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  • Transform between two 3d cartesian coordinate systems

    - by Pris
    I'd like to know how to get the rotation matrix for the transformation from one cartesian coordinate system (X,Y,Z) to another one (X',Y',Z'). Both systems are defined with three orthogonal vectors as one would expect. No scaling or translation occurs. I'm using OpenSceneGraph and it offers a Matrix convenience class, if it makes finding the matrix easier: http://www.openscenegraph.org/documentation/OpenSceneGraphReferenceDocs/a00403.html.

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  • How to implement a simple bullet trajectory

    - by AirieFenix
    I searched and searched and although it's a fair simple question, I don't find the proper answer but general ideas (which I already have). I have a top-down game and I want to implement a gun which shoots bullets that follow a simple path (no physics nor change of trajectory, just go from A to B thing). a: vector of the position of the gun/player. b: vector of the mouse position (cross-hair). w: the vector of the bullet's trajectory. So, w=b-a. And the position of the bullet = [x=x0+speed*time*normalized w.x , y=y0+speed*time * normalized w.y]. I have the constructor: public Shot(int shipX, int shipY, int mouseX, int mouseY) { //I get mouse with Gdx.input.getX()/getY() ... this.shotTime = TimeUtils.millis(); this.posX = shipX; this.posY = shipY; //I used aVector = aVector.nor() here before but for some reason didn't work float tmp = (float) (Math.pow(mouseX-shipX, 2) + Math.pow(mouseY-shipY, 2)); tmp = (float) Math.sqrt(Math.abs(tmp)); this.vecX = (mouseX-shipX)/tmp; this.vecY = (mouseY-shipY)/tmp; } And here I update the position and draw the shot: public void drawShot(SpriteBatch batch) { this.lifeTime = TimeUtils.millis() - this.shotTime; //position = positionBefore + v*t this.posX = this.posX + this.vecX*this.lifeTime*speed*Gdx.graphics.getDeltaTime(); this.posY = this.posY + this.vecY*this.lifeTime*speed*Gdx.graphics.getDeltaTime(); ... } Now, the behavior of the bullet seems very awkward, not going exactly where my mouse is (it's like the mouse is 30px off) and with a random speed. I know I probably need to open the old algebra book from college but I'd like somebody says if I'm in the right direction (or points me to it); if it's a calculation problem, a code problem or both. Also, is it possible that Gdx.input.getX() gives me non-precise position? Because when I draw the cross-hair it also draws off the cursor position. Sorry for the long post and sorry if it's a very basic question. Thanks!

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  • How can I calculate a vertex normal for a hard edge?

    - by K.G.
    Here is a picture of a lovely polygon: Circled is a vertex, and numbered are its adjacent faces. I have calculated the normals of those faces as such (not yet normalized, 0-indexed): Vertex 1 normal 0: 0.000000 0.000000 -0.250000 Vertex 1 normal 1: 0.000000 0.000000 -0.250000 Vertex 1 normal 2: -0.250000 0.000000 0.000000 Vertex 1 normal 3: -0.250000 0.000000 0.000000 Vertex 1 normal 4: 0.250000 0.000000 0.000000 What I'm wondering is, how can I determine, taken as given that I want this vertex to represent a hard edge, whether its normal should be the normal of 1/2 or 3/4? My plan after I glanced at the sketch I used to put this together was "Ha! I'll just use whichever two faces have the same normal!" and now I see that there are two sets of two faces for which this is true. Is there a rule I can apply based on the face winding, angle of the adjacent edges, moon phase, coin flip, to consistently choose a normal direction for this box? For the record, all of the other polygons I plan to use will have their normals dictated in Maya, but after encountering this problem, it made me really curious.

