HTML5 Canvas [146]
context.translate(x,y);
context.rotate(angleInRadians);
//drawShip
context.strokeStyle = '#ffffff';
context.beginPath();
context.moveTo(10,0);
context.lineTo(19,19);
context.lineTo(10,9);
context.moveTo(9,9);
context.lineTo(0,19);
context.lineTo(9,0);
context.stroke();
context.closePath();
//restore context
context.restore(); //pop old state on to screen
//add to rotation
rotation++;
}
As you can see, the player ship rotates clockwise one degree at a time. As we’ve mentioned many times already, we must convert from degrees to radians because the context.rotate() transformations use radians for calculations. In the next section, we’ll take a deeper look at some of the transformations we will use in our Geo Blaster Basic game.
Game Graphic Transformations
As we saw in the previous section, we can easily rotate a game graphic at the top-left corner by using the context.rotate() transformation. However, our game will need to rotate objects at the center rather than the top-left corner. To do this, we must change the transformation point to the center of our game graphic object.
Rotating the Player Ship from the Center
The code to rotate the player ship from its center point is almost exactly like the code used to rotate it at the top-left corner. What we need to modify is the point of the translation. In Example 8-5, we placed the immediate-mode drawing context at the x and y coordinates of our game object (50,50). This had the effect of rotating the object from the top-left corner. Now we must move the translation to the center of our object:
context.translate(x+.5*width,y+.5*height);
NOTE
The width and height variables represent attributes of our drawn player ship. We will create these attributes in Example 8-6.
This is not the only change we need to make; we also need to draw our ship as though it is the center point. To do this, we will subtract half the width from each x attribute in our path draw sequence, and half the height from each y attribute:
context.moveTo(10-.5*width,0-.5*height);
context.lineTo(19-.5*width,19-.5*height);
As you can see, it might get a little confusing trying to draw coordinates in this manner. It is also slightly more processor-intensive than using constants. In that case, we would simply hardcode in the needed values. Remember, the width and height attributes of our ship are both 20. The hardcoded version would look something like this:
context.moveTo(0,−10); //10-10, 0-10
context.lineTo(9,9); //19-10, 19-10
The method where we use the calculated values (using the width and height variables) is much more flexible, while the hardcoded method is much less processor-intensive. Example 8-6 contains all the code to use either method. We have commented out the calculated version of the code.
Example 8-6. Rotating an image from its center point
//canvasApp level variables
var rotation = 0;
var x = 50;
var y = 50; var width = 20;
var height = 20;
function drawScreen() {
// draw background and text
context.fillStyle = '#000000';
context.fillRect(0, 0, 200, 200);
context.fillStyle = '#ffffff';
context.font = '20px _sans';
context.textBaseline = 'top';
context.fillText ("Player Ship - rotate", 0, 180);
//transformation
var angleInRadians = rotation * Math.PI / 180;
context.save(); //save current state in stack
context.setTransform(1,0,0,1,0,0); // reset to identity
//translate the canvas origin to the center of the player
context.translate(x+.5*width,y+.5*height);
context.rotate(angleInRadians);
//drawShip
context.strokeStyle = '#ffffff';
context.beginPath();
//hardcoding in locations
context.moveTo(0,-10);
context.lineTo(9,9);
context.lineTo(0,-1);
context.moveTo(-1,-1);
context.lineTo(-10,9);
context.lineTo(-1,-10);
/*
//using the width and height to calculate
context.moveTo(10-.5*width,0-.5*height);
context.lineTo(19-.5*width,19-.5*height);
context.lineTo(10-.5*width,9-.5*height);
context.moveTo(9-.5*width,9-.5*height);
context.lineTo(0-.5*width,19-.5*height);
context.lineTo(9-.5*width,0-.5*height);