I'm trying to create a thumbnail image on the client side using javascript and a canvas element, but when I shrink the image down, it looks terrible. It looks as if it was downsized in photoshop with the resampling set to 'Nearest Neighbor' instead of Bicubic. I know its possible to get this to look right, because this site can do it just fine using a canvas as well. I've tried using the same code they do as shown in the "[Source]" link, but it still looks terrible. Is there something I'm missing, some setting that needs to be set or something?
编辑:
我正在调整一张jpg图片的大小。我试过在链接的网站和photoshop中调整相同的jpg图片的大小，缩小后看起来很好。
以下是相关代码:
reader.onloadend = function(e)
{
var img = new Image();
var ctx = canvas.getContext("2d");
var canvasCopy = document.createElement("canvas");
var copyContext = canvasCopy.getContext("2d");
img.onload = function()
{
var ratio = 1;
if(img.width > maxWidth)
ratio = maxWidth / img.width;
else if(img.height > maxHeight)
ratio = maxHeight / img.height;
canvasCopy.width = img.width;
canvasCopy.height = img.height;
copyContext.drawImage(img, 0, 0);
canvas.width = img.width * ratio;
canvas.height = img.height * ratio;
ctx.drawImage(canvasCopy, 0, 0, canvasCopy.width, canvasCopy.height, 0, 0, canvas.width, canvas.height);
};
img.src = reader.result;
}
EDIT2:
看来我错了，链接网站并没有做任何更好的工作缩小图像。我尝试了其他建议的方法，没有一个看起来更好。这就是不同方法的结果:
ps:
帆布:
使用图像渲染:optimizeQuality设置并按宽度/高度缩放:
使用-moz-transform设置和缩放:
Canvas在pixastic上调整大小:
我猜这意味着firefox没有像它应该的那样使用双三次抽样。我只能等他们真的加进去了。
EDIT3:
原始图像
快速图像调整/重采样算法使用Hermite过滤器与JavaScript。支持透明，给予优质。预览:
更新:在GitHub上添加2.0版本(更快，web工作者+可转移对象)。终于我让它工作了!
前往： https://github.com/viliusle/Hermite-resize
演示：http://viliusle.github.io/miniPaint/
/**
* Hermite resize - fast image resize/resample using Hermite filter. 1 cpu version!
*
* @param {HtmlElement} canvas
* @param {int} width
* @param {int} height
* @param {boolean} resize_canvas if true, canvas will be resized. Optional.
*/
function resample_single(canvas, width, height, resize_canvas) {
var width_source = canvas.width;
var height_source = canvas.height;
width = Math.round(width);
height = Math.round(height);
var ratio_w = width_source / width;
var ratio_h = height_source / height;
var ratio_w_half = Math.ceil(ratio_w / 2);
var ratio_h_half = Math.ceil(ratio_h / 2);
var ctx = canvas.getContext("2d");
var img = ctx.getImageData(0, 0, width_source, height_source);
var img2 = ctx.createImageData(width, height);
var data = img.data;
var data2 = img2.data;
for (var j = 0; j < height; j++) {
for (var i = 0; i < width; i++) {
var x2 = (i + j * width) * 4;
var weight = 0;
var weights = 0;
var weights_alpha = 0;
var gx_r = 0;
var gx_g = 0;
var gx_b = 0;
var gx_a = 0;
var center_y = (j + 0.5) * ratio_h;
var yy_start = Math.floor(j * ratio_h);
var yy_stop = Math.ceil((j + 1) * ratio_h);
for (var yy = yy_start; yy < yy_stop; yy++) {
var dy = Math.abs(center_y - (yy + 0.5)) / ratio_h_half;
var center_x = (i + 0.5) * ratio_w;
var w0 = dy * dy; //pre-calc part of w
var xx_start = Math.floor(i * ratio_w);
var xx_stop = Math.ceil((i + 1) * ratio_w);
for (var xx = xx_start; xx < xx_stop; xx++) {
var dx = Math.abs(center_x - (xx + 0.5)) / ratio_w_half;
var w = Math.sqrt(w0 + dx * dx);
if (w >= 1) {
//pixel too far
continue;
}
//hermite filter
weight = 2 * w * w * w - 3 * w * w + 1;
var pos_x = 4 * (xx + yy * width_source);
//alpha
gx_a += weight * data[pos_x + 3];
weights_alpha += weight;
//colors
if (data[pos_x + 3] < 255)
weight = weight * data[pos_x + 3] / 250;
gx_r += weight * data[pos_x];
gx_g += weight * data[pos_x + 1];
gx_b += weight * data[pos_x + 2];
weights += weight;
}
}
data2[x2] = gx_r / weights;
data2[x2 + 1] = gx_g / weights;
data2[x2 + 2] = gx_b / weights;
data2[x2 + 3] = gx_a / weights_alpha;
}
}
//clear and resize canvas
if (resize_canvas === true) {
canvas.width = width;
canvas.height = height;
} else {
ctx.clearRect(0, 0, width_source, height_source);
}
//draw
ctx.putImageData(img2, 0, 0);
}
其中一些解决方案的问题是，它们直接访问像素数据并通过它进行循环以执行下采样。根据图像的大小，这可能会非常消耗资源，最好使用浏览器的内部算法。
drawImage()函数使用线性插值、最近邻重采样方法。当您没有将大小调整到原来大小的一半以上时，这种方法效果很好。
如果循环一次只调整最大大小的一半，结果会非常好，而且比访问像素数据快得多。
这个函数每次采样到一半，直到达到所需的大小:
function resize_image( src, dst, type, quality ) {
var tmp = new Image(),
canvas, context, cW, cH;
type = type || 'image/jpeg';
quality = quality || 0.92;
cW = src.naturalWidth;
cH = src.naturalHeight;
tmp.src = src.src;
tmp.onload = function() {
canvas = document.createElement( 'canvas' );
cW /= 2;
cH /= 2;
if ( cW < src.width ) cW = src.width;
if ( cH < src.height ) cH = src.height;
canvas.width = cW;
canvas.height = cH;
context = canvas.getContext( '2d' );
context.drawImage( tmp, 0, 0, cW, cH );
dst.src = canvas.toDataURL( type, quality );
if ( cW <= src.width || cH <= src.height )
return;
tmp.src = dst.src;
}
}
// The images sent as parameters can be in the DOM or be image objects
resize_image( $( '#original' )[0], $( '#smaller' )[0] );
本文致谢
我想从答案中得到一些定义良好的函数，所以最后得到了这些，我希望对其他人也有用，
function getImageFromLink(link) {
return new Promise(function (resolve) {
var image = new Image();
image.onload = function () { resolve(image); };
image.src = link;
});
}
function resizeImageToBlob(image, width, height, mime) {
return new Promise(function (resolve) {
var canvas = document.createElement('canvas');
canvas.width = width;
canvas.height = height;
canvas.getContext('2d').drawImage(image, 0, 0, width, height);
return canvas.toBlob(resolve, mime);
});
}
getImageFromLink(location.href).then(function (image) {
// calculate these based on the original size
var width = image.width / 4;
var height = image.height / 4;
return resizeImageToBlob(image, width, height, 'image/jpeg');
}).then(function (blob) {
// Do something with the result Blob object
document.querySelector('img').src = URL.createObjectURL(blob);
});
为了测试这个，在一个标签中打开的图像上运行它。
