Question

我制作了一个将鼠标坐标转换为画布像素坐标的函数：

/* Returns pixel coordinates according to the pixel that's under the mouse cursor**/
HTMLCanvasElement.prototype.relativeCoords = function(event) {
  var x,y;
  //This is the current screen rectangle of canvas
  var rect = this.getBoundingClientRect();

  //Recalculate mouse offsets to relative offsets
  x = event.clientX - rect.left;
  y = event.clientY - rect.top;
  //Also recalculate offsets of canvas is stretched
  var width = rect.right - rect.left;
  //I use this to reduce number of calculations for images that have normal size 
  if(this.width!=width) {
    var height = rect.bottom - rect.top;
    //changes coordinates by ratio
    x = x*(this.width/width);
    y = y*(this.height/height);
  } 
  //Return as an array
  return [x,y];
}

您可以看到pixel coordinate calculation的演示。问题是图像having border property set的解决方案失败了。

如何从矩形中减去边框宽度？性能确实很重要，因为此计算通常在鼠标移动事件期间执行。

Answer 1

getComputedStyle包含您想要的信息：

在设置了画布边框后，在应用开始时获取一次边框信息。

// get a reference to the canvas element
var canvas=document.getElementById('yourCanvasId');

// get its computed style
var styling=getComputedStyle(canvas,null);

// fetch the 4 border width values
var topBorder=styling.getPropertyValue('border-top-width');
var rightBorder=styling.getPropertyValue('border-right-width');
var bottomBorder=styling.getPropertyValue('border-bottom-width');
var leftBorder=styling.getPropertyValue('border-left-width');

如果您在app-wide范围内设置这些边框宽度变量，则可以在HTMLCanvasElement.prototype.relativeCoords中使用这些预取变量。

祝你的项目好运！

获取没有边框的边界客户端矩形

1 个答案: