Javascript canvas. Draw negative coordinates

2017-07-17 16:07 dennis iversen

In order to use negative coordinates when drawing with canvas, the most simple way to do this is using translate. There is few caveats. I have made some comments in the code

(You can also se the live example on Codepen: https://codepen.io/diversen/pen/ZyZVpL )

<body>
    <script>
    window.onload = function () {
        console.log('loaded');

        var c = document.getElementById('canvas');
        var ctx = c.getContext('2d');

        // Center
        ctx.translate(200, 200);

        // Add some lines
        ctx.beginPath();
        ctx.moveTo(-200,0);
        ctx.lineTo(200,0);
        ctx.stroke();
        ctx.moveTo(0,-200);
        ctx.lineTo(0, 200);
        ctx.stroke();     

        ctx.font = "12px Arial";
        ctx.fillText("+ x",180, -10);
        ctx.fillText("- y",10, 180);

        ctx.fillText("- x",-200, -10);
        ctx.fillText("+ y",10, -180);  

        // From the above I notice that -x coordinates behaves
        // as expected, but the y coordinates jumps on the '
        // wrong side of the line. 

        ctx.fillRect(-100, -100, 4, 4); 
        ctx.fillText("(-100,-100). Incorrect" ,-100, -100); 

        // From 'translate' this is correct, but only the x coordinate
        // looks like it should:
        // x = 200 - 100 = 100
        // y = 200 - 100 = 100

        // In order to draw correct cartesian coordinates we could make
        // a simple function like this 

        function drawCartesianPoint(ctx, x, y) {
            ctx.fillRect(x, -(y), 4, 4); 
        }

        // And for text:
        function drawCartesianText(ctx, x, y, text) {
            ctx.fillText(text , x, -(y));  
        }

        // Draw corrext:
        drawCartesianPoint(ctx, -100, -100);
        drawCartesianText(ctx, -100, -100, '(-100, -100) correct');

    };
    </script>

    <canvas id="canvas" width="400" height="400">
    </canvas>

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