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  • Make an object slide around an obstacle

    - by Isaiah
    I have path areas set up in a game I'm making for canvas/html5 and have got it working to keep the player within these areas. I have a function isOut(boundary, x, y) that returns true if the point is outside the boundary. What I do is check only the new position x/y separately with the corresponding old position x/y. Then if each one is out I assign them the past value from the frame before. The old positions are kept in a variable from a closure I made. like this: opos = [x,y];//old position npos = [x,y];//new position if(isOut(bound, npos[0], opos[1])){ npos[0] = opos[0]; //assign it the old x position } if(isOut(bound, opos[0], npos[1])){ npos[1] = opos[1]; //assign it the old y position } It looks nice and works good at certain angles, but if your boundary has diagonal regions it results in jittery motion. What's happening is the y pos exits the area while x doesn't and continues pushing the player to the side, once it has moved the player to the side a bit the player can move forward and then the y exits again and the whole process repeats. Anyone know how I may be able to achieve a smoother slide? I have access to the player's velocity vector, the angle, and the speed(when used with the angle). I can move the play with either angle/speed or x/yvelocities as I've built in backups to translate one to the other if either have been altered manually.

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  • How can I test if a point lies between two parallel lines?

    - by Harold
    In the game I'm designing there is a blast that shoots out from an origin point towards the direction of the mouse. The width of this blast is always going to be the same. Along the bottom of the screen (what's currently) squares move about which should be effected by the blast that the player controls. Currently I am trying to work out a way to discover if the corners of these squares are within the blast's two bounding lines. I thought the best way to do this would be to rotate the corners of the square around an origin point as if the blast were completely horizontal and see if the Y values of the corners were less than or equal to the width of the blast which would mean that they lie within the effected region, but I can't work out

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  • Moving objects colliding when using unalligned collision avoidance (steering)

    - by James Bedford
    I'm having trouble with unaligned collision avoidance for what I think is a rare case. I have set two objects to move towards each other but with a slight offset, so one of the objects is moving slightly upwards, and one of the objects is moving slightly downwards. In my unaligned collision avoidance steering algorithm I'm finding the points on the object's forward line and the other object's forward line where these two lines are the closest. If these closest points are within a collision avoidance distance, and if the distance between them is smaller than the two radii of the two object's bounding spheres, then the objects should steer away in the appropriate direction. The problem is that for my case, the closest points on the lines are calculated to be really far away from the actual collision point. This is because the two forward lines for each object are moving away from each other as the objects pass. The problem is that because of this, no steering takes place, and the two objects partially collide. Does anyone have any suggestions as to how I can correctly calculate the point of collision? Perhaps by somehow taking into account the size of the two objects?

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  • Best way to implement a simple bullet trajectory

    - by AirieFenix
    I searched and searched and although it's a fair simple question, I don't find the proper answer but general ideas (which I already have). I have a top-down game and I want to implement a gun which shoots bullets that follow a simple path (no physics nor change of trajectory, just go from A to B thing). a: vector of the position of the gun/player. b: vector of the mouse position (cross-hair). w: the vector of the bullet's trajectory. So, w=b-a. And the position of the bullet = [x=x0+speed*time*normalized w.x , y=y0+speed*time * normalized w.y]. I have the constructor: public Shot(int shipX, int shipY, int mouseX, int mouseY) { //I get mouse with Gdx.input.getX()/getY() ... this.shotTime = TimeUtils.millis(); this.posX = shipX; this.posY = shipY; //I used aVector = aVector.nor() here before but for some reason didn't work float tmp = (float) (Math.pow(mouseX-shipX, 2) + Math.pow(mouseY-shipY, 2)); tmp = (float) Math.sqrt(Math.abs(tmp)); this.vecX = (mouseX-shipX)/tmp; this.vecY = (mouseY-shipY)/tmp; } And here I update the position and draw the shot: public void drawShot(SpriteBatch batch) { this.lifeTime = TimeUtils.millis() - this.shotTime; //position = positionBefore + v*t this.posX = this.posX + this.vecX*this.lifeTime*speed*Gdx.graphics.getDeltaTime(); this.posY = this.posY + this.vecY*this.lifeTime*speed*Gdx.graphics.getDeltaTime(); ... } Now, the behavior of the bullet seems very awkward, not going exactly where my mouse is (it's like the mouse is 30px off) and with a random speed. I know I probably need to open the old algebra book from college but I'd like somebody says if I'm in the right direction (or points me to it); if it's a calculation problem, a code problem or both. Also, is it possible that Gdx.input.getX() gives me non-precise position? Because when I draw the cross-hair it also draws off the cursor position. Sorry for the long post and sorry if it's a very basic question. Thanks!

