裁剪画布/导出具有特定宽度和高度的 html5 画布
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【中文标题】裁剪画布/导出具有特定宽度和高度的 html5 画布【英文标题】:Crop Canvas / Export html5 canvas with certain width and height 【发布时间】:2012-10-15 22:26:48 【问题描述】:有数百个教程,介绍如何在画布上通过 drawImage() 裁剪图像。
context.drawImage(imageObj, sourceX, sourceY, sourceWidth, sourceHeight, destX, destY, destWidth, destHeight);
但是,我有一个可以填充用户浏览器的画布。通过将画布导出为图像,我只想从 (0|0) 导出 640px*480px 的区域。
问题:我如何告诉 javascript 只使用 640*480 的画布用于 toDataURL()?
这是我目前所拥有的:
$("#submitGraphic").click( function()
var canvas = document.getElementsByTagName("canvas");
// canvas context
var context = canvas[0].getContext("2d");
// get the current ImageData for the canvas
var data = context.getImageData(0, 0, canvas[0].width, canvas[0].height);
// store the current globalCompositeOperation
var compositeOperation = context.globalCompositeOperation;
// set to draw behind current content
context.globalCompositeOperation = "destination-over";
//set background color
context.fillStyle = "#FFFFFF";
// draw background/rectangle on entire canvas
context.fillRect(0,0,canvas[0].width,canvas[0].height);
// not working, seems to clear the canvas? browser hangs?
// seems that I can click a white image in the background
/*canvas[0].width = 640;
canvas[0].height = 480;*/
// not working either
/*canvas[0].style.width = '640px';
canvas[0].style.height = '480px';*/
// not working at all
/*context.canvas.width = 640;
context.canvas.height = 480;*/
// write on screen
var img = canvas[0].toDataURL("image/png");
document.write('<a href="'+img+'"><img src="'+img+'"/></a>');
)
PS:我不想调整大小或缩放,只是剪裁/裁剪到固定窗口。 Here 我读到您只指定 canvas.width 和 canvas.height - 但这会清除画布。
【问题讨论】:
【参考方案1】:最好的方法是创建一个临时画布以从当前画布上绘制。用户永远不会看到这个临时画布。然后你只需要在临时画布上使用toDataUrl()
。
Live Demo
$("#submitGraphic").click( function()
var canvas = document.getElementsByTagName("canvas");
// canvas context
var context = canvas[0].getContext("2d");
// get the current ImageData for the canvas
var data = context.getImageData(0, 0, canvas[0].width, canvas[0].height);
// store the current globalCompositeOperation
var compositeOperation = context.globalCompositeOperation;
// set to draw behind current content
context.globalCompositeOperation = "destination-over";
//set background color
context.fillStyle = "#FFFFFF";
// draw background/rectangle on entire canvas
context.fillRect(0,0,canvas[0].width,canvas[0].height);
var tempCanvas = document.createElement("canvas"),
tCtx = tempCanvas.getContext("2d");
tempCanvas.width = 640;
tempCanvas.height = 480;
tCtx.drawImage(canvas[0],0,0);
// write on screen
var img = tempCanvas.toDataURL("image/png");
document.write('<a href="'+img+'"><img src="'+img+'"/></a>');
)
【讨论】:
太棒了,效果很好! / 唯一让我恼火的是,当图像已经出现时,Firefox 会指示加载页面(标签显示旋转圆圈)。单击 ESC 在 url 中显示 wyciwyg:// !? ...“所见即所得是用于表示 document.write 结果页面的内部 URI 方案”/但无论如何,很高兴你给了我一个可行的解决方案!【参考方案2】:我创建了一个简单的通用函数,它通过返回一个带有裁剪区域的新画布来进行裁剪。虽然它没有“就地”进行裁剪,但它很简单。 记得在调用后切换到新的上下文。
const cropCanvas = (sourceCanvas,left,top,width,height) =>
let destCanvas = document.createElement('canvas');
destCanvas.width = width;
destCanvas.height = height;
destCanvas.getContext("2d").drawImage(
sourceCanvas,
left,top,width,height, // source rect with content to crop
0,0,width,height); // newCanvas, same size as source rect
return destCanvas;
例如...
let myCanvas = document.createElement('canvas');
myCanvas.width = 200;
myCanvas.height = 200;
let myContext = myCanvas.getContext("2d");
// draw stuff...
