What does canvas.translate do?

Question

An example can be found here compass.java. Api here

Answer 1

Even when I first answered this question a few years ago, I didn't really understand how Canvas transforms (like translate, rotate, etc.) worked. I used to think that translate moved the thing that you were drawing. Actually, translate moves the entire coordinate system. This has the desired effect of also moving the thing you are drawing.

On your screen it looks like you are moving the drawing:

What is actually happening is you are moving the coordinate system to a new place on the canvas:

I draw the tree first at (0,0). Then I translate the origin of the coordinate system to some other spot on the canvas. Then I draw the tree again at (0,0). This way my drawing code doesn't have to change anything at all. Only the coordinate system changes.

Normally (0,0) is at the top left of your view. Doing Canvas.translate moves it to some other part of your view.

Saving and Restoring the Coordinate System

You can do save() and restore() to get back to your original coordinate system.

// draw the tree the first time
canvas.drawBitmap(tree, 0, 0, mPaint);

// draw the tree the second time
canvas.save();
canvas.translate(dx, dy); // dx = change in x, dy = change in y
canvas.drawBitmap(tree, 0, 0, mPaint); // draw still thinks it is at (0,0)
canvas.restore(); // undo the translate 

When you restore, the drawing is already on the canvas. Restoring doesn't change that. It just moves the coordinate system back to wherever it was when you saved it.

Why Translate

Note that you could achieve the same affect by changing the x,y coordinates of the draw method:

// draw the tree the first time
canvas.drawBitmap(tree, x, y, mPaint);

// update the x,y coordinates
x += dx;
y += dy;

// draw the tree the second time
canvas.drawBitmap(tree, x, y, mPaint);

This might be more intuitive coming from a math background. However, when you are translating, rotating, and scaling your image, it is often much easy to keep your drawing logic the same and just transform the canvas. Recalculating x and y for every draw could be very expensive.