javascript - canvas getImageData 返回太多像素

Question

我正在创建一个移动网络应用程序游戏，让用户可以在屏幕上滑动来清除它。我正在尝试编写一个智能函数来检测几乎整个 Canvas 何时是透明的。

我有一个间隔，每秒调用下面的函数来进行检查。它应该工作的方式是我取一 block 20 像素宽、与 Canvas 一样高的 Canvas 部分。

该函数应该运行得相当流畅，但由于某种原因，getImageData 函数返回了许多像素。我的扇区始终是 20 像素宽并且具有相同的高度，因此每次都应该返回相同数量的像素。当然，由于 getImageData 返回的数字越来越大，我的循环变得越来越慢。

有谁知道这是什么原因造成的吗？我是否误解了 getImageData 函数的工作方式？

isCanvasTransparent: function (fromX, toX, currentIteration) {

        if(currentIteration < this.clearedIterations) { // If a sector is already clear, we dont need to check it again
            this.isCanvasTransparent(fromX + 20, toX + 20, currentIteration + 1);
            return;
        } else {
            var data = this.context.getImageData(fromX, 0, toX, parseInt(this.canvas.style.width)).data;
            var counter = 0;
            var i = 0;

            // For some reason, the length increases, but the diff between fromX and toX is always 20
            console.log(data.length); 

            for(i=0;i<(data.length);i+=4) { 
                if(data[i+3]!==0){
                    counter++;
                    // I stop iterating, since there are too many non transparent pixels
                    if(counter > 10) {
                        break;
                    }
                }
            }

            // I accept that 10 pixels in each sector is not transparent
            if((counter < 10) && !this.alerted) {
                this.clearedIterations++; // Here we increase clearedIterations, since this sector is clear

                // There are no more iterations, so we are done
                if(currentIteration === this.maxIterations) {
                     // this is the interval that calls isCanvasTransparent(0, 20, 0)
                     // in the first place. The interval is called each second. But as soon the whole view
                     // is transparent, we clear it, so that isCanvasTransparent is no longer called
                    clearInterval(this.checkCanvasTimeout);
                    this.alerted = true;
                    TouchHelpers.removeTouchEvents();
                    this.levelCompleted();
                    return;
                } else {
                    // this sector is clear, but we need to check the next one, since we are not at the end
                    this.isCanvasTransparent(fromX + 20, toX + 20, currentIteration + 1);
                }
            }
        }
    },

Answer 1

如果没有看到您如何设置 fromX/toX，则不确定，但看起来您没有为 getImageData 提供正确的参数。

context.getImageData 具有以下参数:

• x:要提取的起始x坐标，
• y:要提取的起始 y 坐标，
• 宽度:提取一个这么多像素宽的 block ，
• 高度:提取这么多像素高的 block

[添加:基于附加信息]

如果您想要相同大小的像素数据 block ，您的第一个调用是

var data=getImageData(0,0, 20, canvas.height).data;

你的第二个电话是:

// Note: the width arguement remains at 20, not increase to 40

var data=getImageData(20,0, 20, canvas.height).data;