裁剪畫布/導出具有一定寬度和高度的html5畫布

Question

有數百個教程，如何通過drawImage（）在畫布上裁剪圖像。

context.drawImage(imageObj, sourceX, sourceY, sourceWidth, sourceHeight, destX, destY, destWidth, destHeight); 

但是，我有一個畫布，填充用戶的瀏覽器。通過將畫布導出爲圖像，我想從（0 | 0）僅導出640px * 480px的區域。

問題：如何告訴javascript只使用640 * 480的toDataURL（）畫布？

這是我到目前爲止有：

$("#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：我不想調整大小或規模，只是裁剪/裁剪到固定窗口。 Here我讀到，你只能指定canvas.width和canvas.height - 但是這會清除畫布。

Answer 1

最好的方法是創建一個臨時畫布以從當前畫布上繪製。用戶永遠不會看到這個臨時畫布。那麼你只需要在temp畫布上使用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>'); 
})​ 

Answer 2

您製作第二個離屏畫布，將圖像從第一個畫布複製到第二個畫面（使用第一個作爲圖像對象），然後導出第二個畫布。

Answer 3

純HTML5畫布作物。你可以現場查看here

$('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); 
} 

})