opengl - Page 47 - Developer IT

Rotate triangle so that its tip points in the direction of the point on the screen that we last touched

- by Sid

OpenGL ES - Android. Hello all, I am unable to rotate the triangle accordingly in such a way that its tip always points to my finger. What i did : Constructed a triangle in by GL.GL_TRIANGLES. Added touch events to it. I can rotate the triangle along my Z-axis successfully. Even made the vector class for it. What i need : Each time when I touch the screen, I want to rotate the triangle to face the touch point. Need some help. Here's what i implemented. I wonder that where i am going wrong? My code : public class Graphic2DTriangle { private FloatBuffer vertexBuffer; private ByteBuffer indexBuffer; private float[] vertices = { -1.0f,-1.0f, 0.0f, 2.0f, 0.0f, 0.0f, -1.0f, 1.0f, 0.0f }; private byte[] indices = { 0, 1, 2 }; public Graphic2DTriangle() { ByteBuffer vbb = ByteBuffer.allocateDirect(vertices.length * 4); vbb.order(ByteOrder.nativeOrder()); // Use native byte order vertexBuffer = vbb.asFloatBuffer(); // Convert byte buffer to float vertexBuffer.put(vertices); // Copy data into buffer vertexBuffer.position(0); // Rewind // Setup index-array buffer. Indices in byte. indexBuffer = ByteBuffer.allocateDirect(indices.length); indexBuffer.put(indices); indexBuffer.position(0); } public void draw(GL10 gl) { gl.glEnableClientState(GL10.GL_VERTEX_ARRAY); gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer); gl.glDrawElements(GL10.GL_TRIANGLES, indices.length, GL10.GL_UNSIGNED_BYTE, indexBuffer); gl.glDisableClientState(GL10.GL_VERTEX_ARRAY); } } My SurfaceView class where i've done some Touch Events. public class BallThrowGLSurfaceView extends GLSurfaceView{ MySquareRender _renderObj; View _viewObj; float oldX,oldY,dX,dY; final float TOUCH_SCALE_FACTOR = 0.6f; Vector2 touchPos = new Vector2(); float angle=0; public BallThrowGLSurfaceView(Context context) { super(context); // TODO Auto-generated constructor stub _renderObj = new MySquareRender(context); this.setRenderer(_renderObj); this.setRenderMode(RENDERMODE_WHEN_DIRTY); } @Override public boolean onTouchEvent(MotionEvent event) { // TODO Auto-generated method stub touchPos.x = event.getX(); touchPos.y = event.getY(); Log.i("Co-ord", touchPos.x+"hh"+touchPos.y); switch(event.getAction()){ case MotionEvent.ACTION_MOVE : dX = touchPos.x - oldX; dY = touchPos.y - oldY; if(touchPos.y > getHeight()/2){ dX = dX*-1; } if(touchPos.x < getWidth()/2){ dY = dY*-1; } _renderObj.mAngle += (dX+dY) * TOUCH_SCALE_FACTOR; requestRender(); Log.i("AngleCo-ord", _renderObj.mAngle +"hh"); } oldX = touchPos.x; oldY = touchPos.y; Log.i("OldCo-ord", oldX+" hh "+oldY); return true; } } Last but not the least. My vector2 class. public class Vector2 { public static float TO_RADIANS = (1 / 180.0f) * (float) Math.PI; public static float TO_DEGREES = (1 / (float) Math.PI) * 180; public float x, y; public Vector2() { } public Vector2(float x, float y) { this.x = x; this.y = y; } public Vector2(Vector2 other) { this.x = other.x; this.y = other.y; } public Vector2 cpy() { return new Vector2(x, y); } public Vector2 set(float x, float y) { this.x = x; this.y = y; return this; } public Vector2 set(Vector2 other) { this.x = other.x; this.y = other.y; return this; } public Vector2 add(float x, float y) { this.x += x; this.y += y; return this; } public Vector2 add(Vector2 other) { this.x += other.x; this.y += other.y; return this; } public Vector2 sub(float x, float y) { this.x -= x; this.y -= y; return this; } public Vector2 sub(Vector2 other) { this.x -= other.x; this.y -= other.y; return this; } public Vector2 mul(float scalar) { this.x *= scalar; this.y *= scalar; return this; } public float len() { return FloatMath.sqrt(x * x + y * y); } public Vector2 nor() { float len = len(); if (len != 0) { this.x /= len; this.y /= len; } return this; } public float angle() { float angle = (float) Math.atan2(y, x) * TO_DEGREES; if (angle < 0) angle += 360; return angle; } public Vector2 rotate(float angle) { float rad = angle * TO_RADIANS; float cos = FloatMath.cos(rad); float sin = FloatMath.sin(rad); float newX = this.x * cos - this.y * sin; float newY = this.x * sin + this.y * cos; this.x = newX; this.y = newY; return this; } public float dist(Vector2 other) { float distX = this.x - other.x; float distY = this.y - other.y; return FloatMath.sqrt(distX * distX + distY * distY); } public float dist(float x, float y) { float distX = this.x - x; float distY = this.y - y; return FloatMath.sqrt(distX * distX + distY * distY); } public float distSquared(Vector2 other) { float distX = this.x - other.x; float distY = this.y - other.y; return distX * distX + distY * distY; } public float distSquared(float x, float y) { float distX = this.x - x; float distY = this.y - y; return distX * distX + distY * distY; } } PS : i am able to handle the touch events. I can rotate the triangle with the touch of my finger. But i want that ONE VERTEX of the triangle should point at my finger position respective of the position of my finger.

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