了解画布如何将图像转换为黑白

Question

我发现了这个用于将图像转换为黑白的脚本，效果很好，但我希望能更好地理解代码。我将我的问题以 cmets 的形式放在代码中。

谁能更详细地解释一下这里发生了什么：

function grayscale(src){ //Creates a canvas element with a grayscale version of the color image
    var canvas = document.createElement('canvas');
    var ctx = canvas.getContext('2d');
    var imgObj = new Image();
    imgObj.src = src;
    canvas.width = imgObj.width;
    canvas.height = imgObj.height; 
    ctx.drawImage(imgObj, 0, 0); //Are these CTX functions documented somewhere where I can see what parameters they require / what those parameters mean?
    var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
    for(var y = 0; y < imgPixels.height; y++){
        for(var x = 0; x < imgPixels.width; x++){
            var i = (y * 4) * imgPixels.width + x * 4; //Why is this multiplied by 4?
            var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3; //Is this getting the average of the values of each channel R G and B, and converting them to BW(?)
            imgPixels.data[i] = avg; 
            imgPixels.data[i + 1] = avg; 
            imgPixels.data[i + 2] = avg;
        }
    }
    ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height); 
    return canvas.toDataURL();
}

Answer 1

1. 与大多数函数一样，画布函数在an official specification 中进行了描述。此外，MDC 有助于更多“非正式”文章。例如。 MDC 上的drawImage 函数是here。

2. getImageData 函数返回一个对象，该对象包含一个数组，其中包含所有像素的字节数据。每个像素由 4 个字节描述：r、g、b 和 a。

   r、g 和 b 是颜色分量（红色、绿色和蓝色），alpha 是不透明度。所以每个像素使用4个字节，因此一个像素的数据从pixel_index * 4开始。

3. 是的，它正在平均这些值。因为在接下来的 3 行中，r、g 和 b 都设置为相同的值，您将获得每个像素的灰色（因为所有 3 个分量的数量相同）。

   所以基本上，对于所有像素，这将保持：r === g、g === b 和 r === b。保持此状态的颜色为灰度（0, 0, 0 为黑色，255, 255, 255 为白色）。

Answer 2

function grayscale(src){ //Creates a canvas element with a grayscale version of the color image
    //create canvas
    var canvas = document.createElement('canvas');
    //get its context
    var ctx = canvas.getContext('2d');
    //create empty image
    var imgObj = new Image();
    //start to load image from src url
    imgObj.src = src;
    //resize canvas up to size image size
    canvas.width = imgObj.width;
    canvas.height = imgObj.height;
    //draw image on canvas, full canvas API is described here http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html
    ctx.drawImage(imgObj, 0, 0);
    //get array of image pixels
    var imgPixels = ctx.getImageData(0, 0, canvas.width, canvas.height);
    //run through all the pixels
    for(var y = 0; y < imgPixels.height; y++){
        for(var x = 0; x < imgPixels.width; x++){
            //here is x and y are multiplied by 4 because every pixel is four bytes: red, green, blue, alpha
            var i = (y * 4) * imgPixels.width + x * 4; //Why is this multiplied by 4?
            //compute average value for colors, this will convert it to bw
            var avg = (imgPixels.data[i] + imgPixels.data[i + 1] + imgPixels.data[i + 2]) / 3;
            //set values to array
            imgPixels.data[i] = avg; 
            imgPixels.data[i + 1] = avg; 
            imgPixels.data[i + 2] = avg;
        }
    }
    //draw pixels according to computed colors
    ctx.putImageData(imgPixels, 0, 0, 0, 0, imgPixels.width, imgPixels.height); 
    return canvas.toDataURL();
}

在此函数中使用等于 1/3 的系数，但通常使用的是：0.3R + 0.59G + 0.11B (http://gimp-savvy.com/BOOK/index.html?node54.html) .