OpenCV: Add a Trackbar to Your Application

| OpenCV 3.4.2 Open Source Computer Vision

Add a Trackbar to Your Application

## Goal

- Use HTML DOM Input Range Object to add a trackbar to your application.

## Code Demo

Here, we will create a simple application that blends two images. We will let the user enter the weight by using the trackbar.

First, we need to create three canvas elements: two for input and one for output. Please refer to the tutorial Getting Started with Images.

let src1 = cv.imread('canvasInput1');
let src2 = cv.imread('canvasInput2');

Then, we use HTML DOM Input Range Object to implement the trackbar, which is shown as below.

Note
  <input> elements with type="range" are not supported in Internet Explorer 9 and earlier versions.

You can create an <input> element with type="range" with the document.createElement() method:

let x = document.createElement('INPUT');
x.setAttribute('type', 'range');

You can access an <input> element with type="range" with getElementById():

let x = document.getElementById('myRange');

As a trackbar, the range element need a trackbar name, the default value, minimum value, maximum value, step and the callback function which is executed everytime trackbar value changes. The callback function always has a default argument, which is the trackbar position. Additionally, a text element to display the trackbar value is fine. In our case, we can create the trackbar as below:

Weight: <input type="range" id="trackbar" value="50" min="0" max="100" step="1" oninput="callback()">
<input type="text" id="weightValue" size="3" value="50"/>

Finally, we can use the trackbar value in the callback function, blend the two images, and display the result.

let weightValue = document.getElementById('weightValue');
let trackbar = document.getElementById('trackbar');
weightValue.setAttribute('value', trackbar.value);
let alpha = trackbar.value/trackbar.max;
let beta = ( 1.0 - alpha );
let src1 = cv.imread('canvasInput1');
let src2 = cv.imread('canvasInput2');
let dst = new cv.Mat();
cv.addWeighted( src1, alpha, src2, beta, 0.0, dst, -1);
cv.imshow('canvasOutput', dst);
dst.delete();
src1.delete();
src2.delete();

See also
  cv.addWeighted

## Try it