Quick help needed with 2d rotation of vertex buffer object OpenGL

Alexei Nunez · 2012-10-26T02:30:24

I have a Ship object made up of 4 lines ( looks like the asteroids game ship ). I am rendering it with the vertex buffer object method. It gets drawn to the screen fine but when i go to rotate the ship the whole ship rotates around the (x,y) coordinate and not about the (x,y) coordinate, if that makes sense. CODE SNIPPETS - full code at: https://github.com/Jevi/Asteroids/tree/master/src/org/cstutorials This is how i render the ship object ( which is compromised of 4 lines loaded into a FloatBuffer ( im using LWJGL btw ) that FloatBuffer is loaded in to memory and I draw from that allocated memory. The random GL_POINT is to see where (x,y) currently is. @Override public void render(Graphics g) { glLoadIdentity(); glPushMatrix(); glBegin(GL_POINTS); glVertex2f(x, y); glEnd(); g.rotate(x, y, angle); g.drawLines(); glPopMatrix(); } This is my drawLines() implementation, the variables seen inside are handled by a gameObjectManager which is not in these snippets ( no need ) public void drawLines() { glEnableClientState(GL_VERTEX_ARRAY); glBindBuffer(GL_ARRAY_BUFFER, currentVertexHandle); glVertexPointer(numCoordinates, GL_FLOAT, 0, 0L); glDrawArrays(GL_LINES, 0, numPairVertices); glBindBuffer(GL_ARRAY_BUFFER, 0); glDisableClientState(GL_VERTEX_ARRAY); } Finally here is my rotatation implementation, it use to work fine when I used immediate mode rendering but now that I have VBOs it does not work the same public void rotate(float x, float y, float angle) { glTranslatef(x, y, 0); glRotatef(angle, 0, 0, 1); glTranslatef(-x, -y, 0); }

Graphics and GPU Programming Programming OpenGL

Started by Jevi October 25, 2012 10:55 PM

10 comments, last by Jevi 11 years, 6 months ago

Kaptein

2,226

October 26, 2012 02:19 AM

that's good to hear
sorry i got it backwards, yes the order of operations is indeed backwards

this is how matrices work in general
once you get a bit further into all this, and you are writing your own little matrix library (it's an important step to take)
then you will see why ;) not to mention the possibilities

Jevi

131

Author

October 26, 2012 02:30 AM

that's good to hear
sorry i got it backwards, yes the order of operations is indeed backwards
this is how matrices work in general
once you get a bit further into all this, and you are writing your own little matrix library (it's an important step to take)
then you will see why ;) not to mention the possibilities

thanks again! getting about 1000 fps more with VBOs as compared to intermediate rendering even though its just a couple of lines on the screen I can already see how this is going to be crucial later on

Quick help needed with 2d rotation of vertex buffer object OpenGL

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Quick help needed with 2d rotation of vertex buffer object OpenGL

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