It caught my attention that circles do not require any offsetting to be centered. Is this automatic? If so, can you explain why it differs from the rect, fillRect, fillText, and strokeText methods where centering requires offsetting?
It caught my attention that circles do not require any offsetting to be centered. Is this automatic? If so, can you explain why it differs from the rect, fillRect, fillText, and strokeText methods where centering requires offsetting?
Indeed, a complete arc() (also known as a circle) will be positioned at the x and y coordinates you provide to it. These two values determine the center of the ellipse that is going to be rendered.
As you mentioned, this behavior differs from the rect() method (as well as its fill / stroke counterparts), which essentially represents a sequence of points:
lineTo(x, y);
lineTo(x + width, y);
lineTo(x + width, y + height);
lineTo(x, y + height);
lineTo(x, y);
Hence, we can say that the starting point of the rect method aligns with its top-left corner, similar to other methods like drawImage and putImageData.
Conversely, the origin for fillText() is adjustable by configuring the textAlign and textBaseline attributes of your context, offering a distinct operation approach.
In addition, path methods utilize control points, making the concept of an origin less relevant in those cases.
var x = 250;
var y = 100;
var txt = ['click to redraw the rect', 'using the long-hand version'];
var ctx = canvas.getContext("2d");
ctx.font = '14px sans-serif';
var w = ctx.measureText(txt[1]).width;
// fillText origin can be set
ctx.textBaseline = 'middle';
ctx.textAlign = 'center';
ctx.fillText(txt[0], x, y - 8);
ctx.fillText(txt[1], x, y + 8);
// rect / strokeRect / fillRect is top-left
ctx.strokeRect(x, y, w, 16);
// arc is x,y
ctx.arc(x, y, w / 2, 0, Math.PI * 2);
ctx.stroke();
// show that rect is just a shorthand
onclick = function() {
ctx.beginPath();
ctx.strokeStyle = 'red';
ctx.lineTo(x, y);
ctx.lineTo(x + w, y);
ctx.lineTo(x + w, y + 16);
ctx.lineTo(x, y + 16);
ctx.lineTo(x, y);
ctx.stroke();
}<canvas id="canvas" width="500" height="200"></canvas>It's important to note that the circle is not automatically centered. For more information, I recommend checking out this helpful article on MDN:
Key Parameters:
xThe x-coordinate of the circle's center
Thus, specifying the center position is necessary unless it aligns with your current path.
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