myCanvas = cropCanvas(myCanvas,50,50,100,100);
myContext = myCanvas.getContext("2d");
// now using the cropped 100x100 canvas
【讨论】:
【参考方案3】:制作第二个屏幕外画布,将图像从第一个画布复制到第二个画布(使用第一个画布作为图像对象),然后导出第二个画布。
【讨论】:
【参考方案4】:纯html5画布裁剪:
$('document').ready( function()
const divOffset = 1
var x1,x2,y1,y2, xDif, yDif = 0;
var isSelection,
isBottomRight,
isTopRight,
isTopLeft,
isBottomLeft = false
var r = document.getElementById('source').getBoundingClientRect();
var pos = [0, 0];
pos[0] = r.left;
pos[1] = r.top; //got position coordinates of canvas
var sel = document.getElementById('sel')
var canvasSource = document.getElementById("source");
var ctxSource = canvasSource.getContext("2d");
var img = new Image()
img.src = "http://bohdaq.name/assets/localImage.jpg"
img.onload = function()
ctxSource.drawImage(img, 0, 0)
$( "#source" ).mousedown(function(event)
isSelection = true
x1 = event.pageX - pos[0]
y1 = event.pageY - pos[1]
sel.style.setProperty('display', 'block')
sel.style.setProperty('left', event.pageX + "px")
sel.style.setProperty('top', event.pageY + "px")
sel.style.setProperty('width', '0px')
sel.style.setProperty('height', '0px')
);
$( "#source" ).mouseup(function(event)
isSelection = false
if(isBottomRight)
x2 = event.pageX - pos[0]
y2 = event.pageY - pos[1]
xDif = x2-x1
yDif = y2-y1
else if (isBottomLeft)
y2 = event.pageY - pos[1]
yDif = y2 - y1
xDif = x1 - x2
x1 = x1 - xDif
else if(isTopRight)
x2 = event.pageX - pos[0]
xDif = x2 - x1
yDif = y1 - y2
y1 = y1 - yDif
else if (isTopLeft)
xDif = x1 - x2
x1 = x1 - xDif
yDif = y1 - y2
y1 = y1 - yDif
sel.style.setProperty('display', 'none')
crop(x1, y1, xDif, yDif)
);
$('#source').mousemove(function(event)
if(isSelection)
x2 = event.pageX - pos[0]
y2 = event.pageY - pos[1]
if(x2>x1 && y2>y1) //moving right bottom selection
isBottomRight = true
isBottomLeft = false
isTopLeft = false
isTopRight = false
xDif = x2 - x1
yDif = y2 - y1
sel.style.setProperty('width', xDif + 'px')
sel.style.setProperty('height', yDif + 'px')
else if(x2<x1 && y2>y1) //moving left bottom selection
isBottomLeft = true
isTopLeft = false
isTopRight = false
isBottomRight = false
xDif = x1 - x2
yDif = y2 - y1
sel.style.setProperty('left', x2 + 'px')
sel.style.setProperty('width', xDif + 'px')
sel.style.setProperty('height', yDif + 'px')
else if(x2>x1 && y2<y1)
isTopRight = true
isTopLeft = false
isBottomLeft = false
isBottomRight = false
xDif = y1 - y2
yDif = x2 - x1
sel.style.setProperty('top', y2 + 'px')
sel.style.setProperty('width', yDif + 'px')
sel.style.setProperty('height', xDif + 'px')
else if (x2<x1 && y2<y1)
isTopLeft = true
isTopRight = false
isBottomLeft = false
isBottomRight = false
yDif = y1 - y2
xDif = x1 - x2
sel.style.setProperty('left', x2 + pos[0] + divOffset + 'px')
sel.style.setProperty('top', y2 + pos[1] + divOffset + 'px')
sel.style.setProperty('width', xDif + 'px')
sel.style.setProperty('height', yDif + 'px')
)
function crop(x, y, xDif, yDif)
canvasSource.width = xDif
canvasSource.height = yDif
ctxSource.drawImage(img, x, y, xDif, yDif, 0, 0, xDif, yDif);
)
【讨论】:
【参考方案5】:基于@GarySkiba 的回答:
// crop canvas function
const cropCanvas = (
canvas: any,
x: number,
y: number,
width: number,
height: number
) =>
// create a temp canvas
const newCanvas = document.createElement('canvas');
// set its dimensions
newCanvas.width = width;
newCanvas.height = height;
// draw the canvas in the new resized temp canvas
newCanvas
.getContext('2d')!
.drawImage(canvas, x, y, width, height, 0, 0, width, height);
return newCanvas
;
像这样使用它:
// get canvas from somewhere
const canvas = ....;
// crop the left top 50x50 rect
const newCanvas = cropCanvas(canvas, 0, 0, 50, 50 );
// get the relative image
newCanvas.toDataURL('image/png')
【讨论】:
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