Particles placed in the middle of the screen (OpenGL ES 2 / Android)

Question

It seems like each of my particles is placed at (0, 0, 0) even though I'm setting different positions for each of them (debugger also confirms this). It must be something really silly that I'm missing. My intention is to paint 250 particles at random positions on the screen. Here's my code:

@Override
public void onDrawFrame(GL10 gl) {
    if(time++ == MAXTIME) {
        time = 0;
    }

    //my own helper methods
    GLUtils.sendBufferData(a_Position, 3, position);
    GLUtils.sendBufferData(a_Velocity, 3, velocity);
    GLUtils.sendBufferData(a_Offset, 1, offset);

    Matrix.setIdentityM(mModelMatrix, 0);
    Matrix.multiplyMM(mMVPMatrix, 0, camera.getViewMatrix(), 0, mModelMatrix, 0);
    Matrix.multiplyMM(mMVPMatrix, 0, camera.getProjectionMatrix(), 0, mMVPMatrix, 0);

    GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mMVPMatrix, 0);

    GLES20.glUniform1f(u_Time, time);

    GLES20.glActiveTexture(GLES20.GL_TEXTURE0);
    GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, texture);
    GLES20.glUniform1i(u_Texture, 0);

    GLES20.glDrawArrays(GLES20.GL_POINTS, 0, particlesCount); //particlesCount = 250
}

@Override
protected void initData() {
    float[] positions = new float[3 * count];
    float[] velocities = new float[3 * count];
    float[] offsets = new float[count];

    int index = 0;

    for(int i = 0; i < count; i++) {
        offsets[i] = MathUtils.random() * 60 + 20;

        positions[index] = MathUtils.random() * 2 - 1;
        velocities[index] = -0.025f;
        index++;

        positions[index] = MathUtils.random() * 2 - 1;
        velocities[index] = -0.035f;
        index++;

        positions[index] = 0;
        velocities[index] = 0;
        index++;
    }
    //my own helper method
    texture = GLUtils.loadTexture(mContext, R.drawable.snowflake);

    position = ByteBuffer.allocateDirect(positions.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();
    velocity = ByteBuffer.allocateDirect(velocities.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();
    offset = ByteBuffer.allocateDirect(offsets.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();

    position.put(positions).position(0);
    velocity.put(velocities).position(0);
    offset.put(offsets).position(0);
}

private static final String vertexShader() {
    return "" +
            "uniform mat4 u_MVPMatrix;\n" +
            "uniform float u_Lifetime;\n" +
            "uniform float u_CurrentTime;\n" +
            "\n" +
            "attribute vec4 a_Position;\n" +
            "attribute vec3 a_Velocity;\n" +
            "attribute float a_Offset;\n" +
            "\n" +
            "void main() {\n" +
            "    gl_PointSize = 10.0;\n" +
            "    gl_Position = u_MVPMatrix * a_Position;\n" +
            "}";
}

private static final String fragmentShader() {
    return "precision mediump float;\n" +
            "\n" +
            "uniform sampler2D u_Texture;\n" +
            "\n" +
            "void main() {\n" +
            "    gl_FragColor = texture2D(u_Texture, gl_PointCoord);\n" +
            "}";
}

Note: I deliberately deleted some lines from the vertex shader since I don't think they are relevant in this case.

Edit: It may be that gl_PointCoord is always set to 0. I'm looking into this now.

Answer 1

you allocate and initialize positions and velocities but never fill in the allocated buffers:

position = ByteBuffer.allocateDirect(positions.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();
velocity = ByteBuffer.allocateDirect(velocities.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();
offset = ByteBuffer.allocateDirect(offsets.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();

position.put(positions);
velocity.put(velocities);
offset.put(offsets);