE[1] rate=20 flags=0x6 RATE[2] rate=55 flags=0x6 RATE[3] rate=110 flags=0x6 RATE[4] rate=60 flags=0x0 RATE[5] rate=90 flags=0x0 RATE[6] rate=120 flags=0x0 RATE[7] rate=180 flags=0x0 RATE[8] rate=240 flags=0x0 RATE[9] rate=360 flags=0x0 RATE[10] rate=480 flags=0x0 RATE[11] rate=540 flags=0x0 Passive scanning not supported Mode: IEEE 802.11g Channel: 2 Frequency: 2417 MHz Could not set channel for kernel driver wlan1: Unable to setup interface. My wireless card is listed as 02:00.0 Network controller: Atheros Communications Inc. AR5008 Wireless Network Adapter (rev 01) by lspci. Am I doing it wrong and there's a new way of doing it? I'm holding off upgrading to Natty because of this. What changed between the versions that would cause this? Should I report it as a bug?

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  • Is there any hueristic to polygonize a closed 2d-raster shape with n triangles?

    - by Arthur Wulf White
    Lets say we have a 2d image black on white that shows a closed geometric shape. Is there any (not naive brute force) algorithm that approximates that shape as closely as possible with n triangles? If you want a formal definition for as closely as possible: Approximate the shape with a polygon that when rendered into a new 2d image will match the largest number of pixels possible with the original image.

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  • box2d resize bodies arround point

    - by philipp
    I have a compound object, consisting of a b2Body, vector-graphics and a list polygons which describe the b2body's shapes. This object has its own transformation matrix to centralize the storage of transformations. So far everything is working quiet fine, even scaling works, but not if i scale around a point. In the initialization phase of the object it is scaled around a point. This happens in this order: transform the main matrix transform the vector graphics and the polygons recreate the b2Body After this function ran, the shapes and all the graphics are exactly where they should be, BUT: after the first steps of the b2World the graphical stuff moves away from the body. When I ran the debugger I found out that the position of the body is 0/0 the red dot shows the center of scaling. the first image shows the basic setup and the second the final position of the graphics. This distance stays constant for the rest of the simulation. If I set the position via myBody.SetPosition( sx, sy ); the whole scenario just plays a bit more distant for the origin. Any Idea how to fix this? EDIT:: I came deeper down to the problem and it lies in the fact that i must not scale the transform matrix for the b2body shapes around the center, but set the b2body's position back to the point after scaling. But how can I calculate that point? EDIT 2 :: I came ever deeper down to it, even solved it, but this is a slow solution and i hope that there is somebody who understands what formula I need. assuming to have a set polygons relative to an origin as basis shapes for a b2body: scaling the whole object around a certain point is done in the following steps: i scale everything around the center except the polygons i create a clone of the polygons matrix i scale this clone around the point i calculate dx, dy as difference of clone.tx - original.tx and clone.ty - original.ty i scale the original polygon matrix NOT around the point i recreate the body i create the fixture i set the position of the body to dx and dy done! So what i an interested in is a formula for dx and dy without cloning matrices, scaling the clone around a point, getting dx and dy and finally scale the vertex matrix.

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