Magnifying part of the canvas when touched

Question

I have a custom view that extends SurfaceView. I am drawing an image on the canvas. When the user touches a point on the canvas, i want to show a magnified view of the area

Answer 1

To zoom the image you're drawing on the canvas:

Create a BitmapShader (using the bitmap of the image you're drawing), a Matrix and a Paint:

shader = new BitmapShader(bmp, TileMode.CLAMP, TileMode.CLAMP);
matrix = new Matrix();
shaderPaint = new Paint();
shaderPaint.setShader(shader);

On a touch event record the touch position (e.g. in a PointF):

zoomPos.x = event.getX();
zoomPos.y = event.getY();

...and set up the shader's matrix (I do this on each touch, there's probably a better way):

matrix.reset();
matrix.postScale(2f, 2f);
matrix.postTranslate(-zoomPos.x, -zoomPos.y);
shader.setLocalMatrix(matrix);

Then in the drawing code, draw a circle using the shader Paint.

canvas.drawCircle(zoomPos.x, zoomPos.y, size_of_the_circle, shaderPaint);

Edit

The two lines:

matrix.postScale(2f, 2f);
matrix.postTranslate(-zoomPos.x, -zoomPos.y);

Can be replaced with one:

matrix.postScale(2f, 2f, zoomPos.x, zoomPos.y);

This allows the scale factor to be changed without breaking the offset.