畫布drawImage縮放

Question

我試圖按比例縮放圖像到畫布。我能夠有固定的寬度和高度擴展它像這樣：

context.drawImage(imageObj, 0, 0, 100, 100) 

但我只想要調整寬度和具有高度調整比例。類似以下內容：

context.drawImage(imageObj, 0, 0, 100, auto) 

我已經到處看了，我能想到並且沒有看到這是否可能。

Answer 1

context.drawImage(imageObj, 0, 0, 100, 100 * imageObj.height/imageObj.width) 

Answer 2

如果你想正確地縮放畫面按屏幕大小，這裏是你可以算一算： 如果你不使用jQuery，替換$（窗口）.WIDTH適當的等效選項。

   var imgWidth = imageObj.naturalWidth; 
       var screenWidth = $(window).width() - 20; 
       var scaleX = 1; 
       if (imageWdith > screenWdith) 
        scaleX = screenWidth/imgWidth; 
       var imgHeight = imageObj.naturalHeight; 
       var screenHeight = $(window).height() - canvas.offsetTop-10; 
       var scaleY = 1; 
       if (imgHeight > screenHeight) 
        scaleY = screenHeight/imgHeight; 
       var scale = scaleY; 
       if(scaleX < scaleY) 
        scale = scaleX; 
       if(scale < 1){ 
        imgHeight = imgHeight*scale; 
        imgWidth = imgWidth*scale;   
       } 
       canvas.height = imgHeight; 
       canvas.width = imgWidth; 
       ctx.drawImage(imageObj, 0, 0, imageObj.naturalWidth, imageObj.naturalHeight, 0,0, imgWidth, imgHeight); 

Answer 3

@TechMaze的解決方案是相當不錯的。

這裏是一些正確性和引入image.onload事件後的代碼。 image.onload對於避免任何形式的失真來說太重要了。

function draw_canvas_image() { 

var canvas = document.getElementById("image-holder-canvas"); 
var context = canvas.getContext("2d"); 
var imageObj = document.getElementById("myImageToDisplayOnCanvas"); 

imageObj.onload = function() { 
    var imgWidth = imageObj.naturalWidth; 
    var screenWidth = canvas.width; 
    var scaleX = 1; 
    if (imgWidth > screenWidth) 
     scaleX = screenWidth/imgWidth; 
    var imgHeight = imageObj.naturalHeight; 
    var screenHeight = canvas.height; 
    var scaleY = 1; 
    if (imgHeight > screenHeight) 
     scaleY = screenHeight/imgHeight; 
    var scale = scaleY; 
    if(scaleX < scaleY) 
     scale = scaleX; 
    if(scale < 1){ 
     imgHeight = imgHeight*scale; 
     imgWidth = imgWidth*scale;   
    } 

    canvas.height = imgHeight; 
    canvas.width = imgWidth; 

    context.drawImage(imageObj, 0, 0, imageObj.naturalWidth, imageObj.naturalHeight, 0,0, imgWidth, imgHeight); 
} 
}