Question

Good evening.

I’m trying to rotate a line on a canvas using setRotation -method and it works perfectly, unless you want to draw another shape on the same canvas. After using Canvas’s concat -method, the entire canvas will be rotated, let’s say, by 30 degrees counterclockwise/clockwise. And this is the problem. I would like to rotate only the line and I don’t want to rotate any other shapes on this canvas or the entire canvas. I found out that the bitmap could be drawn with matrices, but it looks cumbersome and clumsy. Also, there was a suggestion to setup a new matrix for the Canvas, in fact, this proposition works neither.

So, the question sounds simple enough, how could the only one shape on the canvas be rotated without using OpenGl and affecting on other shapes on the canvas?

Thank you for your answers in advance!

Here is the code with comments and other stuff:

@Override
public void onDraw(Canvas canvas)
{
    int startX, startY, stopX, stopY;
    startY = stopY = 100;
    startX = 100;
    stopX = 200;
    this.paint = new Paint();
    //this.path = new Path();
    this.matrix = canvas.getMatrix();
    this.paint.setColor(Color.BLUE);
    this.paint.setStrokeWidth(4);

    this.matrix.setRotate(180, startX, startY);
    canvas.concat(this.matrix);
    /*this.matrix.setTranslate(startX, 0);
    canvas.concat(this.matrix);*/

    canvas.drawLine(startX, startY, stopX, stopY, this.paint);

    canvas.setMatrix(new Matrix());
    //canvas.drawCircle(200, 200, 50, paint);
}

Answer 1

You could try Canvas.save() and Canvas.restore() for this. They supposedly put current matrix into stack and you can return back to previous one once done with the modified matrix.

this.matrix.setRotate(180, startX, startY);
canvas.save();
canvas.concat(this.matrix);
canvas.drawLine(startX, startY, stopX, stopY, this.paint);
canvas.restore();