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  • Understanding dot notation

    - by Starkers
    Here's my interpretation of dot notation: a = [2,6] b = [1,4] c = [0,8] a . b . c = (2*6)+(1*4)+(0*8) = 12 + 4 + 0 = 16 What is the significance of 16? Apparently it's a scalar. Am I right in thinking that a scalar is the number we times a unit vector by to get a vector that has a scaled up magnitude but the same direction as the unit vector? So again, what is the relevance of 16? When is it used? It's not the magnitude of all the vectors added up. The magnitude of all of them is calculated as follows: sqrt( ax * ax + ay * ay ) + sqrt( bx * bx + by * by ) + sqrt( cx * cx + cy * cy) sqrt( 2 * 2 + 6 * 6 ) + sqrt( 1 * 1 + 4 * 4 ) + sqrt( 0 * 0 + 8 * 8) sqrt( 4 + 36 ) + sqrt( 1 + 16 ) + sqrt( 0 + 64) sqrt( 40 ) + sqrt( 17 ) + sqrt( 64) 6.3 + 4.1 + 8 10.4 + 8 18.4 So I don't really get this diagram: Attempting with sensible numbers: a = [1,0] b = [4,3] a . b = (1*0) + (4*3) = 0 + 12 = 12 So what exactly is a . b describing here? The magnitude of that vector? Because that isn't right: the 'a.b' vector = [4,0] sqrt( x*x + y*y ) sqrt( 4*4 + 0*0 ) sqrt( 16 + 0 ) 4 So what is 12 describing?

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  • Lerping to a center point while in motion

    - by Fibericon
    I have an enemy that initially flies in a circular motion, while facing away from the center point. This is how I achieve that: position.Y = (float)(Math.Cos(timeAlive * MathHelper.PiOver4) * radius + origin.Y); position.X = (float)(Math.Sin(timeAlive * MathHelper.PiOver4) * radius + origin.X); if (timeAlive < 5) { angle = (float)Math.Atan((0 - position.X) / (0 - position.Y)); if (0 < position.Y) RotationMatrix = Matrix.CreateRotationX(MathHelper.PiOver2) * Matrix.CreateRotationZ(-1 * angle); else RotationMatrix = Matrix.CreateRotationX(MathHelper.PiOver2) * Matrix.CreateRotationZ(MathHelper.Pi - angle); } That part works just fine. After five seconds of this, I want the enemy to turn inward, facing the center point. However, I've been trying to lerp to that point, since I don't want it to simply jump to the new rotation. Here's my code for trying to do that: else { float newAngle = -1 * (float)Math.Atan((0 - position.X) / (0 - position.Y)); angle = MathHelper.Lerp(angle, newAngle, (float)gameTime.ElapsedGameTime.Milliseconds / 1000); if (0 < position.Y) RotationMatrix = Matrix.CreateRotationX(MathHelper.PiOver2) * Matrix.CreateRotationZ(MathHelper.Pi - angle); else RotationMatrix = Matrix.CreateRotationX(MathHelper.PiOver2) * Matrix.CreateRotationZ(-1 * angle); } That doesn't work so fine. It seems like it's going to at first, but then it just sort of skips around. How can I achieve what I want here?

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  • How to get xy coordinates along a given path

    - by netbrain
    Say i have two points (x,y), (0,0) and (10,10). Now i wan´t to get coordinates along the line by stepping through values of x and y. I thought i solved it with the following functions: fy = startY + (x - startX) * ((destY-startY)/(destX-startX)); fx = (y + startY) / ((destY-startY)/(destX-startX)) + startX; taken from http://en.wikipedia.org/wiki/Linear_interpolation However, it seems that im getting a problem when destX and startX is the same value, so you get division by zero. Is there a better way of getting coordinates along a line when knowing the start and endpoint of the line?

