2d rotation around a point using Canvas and Path

The issue involves an Android Path shape. It's a triangle that I'm using as an arrow to point towards objects on a screen Canvas. This is for a 2d game. player in the middle of the screen, objects around him and offscreen.

These arrows are supposed to rotate around the center of the screen, with a radius so that they rotate in a circle around the player. The arrows point towards objects that the player needs to move towards.

What I have right now is somewhat working, but the arrows 开发者_运维技巧are zipping around the circle at ridiculous speeds. Funny enough, they're pointing in the right direction, but they aren't staying at the right point on the circle. (if arrow is pointing northeast, arrow should be on the northeast part of the circle, etc)

I'm sure it's because of the math. I'm probably using atan2 wrong. Or canvas.translate wrong. Or maybe I shouldn't be using atan2 at all. Help! :)

Here is the code:

    // set the shape of our radar blips
    oBlipPath.moveTo(0, -5);
    oBlipPath.lineTo(5, 0);
    oBlipPath.lineTo(0, 5);

    // Paint all the enemies and radar blips!
    for(int i=0; i<iNumEnemies; i++){
        if (oEnemies[i].draw(canvas, (int)worldX, (int)worldY)){

            //calculate the degree the object is from the center of the screen. 
            //(user is the player. this could be done easier using iWidth and iHeight probably)
            //we use a world coordinate system. worldY and worldX are subtracted
            fDegrees = (float)Math.toDegrees(Math.atan2((oEnemies[i].getEnemyCenterY()-worldY)-user.getShipCenterY(), (oEnemies[i].getEnemyCenterX()-worldX)-user.getShipCenterX()));

            canvas.save();

            //get to the center
            canvas.translate((iWidth / 2) , (iHeight / 2) );

            //move a little bit depending on direction (trying to make arrows appear around a circle)
            canvas.translate((float)(20 * Math.cos(fDegrees)), (float)(20* Math.sin(fDegrees)));

            //rotate canvas so arrows will rotate and point in the right direction
            canvas.rotate(fDegrees);

            //draw arrows
            canvas.drawPath(oBlipPath, oBlipPaint);
            canvas.restore();

        }
    }

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Affine transformations are are not commutative. They are typically applied in an apparent last-specified-first-applied order. As an alternative, consider the rotate() variation that rotates about a point.

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Well, I've got it doing what I wanted, but I don't really know how. I threw in some random numbers until things showed up on the screen the way I wanted. If anyone wants to clue me in as to a better way to do this, I'm all ears.

The code:

    // set the shape of our radar blips
    oBlipPath.moveTo(0, -5);
    oBlipPath.lineTo(6, 0);
    oBlipPath.lineTo(0, 5);


    oBlipMatrix.setRotate(45, 0, 0);
    oBlipPath.transform(oBlipMatrix);

    // Paint all the enemies and radar blips!
    for(int i=0; i<iNumEnemies; i++){
        oEnemies[i].draw(canvas, (int)worldX, (int)worldY);
        if (oEnemies[i].bActive){
            //calculate the degree the object is from the center of the screen. 
            //(user is the player. this could be done easier using iWidth and iHeight probably)
            //we use a world coordinate system. worldY and worldX are subtracted
            fDegrees = (float)Math.toDegrees(Math.atan2((oEnemies[i].getEnemyCenterY()-worldY)-(iHeight / 2), (oEnemies[i].getEnemyCenterX()-worldX)-(iWidth / 2)));

            canvas.save();

            //get to the center
            canvas.translate((iWidth / 2 + 50) , (iHeight / 2 + 50) );

            //move a little bit depending on direction (trying to make arrows appear around a circle)
            //canvas.translate((float)(20 * Math.cos(fDegrees)), (float)(20* Math.sin(fDegrees)));

            //rotate canvas so arrows will rotate and point in the right direction
            canvas.rotate(fDegrees-45, -50, -50);

            //draw arrows
            canvas.drawPath(oBlipPath, oBlipPaint);
            canvas.restore();
        }

    }

For whatever reason, I have to subtract 45 degrees from the canvas rotation, but add 45 degrees to the matrix rotation of the path shape. It works, but why?! :)