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javaandroidopengl-escoordinate-transformation

Open GL Rotate Object

I am trying to rotate an object in Open GL. I can successfully draw a square, but when I try to rotate it stays in the same place. I have tried to move around the order of the lines below but still doesn't work, (doesn't draw at all with certain order or certain lines removed)... here's the code i have so far, mAngle is a random float between 0 and 360.

    Matrix.setIdentityM(mModelMatrix, 0);
    Matrix.setRotateM(mModelMatrix, 0, mAngle, 0f, 0f, 1.0f);
    Matrix.setLookAtM(mModelMatrix, 0, 0, 0, 5f, 0f, 0f, 0f, 0f, 10.0f, 0.0f);
    Matrix.translateM(mModelMatrix, 0, 0f, 0f, 0f);
    Matrix.multiplyMM(mMVPMatrix, 0, mProjectionMatrix, 0, mModelMatrix, 0);

    mSquare.draw(mMVPMatrix);
Solution

  • Matrix.setRotateM creates a new matrix for rotation by angle a (in degrees) around the axis (x, y, z).
    Matrix.setLookAtM defines a new viewing transformation in terms of an eye point, a center of view, and an up vector.
    Both operations do not manipulate a input matrix, they create a complete new matrix by their parameters and write it to the output rm.

    In compare, with Matrix.rotateM a given matrix is rotated.

    See android.opengl - Matrix


    You have to create a separated view matrix by Matrix.setLookAtM:

    Matrix viewM = new Matrix();
Matrix.setLookAtM(viewM, 0, 0, 0, 5f, 0f, 0f, 0f, 0f, 10.0f, 0.0f);

    and you have to multiply it to the roation matrix (model matrix) by Matrix.multiplyMM:

    Matrix.multiplyMM(mModelMatrix, 0, viewM, 0, modelM, 0);


    I recommend writing the code like this: 

    Matrix modelM= new Matrix();
Matrix.setRotateM(modelM, 0, mAngle, 0f, 0f, 1.0f);

Matrix viewM = new Matrix();
Matrix.setLookAtM(viewM, 0, 0, 0, 5f, 0f, 0f, 0f, 0f, 10.0f, 0.0f);

Matrix.multiplyMM(mModelMatrix, 0, viewM, 0, modelM, 0);
Matrix.multiplyMM(mMVPMatrix, 0, mProjectionMatrix, 0, mModelMatrix, 0);