在openGL android中旋转三角形

Question

I did a simple app based on the sample of getting started at developer.android.com. It was working fine, but I changed the rotation logic and now it doesn't show the triangle. I'm not sure I understood the matrix stuff, so I would like if someone could check my code. This is the SurfaceView method I think may have a problem:

@Override
public boolean onTouchEvent(MotionEvent e){
    float x = e.getX();
    float y = e.getY();

    switch(e.getAction()){
    case MotionEvent.ACTION_MOVE:
        float rotateX = 0.0f;
        float rotateY = 0.0f;
        float rotateZ = 0.0f;

        //na coluna da esquerda, rotação no eixo Z
        if(x < getWidth() / 3){
            rotateZ = (y - previousY);
        }
        //na coluna do meio, rotação no eixo X
        if(getWidth()/3 < x && x < 2*getWidth()/3){
            rotateX = (y - previousY);
        }
        //na coluna da direita, rotação no eixo Z invertido
        if(x > getWidth() / 3){
            rotateZ = - (y - previousY);
        }

        //na linha superior, rotação no eixo Z
        if(y < getHeight() / 3){
            rotateZ = (x - previousX);
        }
        //na linha do meio, rotação no eixo Y
        if(getHeight()/3 < y && y < 2*getHeight()/3){
            rotateY = (x - previousY);
        }
        //na linha inferior, rotação no eixo Z invertido
        if(y > 2*getHeight() / 3){
            rotateZ = - (x - previousX);
        }

        mRenderer.setAngulo(mRenderer.getAnguloX() + (rotateX) * TOUCH_SCALE_FACTOR,
                            mRenderer.getAnguloY() + (rotateY) * TOUCH_SCALE_FACTOR,
                            mRenderer.getAnguloZ() + (rotateZ) * TOUCH_SCALE_FACTOR);
        requestRender();

    }
    previousX = x;
    previousY = y;
    return true;
}

This is the renderer method:

@Override
public void onDrawFrame(GL10 unused) {

    //desenha o fundo
    GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);

    //define a posição da camera
    //Matrix.setLookAtM(mViewMatrix, 0, eyeX, eyeY, eyeZ, lookX, lookY, lookZ, upX, upY, upZ);
    Matrix.setLookAtM(mViewMatrix, 0, 0, 0, -5f, 0f, 0f, 0f, 0f, 1.0f, 0.0f);

    //cria uma matrix para a rotação de cada angulo
    Matrix.setRotateM(xRotationMatrix, 0, anguloX, 1, 0, 0);
    Matrix.setRotateM(yRotationMatrix, 0, anguloY, 0, 1, 0);
    Matrix.setRotateM(zRotationMatrix, 0, anguloZ, 0, 0, 1);

    float[] mMatrix = new float[16];
    //aplica todas as rotações na matrix scratch
    Matrix.multiplyMM(mMatrix, 0, xRotationMatrix, 0, mMatrix, 0);
    Matrix.multiplyMM(mMatrix, 0, yRotationMatrix, 0, mMatrix, 0);
    Matrix.multiplyMM(mMatrix, 0, zRotationMatrix, 0, mMatrix, 0);

    // Calcula a view e depois a projeção
    Matrix.multiplyMM(resultingMatrix, 0, mViewMatrix, 0, mMatrix, 0);
    Matrix.multiplyMM(resultingMatrix, 0, mProjectionMatrix, 0, resultingMatrix, 0);

    // Draw shape
    triangulo.draw(resultingMatrix);
}

The Triangulo class must be right because I didn't changed it since the last time the app was working.

Answer 1

Well, i analysed your code thoroughly and there does not seem to be any problem in it.

But, i guess the problem is this :

When you call

Matrix.setLookAtM(mViewMatrix, 0, 0, 0, -5f, 0f, 0f, 0f, 0f, 1.0f, 0.0f);

You are setting the camera at Z = -5 and looking towards origin ie towards the +ve Z direction.

(I guess initially the vertices of the triangle must be in XY plane and hence the triangle is visible.)

Then when you call

Matrix.setRotateM(zRotationMatrix, 0, anguloZ, 0, 0, 1);

It rotates the triangle about Z axis which is fine.

But then you call

Matrix.setRotateM(xRotationMatrix, 0, anguloX, 1, 0, 0);
Matrix.setRotateM(yRotationMatrix, 0, anguloY, 0, 1, 0);

Here you are rotating the triangle about X and Y axis and since triangles have no thickness, It becomes invisible.

Possible fixes :

1) Remove the rotations about X and Y axis and check if it works.

                  OR

2) Put a cube instead of triangle and see if it works.

                  OR

3) If neither works, give me the full code.I will debug and run it.

Best of Luck