Javascript 如何使用动态旋转值在HTML5画布中绘制水印？_Javascript_Html_Html5 Canvas

Javascript 如何使用动态旋转值在HTML5画布中绘制水印？

javascript html html5-canvas

Javascript 如何使用动态旋转值在HTML5画布中绘制水印？,javascript,html,html5-canvas,Javascript,Html,Html5 Canvas,我在HTML5中工作，我正在尝试使用文本在图像上绘制水印。我有以下代码： <html> <body onload="addWaterMarkOnPage();"> <div id="pageContainer" style="width: 612px; height: 792px;"> <canvas id="page1" width="612" height="792"></canvas&

我在HTML5中工作，我正在尝试使用文本在图像上绘制水印。我有以下代码：

<html>
    <body onload="addWaterMarkOnPage();">
        <div id="pageContainer" style="width: 612px; height: 792px;">
             <canvas id="page1" width="612" height="792"></canvas> //image loading canvas  
             </canvas>
        </div>
    </body>
<html>

}

我想在页面的对角线处绘制文本（“水印演示”），我尝试了很多方法，但都没有成功。

下面是一些改进的代码：

var container=document.getElementById("pageContainer")
var origCanvas=document.getElementById("page1");
var wmCanvas=document.createElement("canvas");
wmCanvas.id="watermark";
wmCanvas.width=origCanvas.width;
wmCanvas.height=origCanvas.height;
wmCanvas.setAttribute("style","position:absolute;border:1px solid black")

if(container.firstChild)
    container.insertBefore(wmCanvas, container.firstChild);
else
    container.appendChild(wmCanvas);

var wmContext=wmCanvas.getContext('2d');
wmContext.globalAlpha=0.5;
// setup text for filling
wmContext.font = "72px Comic Sans MS" ;
wmContext.fillStyle = "red";
// get the metrics with font settings
var metrics = wmContext.measureText("WaterMark Demo");
var width = metrics.width;
// height is font size
var height = 72;

// change the origin coordinate to the middle of the context
wmContext.translate(origCanvas.width/2, origCanvas.height/2);
// rotate the context (so it's rotated around its center)
wmContext.rotate(-Math.atan(origCanvas.height/origCanvas.width));
// as the origin is now at the center, just need to center the text
wmContext.fillText("WaterMark Demo",-width/2,height/2);

工作地点：

灵感来源于：

取决于你所说的“完全对角”。这里的旋转为π/4，表示半个四分之一圈。如果你想从一个角到另一个角，你需要计算角度。好的，这意味着每当我改变角度时，我也会改变平移坐标。这是正确的。对于角度，你可以这样读：那么有没有任何计算来获得平移坐标？我更新了小提琴：不是重新转换原点，而是调整了文本。由于之后不再使用上下文，因此不需要重置它。

var container=document.getElementById("pageContainer")
var origCanvas=document.getElementById("page1");
var wmCanvas=document.createElement("canvas");
wmCanvas.id="watermark";
wmCanvas.width=origCanvas.width;
wmCanvas.height=origCanvas.height;
wmCanvas.setAttribute("style","position:absolute;border:1px solid black")

if(container.firstChild)
    container.insertBefore(wmCanvas, container.firstChild);
else
    container.appendChild(wmCanvas);

var wmContext=wmCanvas.getContext('2d');
wmContext.globalAlpha=0.5;
// setup text for filling
wmContext.font = "72px Comic Sans MS" ;
wmContext.fillStyle = "red";
// get the metrics with font settings
var metrics = wmContext.measureText("WaterMark Demo");
var width = metrics.width;
// height is font size
var height = 72;

// change the origin coordinate to the middle of the context
wmContext.translate(origCanvas.width/2, origCanvas.height/2);
// rotate the context (so it's rotated around its center)
wmContext.rotate(-Math.atan(origCanvas.height/origCanvas.width));
// as the origin is now at the center, just need to center the text
wmContext.fillText("WaterMark Demo",-width/2,height/2);