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  • Trouble with Collada bones

    - by KyleT
    I have a Collada file with a rigged mesh. I've read the node tags in the library_visual_scenes tag and extracted the matrix for each node and stored everything in a hierarchical bone structure. My Matrix container is "row major", so I'd store the first float of a matrix tag in the 1st row, 1st column, the second in the 1st row, 2nd column, etc. From what I gather this is the Bind Pose Matrix. After that I went through the tag and extracted the float array in the source tag of the skin tag of the controller for the mesh. I stored each matrix from this float array in their corresponding Bone as the Inverse Bind Matrix. I also extracted the bind-shape-matrix and stored it. Now I'd like to draw the skeleton with OpenGL to see if everything is working correctly before I go about skinning. I iterate once over my bones and multiply a bone's Bind Pose Matrix by it's parents and store that. After that I iterate again over the bones and multiply the result of the previous matrix multiplication by the Inverse Bind Matrix and then by the Bind Shape Matrix. The results look something like this: [0.2, 9.2, 5.8, 1.2 ] [4.6, -3.3, -0.2, -0.1 ] [-1.8, 0.2, -4.2, -3.9 ] [0, 0, 0, 1 ] I've had to go to various sources to get the little understanding of Collada I have and books about 3d transform matricies can get pretty intense. I've hit a brick wall and if you could please read through this and see if there is something I'm doing wrong, and how I'd go about getting an X,Y,Z to draw a point for each of these joints once I've calculated the final transform, I'd really appreciate it.

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  • Matrices: Arrays or separate member variables?

    - by bjz
    I'm teaching myself 3D maths and in the process building my own rudimentary engine (of sorts). I was wondering what would be the best way to structure my matrix class. There are a few options: Separate member variables: struct Mat4 { float m11, m12, m13, m14, m21, m22, m23, m24, m31, m32, m33, m34, m41, m42, m43, m44; // methods } A multi-dimensional array: struct Mat4 { float[4][4] m; // methods } An array of vectors struct Mat4 { Vec4[4] m; // methods } I'm guessing there would be positives and negatives to each. From 3D Math Primer for Graphics and Game Development, 2nd Edition p.155: Matrices use 1-based indices, so the first row and column are numbered 1. For example, a12 (read “a one two,” not “a twelve”) is the element in the first row, second column. Notice that this is different from programming languages such as C++ and Java, which use 0-based array indices. A matrix does not have a column 0 or row 0. This difference in indexing can cause some confusion if matrices are stored using an actual array data type. For this reason, it’s common for classes that store small, fixed size matrices of the type used for geometric purposes to give each element its own named member variable, such as float a11, instead of using the language’s native array support with something like float elem[3][3]. So that's one vote for method one. Is this really the accepted way to do things? It seems rather unwieldy if the only benefit would be sticking with the conventional math notation.

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  • Portal View/Projection Matrix near plane

    - by melak47
    For RenderToTexture/Camera based portal rendering, the basics seems simple enough. However, with a free camera, most of the time it is going to be looking at such portals at an angle: Now a regular near clipping plane will not always work here, it will either intersect with the wall the portal is sitting on, or possibly with objects in front of the wall. The desired near clipping plane would be aligned like the portal, producing a view volume more like this: or this in 3D: So here is my question: How does one construct or "truncate" a view/projection matrix to achieve such an off-camera-normal (near) clipping plane?

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  • Mouse pointer position to screen space

    - by Ylisar
    If I have a mouse pointer position in pixels of canvas, I can easily convert it to the -1..1 range for both X & Y by lerping by dividing with canvas dimensions. However, the problem is what I should put in Z & W if I want my screen space position to be on the near plane? The step afterwards would be for me to multiply by the inverse of view-projection to take me to world space, where I easily can construct a ray from the cameras world space position.

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  • Issue with angular gradient algorithm

    - by user146780
    I have an angular gradient algorithm: GLuint OGLENGINEFUNCTIONS::CreateAngularGradient( std::vector<ARGBCOLORF> &input,POINTFLOAT start, POINTFLOAT end, int width, int height ) { std::vector<GLubyte> pdata(width * height * 4); float pi = 3.1415; float cx; float cy; float r1; float r2; float ang; float px; float py; float t; cx = start.x; cy = start.y; r1 = 0; r2 = 500; ang = end.x / 100; ARGBCOLORF color; for (unsigned int i = 0; i < height; i++) { for (unsigned int j = 0; j < width; j++) { px = j; py = i; if( px * px + py * py <= r2 * r2 && px * px + py * py >= r1 * r1 ) { t= atan2(py-cy,px-cx) + ang; t= t+ pi; if (t > 2* pi) { t=t-2*pi; t=t/(2*pi); } } //end + start color.r = (0 * t) + (120 * (1 - t)); color.g = (50 * t) + (255 * (1 - t)); color.b = (0 * t) + (50 * (1 - t)); color.a = (255 * t) + (200 * (1 - t)); pdata[i * 4 * width + j * 4 + 0] = (GLubyte) color.r; pdata[i * 4 * width + j * 4 + 1] = (GLubyte) color.g; pdata[i * 4 * width + j * 4 + 2] = (GLubyte) 12; pdata[i * 4 * width + j * 4 + 3] = (GLubyte) 255; } } It works fine except the ang variable only controls the end sweep, not the start sweep. The start sweep is always facing the middle left as seen here: http://img810.imageshack.us/img810/9623/uhoh.png Basically I have no control over the end one going this <- way. How could I control this one? Thanks

