OpenGL ES rotating shape around its center-CodePudding

I tried to rotate rectangle using rotateMatrix, following tutorial on https://developer.android.com/training/graphics/opengl/motion, but when I runned it, the rectangle is rotating around (I think) point 0,0. What do I need to change to make it rotate around it's center?

@Override
public void onSurfaceChanged(GL10 gl, int width, int height) {
    MainGLRenderer.width = width;
    MainGLRenderer.height = height;

    loadLevel();

    GLES20.glViewport(0, 0, width, height);

    Matrix.orthoM(projectionMatrix, 0, 0, width, height, 0, 1, -1);
    Matrix.setLookAtM(viewMatrix, 0, 0, 0, 1, 0f, 0f, 0f, 0f, 1.0f, 0.0f);
    Matrix.multiplyMM(mvpMatrix, 0, projectionMatrix, 0, viewMatrix, 0);
}

@Override
public void onDrawFrame(GL10 gl) {
    GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT | GLES20.GL_DEPTH_BUFFER_BIT);

    float[] scratch = new float[16];
    long time = SystemClock.uptimeMillis() % 4000L;
    float angle = 0.090f * ((int) time);
    Matrix.setRotateM(rotationMatrix, 0, angle, 0, 0, -1.0f);
    Matrix.multiplyMM(scratch, 0, mvpMatrix, 0, rotationMatrix, 0);

    rectangle.x = 200;
    rectangle.y = 200;
    rectangle.w = 200;
    rectangle.h = 200;
    rectangle.draw(scratch);
}

CodePudding user response：

If you want to rotate around a pivot you have to:

Translate the object so that the pivot point is moved to (0, 0).
Rotate the object.
Move the object so that the pivot point moves in its original position.

float pivotX = 300;
float pivotY = 300;
Matrix.setIdentityM(rotationMatrix, 0);
Matrix.translateM(rotationMatrix, 0, pivotX, pivotY, 0);
Matrix.rotateM(rotationMatrix, 0, angle, 0, 0, -1.0f);
Matrix.translateM(rotationMatrix, 0, -pivotX, -pivotY, 0);

However, I recommend drawing the rectangle so that the center of the rectangle is at position (0, 0). Finally move the rectangle to its target position in the scene:

Matrix.setIdentityM(rotationMatrix, 0);
Matrix.translateM(rotationMatrix, 0, pivotX, pivotY, 0);
Matrix.rotateM(rotationMatrix, 0, angle, 0, 0, -1.0f);
Matrix.multiplyMM(scratch, 0, mvpMatrix, 0, rotationMatrix, 0);

rectangle.x = -100;
rectangle.y = -100;
rectangle.w = 200;
rectangle.h = 200;
rectangle.draw(scratch);