Checking for collision for maze game in canvas

Question

I am trying to develop a maze game in canvas. I have drawn the maze manually such as

    context.moveTo(100,700);
    context.lineTo(100,600);
    context.lineTo(200,600);
    context.lineTo(200,500);

and there are many more. I want to move the object through these lines without moving beyond those border.

For this I have tried the following code :

function right() {
    context.clearRect(0, 0, canvas.width, canvas.height);

    art();
    myImage11.src = "Point.png";
    x += 20;

    if ((x + 20 < 50)) {
        context.clearRect(0, 0, canvas.width, canvas.height);
        checkcollision();
        if (collision == 1) {
            x -= 20;
            collision = 0;
        }

        context.drawImage(myImage11, x, y, 50, 50);
    }

my checkcollision function does

function checkcollision() {
    var myImage11 = context.getImageData(x, y, 10, 10);
    var pix = myImage11.data;
    for (var i = 0; n = pix.length, i < n; i += 4) {
        if (pix[i] == 0) {
            collision = 1;
        }
    }
}

But it doesn't work. Please help to solve this!!! Thanks in advance

Answer 1

It's difficult to extrapolate the exact workings of the current scenario, like the collision only checked when x + 20 < 50 , but the problem probably is in the area from which the getImageData is obtained: the x and y positions of the 'avatar' with a width of 10 ( context.getImageData(x, y, 10, 10); ) This would check if the left side of the newavatar position has a collision, but not the right side in which direction the move was made. For that something like x + avatarWidth- XMovement and a width of XMovement should be checked.

Perhaps not entirely corresponding with your current setup, but made a fiddle that emulates the movement. In it the entire avatar target position is checked for a quicker setup, but could be optimized to use the new overlaps if needed.

var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var x = 100, y = 200;
var imgWidth = 50, imgHeight = 50;

var checkcollision = function(){ //checks collision of the entire 'avatar' rectangle. This can be done more efficiently by only checking the moved to area, but that would require different parameters per direction.
    var data = context.getImageData(x , y, imgWidth, imgHeight).data;
    return data.some(function(p){ return  p != 0;});
} 

function right() {
    context.clearRect(0,0, canvas.width, canvas.height);

    art(); //draw maze
    //myImage11.src = "Point.png";

    var curx= x;
    x+=20;   

    while(checkcollision() && x > curx){ //while collision decrease x (note, this could be more efficient by just checking the new position overlap)
        x--;
    } 

    drawAvatar();
}


function art(){
    context.moveTo(100,400); //decreased y a bit to keep it easier viewable while testing
    context.lineTo(100,300);
    context.lineTo(200,300);
    context.lineTo(200,200);
    context.stroke();
}

function drawAvatar(){
    //context.drawImage(myImage11, x, y, imgWidth, imgHeight);
    context.fillRect(x,y,imgWidth,imgHeight);  
}

art();drawAvatar();

Answer 2

Image data does do some weird stuff Sometimes the color you draw is off a little. Check the diffrence between y=98,99,100 http://jsfiddle.net/15wsszw4/

the formula gets the RED value of a pixel at x,y

console.log('R:'+imgData.data[y*4*c.width+x*4])