Scaling an image to fit on canvas

javascript html canvas html5-canvas

I have a form that allows a user to upload an image.

Once the image is loaded, we perform some scaling on it in order to reduce its filesize before we pass it back to the server.

To do this, we place it on the canvas and manipulate it there.

This code will render the scaled image on the canvas, with the canvas of size 320 x 240px:

ctx.drawImage(img, 0, 0, canvas.width, canvas.height)

... where canvas.width and canvas.height is the image height and width x a scaling factor based on the size of the original image.

But when I go to use the code:

ctx.drawImage(img, 0, 0, canvas.width, canvas.height, 0, 0, canvas.width, canvas.height

... I only get part of the image on the canvas, in this case the top left corner. I need the whole image 'scaled' to fit on the canvas, despite the actual image size being larger than the 320x240 canvas size.

So for the code above, the width and heights are 1142x856, as that is the final image size. I need to maintain that size to pass beck to the server when the form is submitted, but only want a smaller view of it to appear in the canvas for the user.

What am I missing here? Can anyone point me in the right direction please?

Many thanks in advance.

GameAlchemist

You made the error, for the second call, to set the size of source to the size of the target. Anyway i bet that you want the same aspect ratio for the scaled image, so you need to compute it :

var hRatio = canvas.width / img.width    ;
var vRatio = canvas.height / img.height  ;
var ratio  = Math.min ( hRatio, vRatio );
ctx.drawImage(img, 0,0, img.width, img.height, 0,0,img.width*ratio, img.height*ratio);

i also suppose you want to center the image, so the code would be :

function drawImageScaled(img, ctx) {
   var canvas = ctx.canvas ;
   var hRatio = canvas.width  / img.width    ;
   var vRatio =  canvas.height / img.height  ;
   var ratio  = Math.min ( hRatio, vRatio );
   var centerShift_x = ( canvas.width - img.width*ratio ) / 2;
   var centerShift_y = ( canvas.height - img.height*ratio ) / 2;  
   ctx.clearRect(0,0,canvas.width, canvas.height);
   ctx.drawImage(img, 0,0, img.width, img.height,
                      centerShift_x,centerShift_y,img.width*ratio, img.height*ratio);  
}

you can see it in a jsbin here : http://jsbin.com/funewofu/1/edit?js,output

Thank you! I changed Math.min() to Math.max() to scale and crop the image to fit the canvas. This was very helpful!

but this generates blur image when i generate big image from small size. it gets blur.. any solution ? please

Much appreciated @gamealchemist. Reading through this helped me understand how to get to a scaled down image and it would have taken me forever to figure this out on my own. Thanks!

@saadk knowyourmeme.com/photos/996305-zoom-and-enhance ;)

佚

佚名

Provide the source image (img) size as the first rectangle:

ctx.drawImage(img, 0, 0, img.width,    img.height,     // source rectangle
                   0, 0, canvas.width, canvas.height); // destination rectangle

The second rectangle will be the destination size (what source rectangle will be scaled to).

Update 2016/6: For aspect ratio and positioning (ala CSS' "cover" method), check out:
Simulation background-size: cover in canvas

This is perfect, +1. Quick point - you're missing the closing bracket on the drawImage() function. Messing with my OCD a little ;)

Perfect for my needs. It saved me from having to fiddle with my image file's size/scale when form margins changed. Thank you ;)

Coder_Naveed

I guess that you want the image to be scaled to a smaller size, without losing the ratio of the dimensions. I have a solution.

var ratio = image.naturalWidth / image.naturalHeight;
var width = canvas.width;
var height = width / ratio;
ctx.drawImage(image, 0, 0, width, height);

the ratio will be maintained. And the image drawn on the canvas will be of the same ratio. you can use the if loop if the height of the image is long, you can replace the canvas.width to some other width

Wilson

You can call ctx.scale() before calling ctx.drawImage:

var factor  = Math.min ( canvas.width / img.width, canvas.height / img.height );
ctx.scale(factor, factor);
ctx.drawImage(img, 0, 0);
ctx.scale(1 / factor, 1 / factor);

This should preserve the aspect ratio.

Scaling an image to fit on canvas

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