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  • Resilient backpropagation neural network - question about gradient

    - by Raf
    Hello Guys, First I want to say that I'm really new to neural networks and I don't understand it very good ;) I've made my first C# implementation of the backpropagation neural network. I've tested it using XOR and it looks it work. Now I would like change my implementation to use resilient backpropagation (Rprop - http://en.wikipedia.org/wiki/Rprop). The definition says: "Rprop takes into account only the sign of the partial derivative over all patterns (not the magnitude), and acts independently on each "weight". Could somebody tell me what partial derivative over all patterns is? And how should I compute this partial derivative for a neuron in hidden layer. Thanks a lot UPDATE: My implementation base on this Java code: www_.dia.fi.upm.es/~jamartin/downloads/bpnn.java My backPropagate method looks like this: public double backPropagate(double[] targets) { double error, change; // calculate error terms for output double[] output_deltas = new double[outputsNumber]; for (int k = 0; k < outputsNumber; k++) { error = targets[k] - activationsOutputs[k]; output_deltas[k] = Dsigmoid(activationsOutputs[k]) * error; } // calculate error terms for hidden double[] hidden_deltas = new double[hiddenNumber]; for (int j = 0; j < hiddenNumber; j++) { error = 0.0; for (int k = 0; k < outputsNumber; k++) { error = error + output_deltas[k] * weightsOutputs[j, k]; } hidden_deltas[j] = Dsigmoid(activationsHidden[j]) * error; } //update output weights for (int j = 0; j < hiddenNumber; j++) { for (int k = 0; k < outputsNumber; k++) { change = output_deltas[k] * activationsHidden[j]; weightsOutputs[j, k] = weightsOutputs[j, k] + learningRate * change + momentumFactor * lastChangeWeightsForMomentumOutpus[j, k]; lastChangeWeightsForMomentumOutpus[j, k] = change; } } // update input weights for (int i = 0; i < inputsNumber; i++) { for (int j = 0; j < hiddenNumber; j++) { change = hidden_deltas[j] * activationsInputs[i]; weightsInputs[i, j] = weightsInputs[i, j] + learningRate * change + momentumFactor * lastChangeWeightsForMomentumInputs[i, j]; lastChangeWeightsForMomentumInputs[i, j] = change; } } // calculate error error = 0.0; for (int k = 0; k < outputsNumber; k++) { error = error + 0.5 * (targets[k] - activationsOutputs[k]) * (targets[k] - activationsOutputs[k]); } return error; } So can I use change = hidden_deltas[j] * activationsInputs[i] variable as a gradient (partial derivative) for checking the sing?

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  • Use a vector to index a matrix without linear index

    - by David_G
    G'day, I'm trying to find a way to use a vector of [x,y] points to index from a large matrix in MATLAB. Usually, I would convert the subscript points to the linear index of the matrix.(for eg. Use a vector as an index to a matrix in MATLab) However, the matrix is 4-dimensional, and I want to take all of the elements of the 3rd and 4th dimensions that have the same 1st and 2nd dimension. Let me hopefully demonstrate with an example: Matrix = nan(4,4,2,2); % where the dimensions are (x,y,depth,time) Matrix(1,2,:,:) = 999; % note that this value could change in depth (3rd dim) and time (4th time) Matrix(3,4,:,:) = 888; % note that this value could change in depth (3rd dim) and time (4th time) Matrix(4,4,:,:) = 124; Now, I want to be able to index with the subscripts (1,2) and (3,4), etc and return not only the 999 and 888 which exist in Matrix(:,:,1,1) but the contents which exist at Matrix(:,:,1,2),Matrix(:,:,2,1) and Matrix(:,:,2,2), and so on (IRL, the dimensions of Matrix might be more like size(Matrix) = (300 250 30 200) I don't want to use linear indices because I would like the results to be in a similar vector fashion. For example, I would like a result which is something like: ans(time=1) 999 888 124 999 888 124 ans(time=2) etc etc etc etc etc etc I'd also like to add that due to the size of the matrix I'm dealing with, speed is an issue here - thus why I'd like to use subscript indices to index to the data. I should also mention that (unlike this question: Accessing values using subscripts without using sub2ind) since I want all the information stored in the extra dimensions, 3 and 4, of the i and jth indices, I don't think that a slightly faster version of sub2ind still would not cut it..

