javascript - 如何计算图像在整个区域的透明性？

就像这张照片一样，红色边框表示图像的大小，绿色边框表示图像的可见大小，绿色边框外面是alpha像素。如何计算可见帧？

两种计算机语言都可以。

最佳答案

// This function will return object with properties like left, top position and width & height of green box.
//@imgObj : Image Object of target image (.png)

 function getImageDetails(imgObj)
 {
    var cns = document.createElement("canvas");

    cns.width = imgObj.width;
    cns.height = imgObj.height;

    var ctx = canvas.getContext("2d");
    ctx.drawImage(imgObj,0,0,cns.width,cns.height);

    var imgData = ctx.getImageData(0, 0, cns.width, cns.height).data;
    var minCol,maxCol,curCol,
        minRow,maxRow,curRow;

    for (var i = 0; i < imgData.length; i += 4) {
         var index = Math.floor(i/4);
         curCol = index%cns.width;
         curRow = Math.floor(index/cns.width);
         if( imgData[i] != 0 ||
             imgData[i + 1] != 0 ||
             imgData[i + 2] != 0 ||
             imgData[i + 3] != 0)
         {
             minCol = minCol == undefined || minCol > curCol ? curCol : minCol;
             maxCol = maxCol == undefined || maxCol < curCol ? curCol : maxCol;
             minRow = minRow == undefined || minRow > curRow ? curRow : minRow;
             maxRow = maxRow == undefined || maxRow < curRow ? curRow : maxRow;
         }
    }

    var details = new Object();

    details.x = minCol;
    details.y = minRow;
    details.width = maxCol-minCol+1;
    details.height = maxRow-minRow+1;

    return details
 }