S2JS - Rotating, flipping, pointing - another game : Moving first attempt

To express that idea as code, first we calculate the x and y distances. We do this using the centre point of the sprite (its top-left plus half its height-width):

var distance_x = target_x - (this.x + this.MyImg.width / 2); var distance_y = target_y - (this.y + this.MyImg.height / 2);

This all happens inside our Move_Towards method, of course.

Then we set the velocity equal to the distance. Only thing is, we don't want it to make the journey in a single frame -- that would be too quick and we wouldn't see a thing. We want it to take some time so we can watch it happen. We divide the distance by ten, so it will take ten frames to travel the distance.

this.velocity_x = distance_x / 10; this.velocity_y = distance_y / 10;

Try waving your mouse or finger around and watch the jellyfish move.

Try changing the 10 to a larger or smaller number. What happens to the speed of motion?

<html> <body> <canvas id="myCanvas" width=300 height=300 style="background-color:MediumSeaGreen"> </canvas> <script> var ctx = myCanvas.getContext("2d"); // Get the drawing context for the canvas var FPS = 40; // How many frames per second var mouse_x = 0; var mouse_y = 0; // Where is the mouse function MySprite (img_url) { this.x = 0; this.y = 0; this.visible= true; this.velocity_x = 0; this.velocity_y = 0; this.MyImg = new Image(); this.MyImg.src = img_url ; this.angle = 0; // How many degrees we are rotated this.flipV = false; // Are we flipped vertically this.flipH = false; // Are we flipped horizontally } MySprite.prototype.Do_Frame_Things = function() { ctx.save(); ctx.translate(this.x + this.MyImg.width/2, this.y + this.MyImg.height/2); ctx.rotate(this.angle * Math.PI / 180); if (this.flipV) ctx.scale(1,-1); if (this.flipH) ctx.scale(-1,1); if (this.visible) ctx.drawImage(this.MyImg, -this.MyImg.width/2, -this.MyImg.height/2); ctx.restore(); this.x = this.x + this.velocity_x; this.y = this.y + this.velocity_y; // move the thing } function MyTouchMoveHandler(MyEvent) { var rect = myCanvas.getBoundingClientRect(); mouse_x = MyEvent.touches[0].clientX - rect.left; mouse_y = MyEvent.touches[0].clientY - rect.top; MyEvent.preventDefault() } addEventListener("touchmove", MyTouchMoveHandler); addEventListener("touchstart", MyTouchMoveHandler); function MyMouseMoveHandler (MyEvent) { var rect = myCanvas.getBoundingClientRect(); mouse_x= MyEvent.clientX - rect.left; mouse_y= MyEvent.clientY - rect.top; } addEventListener("mousemove", MyMouseMoveHandler); MySprite.prototype.Point_Towards = function(target_x, target_y) { var distance_x = target_x - (this.x + this.MyImg.width/2); var distance_y = target_y - (this.y + this.MyImg.height/2); this.angle = (Math.atan2(distance_y, distance_x) * 180 / Math.PI) + 90; } MySprite.prototype.Move_Towards = function(target_x, target_y) { var distance_x = target_x - (this.x + this.MyImg.width / 2); var distance_y = target_y - (this.y + this.MyImg.height / 2); this.velocity_x = distance_x / 10; this.velocity_y = distance_y / 10; } function Do_a_Frame () { ctx.clearRect(0, 0, myCanvas.width, myCanvas.height); // clear the frame jelly.Point_Towards(mouse_x, mouse_y); // tell jelly to point to mouse jelly.Move_Towards(mouse_x, mouse_y); // and move that way jelly.Do_Frame_Things(); // let the jelly do its thing } var jelly= new MySprite("http://s2js.com/img/etc/jellyfish2.png"); jelly.x=120; jelly.y=120; setInterval(Do_a_Frame, 1000/FPS); // set my frame renderer </script> </body> </html>