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  • How to partition bits in a bit array with less than linear time

    - by SiLent SoNG
    This is an interview question I faced recently. Given an array of 1 and 0, find a way to partition the bits in place so that 0's are grouped together, and 1's are grouped together. It does not matter whether 1's are ahead of 0's or 0's are ahead of 1's. An example input is 101010101, and output is either 111110000 or 000011111. Solve the problem in less than linear time. Make the problem simpler. The input is an integer array, with each element either 1 or 0. Output is the same integer array with integers partitioned well. To me, this is an easy question if it can be solved in O(N). My approach is to use two pointers, starting from both ends of the array. Increases and decreases each pointer; if it does not point to the correct integer, swap the two. int * start = array; int * end = array + length - 1; while (start < end) { // Assume 0 always at the end if (*end == 0) { --end; continue; } // Assume 1 always at the beginning if (*start == 1) { ++start; continue; } swap(*start, *end); } However, the interview insists there is a sub-linear solution. This makes me thinking hard but still not get an answer. Can anyone help on this interview question?

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  • Linear Search with Jagged Array?

    - by Nerathas
    Hello, I have the following program that creates 100 random elements trough a array. Those 100 random value's are unique, and every value only gets displayed once. Although with the linear search it keeps looking up the entire array. How would i be able to get a Jagged Array into this, so it only "scans" the remaining places left? (assuming i keep the table at 100 max elements, so if one random value is generated the array holds 99 elements with linear search scans and on...) I assume i would have to implent the jagged array somewhere in the FoundLinearInArray? Hopefully this made any sence. Regards. private int ValidNumber(int[] T, int X, int Range) { Random RndInt = new Random(); do { X = RndInt.Next(1, Range + 1); } while (FoundLinearInArray(T, X)); return X; }/*ValidNumber*/ private bool FoundLinearInArray(int[] A, int X) { byte I = 0; while ((I < A.Length) && (A[I] != X)) { I++; } return (I < A.Length); }/*FoundInArray*/ public void FillArray(int[] T, int Range) { for (byte I = 0; I < T.Length; I++) { T[I] = ValidNumber(T, I, Range); } }/*FillArray*/

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  • Admob in xml not showing in Linear

    - by NoobMe
    i am implementing admob on my app it appears when the parent is in relative layout but i must not use the alignparentbottom so i am changing it to linear but it doesnt show when i change it to linear.. any tips? help? thanks in advance here it is in xml: <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="vertical" > <RelativeLayout android:id="@+id/banner_holder" android:layout_width="match_parent" android:layout_height="wrap_content" > <ImageView android:id="@+id/offline_banner" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_centerInParent="true" android:background="@color/black" android:src="@drawable/offline_banner" /> <com.google.ads.AdView xmlns:ads="http://schemas.android.com/apk/lib/com.google.ads" android:id="@+id/adView" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_centerInParent="true" ads:adSize="SMART_BANNER" ads:adUnitId="@string/unit_id" ads:loadAdOnCreate="true" /> </RelativeLayout> <FrameLayout android:id="@+id/fragmentContainer" android:layout_width="match_parent" android:layout_height="wrap_content" /> </LinearLayout> i want the admob to be at the bottom part of the screen without using the alignparentbottom of relative layout thanks~

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  • Moving from Linear Probing to Quadratic Probing (hash collisons)

