已解决。我不得不在画布创建之后更改画布大小：

Solved. I had to change the canvas size just after canvas creation:

var canvas = document.createElement('canvas');
var context = canvas.getContext('2d');

var size = 56;
canvas.height = size;
canvas.width = size;

https://jsfiddle.net/03h4fyka/1/

对WestLangley的信用。我有另一个问题，我会把它发布在另一个问题。

Credits to WestLangley. I have another problem, I'll post it in another question.