了解画布如何将图像转换为黑白图像

Question

我发现这个脚本用于将图像转换为黑白图像,效果很好,但我希望能够更好地理解代码.我以评论的形式将我的问题放在代码中.

任何人都可以更详细地解释这里发生的事情:

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,canvas函数在官方规范中有描述.此外,MDC有助于更多"非正式"文章.例如drawImage,MDC上的功能就在这里.

2. 该getImageData函数返回一个对象,该对象包含一个包含所有像素字节数据的数组.每个像素由4个字节描述:r,g,b和a.

   r,g并且b是色分量(红,绿和蓝)和α是不透明度.因此每个像素使用4个字节,因此像素的数据开始于pixel_index * 4.

3. 是的,这是对价值的平均值.因为在接下来的3行中r,g并且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个组件的数量都相同).

   所以基本上,对于所有像素,这将保持:r === g,g === b因此也是如此r === b.它所持有的颜色是灰度(0, 0, 0黑色和255, 255, 255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).