Smoothing Resized Images with JavaScript Canvas

Resizing images with HTML canvas provides a convenient way to manipulate visuals within a web application. However, the default resizing algorithm may result in pixelated or jagged images, lacking the smoothness found in image editing software. This issue can be addressed by incorporating image smoothing techniques.

One approach to achieve smoothness is downscaling in steps. This method involves resizing the image gradually, using smaller increments. For instance, instead of directly resizing the original image to 50%, you can first reduce it to 75%, then 62.5%, and so on. This incremental approach reduces the impact of pixel interpolation, resulting in a smoother result.

var oc = document.createElement('canvas'),
    octx = oc.getContext('2d');

oc.width = img.width * 0.5;
oc.height = img.height * 0.5;
octx.drawImage(img, 0, 0, oc.width, oc.height);

// ...

ctx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5,
    0, 0, canvas.width, canvas.height);

Another method is to set the imageSmoothingQuality property, which is supported in modern browsers such as Chrome. This property controls the interpolation algorithm used for image resizing. By setting it to "high", browsers may switch to a more advanced bicubic interpolation method, improving image smoothness.

canvas.getContext('2d', { imageSmoothingQuality: "high" });

Utilizing downscaling in steps or adjusting the image smoothing quality allows developers to enhance the appearance of resized images with JavaScript canvas, creating visually appealing results.