HTML5 Canvas, 2nd Edition

Rotation and Translation Transformations

An object on the canvas is said to be at the 0 angle rotation when it is facing to the left. (This is important if an object has a facing side; otherwise, we will use this as a guide.) Consequently, if we draw an equilateral box (all four sides are the same length), it doesn’t have an initial facing side other than one of the flat sides facing to the left. Let’s draw that box for reference:

//now draw a red square
context.fillStyle = "red";
context.fillRect(100,100,50,50);

Now, if we want to rotate the entire canvas 45 degrees, we need to do a couple simple steps. First, we always set the current Canvas transformation to the “identity” (or “reset”) matrix:

context.setTransform(1,0,0,1,0,0);

Because Canvas uses radians, not degrees, to specify its transformations, we need to convert our 45-degree angle into radians:

var angleInRadians = 45 * Math.PI / 180;
context.rotate(angleInRadians);

Lesson 1: Transformations are applied to shapes and paths drawn after the setTransform() or other transformation function is called

If you use this code verbatim, you will see a funny result...nothing! This is because the setTransform() function call affects only shapes drawn to the canvas after it is applied. We drew our square first and then set the transformation properties. This resulted in no change (or transform) to the drawn square. Example 2-7 gives the code in the correct order to produce the expected result, as illustrated in Figure 2-12.

Example 2-7. Simple ...