了解画布如何将图像转换为黑色和白色

Understanding how canvas converts an image to black and white

本文关键字：黑色 白色 转换 图像 布如何 了解      更新时间：2023-09-26

我发现这个脚本用于将图像转换为黑色和白色，它工作得很好，但我希望更好地理解代码。我把我的问题放在代码中，以注释的形式。

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();
}

  1. 画布函数和大多数函数一样，在官方规范中描述。此外，MDC对更"非正式"的文章也很有帮助。例如，MDC上的drawImage函数在这里

  2. 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。每个像素用4个字节来描述:r, g, b和a。

    r、g和b是颜色成分(红、绿、蓝)，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，它在取平均值。因为在接下来的3行r, g和b都被设置为相同的值，所以您将获得每个像素的灰色(因为所有3个组件的数量是相同的)。

    所以基本上，对于所有像素，这将保持:r === g, g === b和r === b。
    4. 为灰度(0, 0, 0为黑色，255, 255, 255为白色)。
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();
}

在这个函数中使用系数等于1/3，但通常使用的是:0.3R + 0.59G + 0.11B (http://gimp-savvy.com/BOOK/index.html?node54.html)。

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