    - by Nazgulled
    Hi, My current implementation of an Hash Table is using Linear Probing and now I want to move to Quadratic Probing (and later to chaining and maybe double hashing too). I've read a few articles, tutorials, wikipedia, etc... But I still don't know exactly what I should do. Linear Probing, basically, has a step of 1 and that's easy to do. When searching, inserting or removing an element from the Hash Table, I need to calculate an hash and for that I do this: index = hash_function(key) % table_size; Then, while searching, inserting or removing I loop through the table until I find a free bucket, like this: do { if(/* CHECK IF IT'S THE ELEMENT WE WANT */) { // FOUND ELEMENT return; } else { index = (index + 1) % table_size; } while(/* LOOP UNTIL IT'S NECESSARY */); As for Quadratic Probing, I think what I need to do is change how the "index" step size is calculated but that's what I don't understand how I should do it. I've seen various pieces of code, and all of them are somewhat different. Also, I've seen some implementations of Quadratic Probing where the hash function is changed to accommodated that (but not all of them). Is that change really needed or can I avoid modifying the hash function and still use Quadratic Probing? EDIT: After reading everything pointed out by Eli Bendersky below I think I got the general idea. Here's part of the code at http://eternallyconfuzzled.com/tuts/datastructures/jsw_tut_hashtable.aspx: 15 for ( step = 1; table->table[h] != EMPTY; step++ ) { 16 if ( compare ( key, table->table[h] ) == 0 ) 17 return 1; 18 19 /* Move forward by quadratically, wrap if necessary */ 20 h = ( h + ( step * step - step ) / 2 ) % table->size; 21 } There's 2 things I don't get... They say that quadratic probing is usually done using c(i)=i^2. However, in the code above, it's doing something more like c(i)=(i^2-i)/2 I was ready to implement this on my code but I would simply do: index = (index + (index^index)) % table_size; ...and not: index = (index + (index^index - index)/2) % table_size; If anything, I would do: index = (index + (index^index)/2) % table_size; ...cause I've seen other code examples diving by two. Although I don't understand why... 1) Why is it subtracting the step? 2) Why is it diving it by 2?

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  • Non-linear regression models in PostgreSQL using R

    - by Dave Jarvis
    Background I have climate data (temperature, precipitation, snow depth) for all of Canada between 1900 and 2009. I have written a basic website and the simplest page allows users to choose category and city. They then get back a very simple report (without the parameters and calculations section): The primary purpose of the web application is to provide a simple user interface so that the general public can explore the data in meaningful ways. (A list of numbers is not meaningful to the general public, nor is a website that provides too many inputs.) The secondary purpose of the application is to provide climatologists and other scientists with deeper ways to view the data. (Using too many inputs, of course.) Tool Set The database is PostgreSQL with R (mostly) installed. The reports are written using iReport and generated using JasperReports. Poor Model Choice Currently, a linear regression model is applied against annual averages of daily data. The linear regression model is calculated within a PostgreSQL function as follows: SELECT regr_slope( amount, year_taken ), regr_intercept( amount, year_taken ), corr( amount, year_taken ) FROM temp_regression INTO STRICT slope, intercept, correlation; The results are returned to JasperReports using: SELECT year_taken, amount, year_taken * slope + intercept, slope, intercept, correlation, total_measurements INTO result; JasperReports calls into PostgreSQL using the following parameterized analysis function: SELECT year_taken, amount, measurements, regression_line, slope, intercept, correlation, total_measurements, execute_time FROM climate.analysis( $P{CityId}, $P{Elevation1}, $P{Elevation2}, $P{Radius}, $P{CategoryId}, $P{Year1}, $P{Year2} ) ORDER BY year_taken This is not an optimal solution because it gives the false impression that the climate is changing at a slow, but steady rate. Questions Using functions that take two parameters (e.g., year [X] and amount [Y]), such as PostgreSQL's regr_slope: What is a better regression model to apply? What CPAN-R packages provide such models? (Installable, ideally, using apt-get.) How can the R functions be called within a PostgreSQL function? If no such functions exist: What parameters should I try to obtain for functions that will produce the desired fit? How would you recommend showing the best fit curve? Keep in mind that this is a web app for use by the general public. If the only way to analyse the data is from an R shell, then the purpose has been defeated. (I know this is not the case for most R functions I have looked at so far.) Thank you!

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