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Cover image for Realtime Collaborative Drawing (part 3): Simulating webkit force and using a color swatch with canvas and javascript

Realtime Collaborative Drawing (part 3): Simulating webkit force and using a color swatch with canvas and javascript

nyxtom profile image Tom Holloway πŸ• Updated on ・6 min read

I promised to give our drawing tool in canvas an upgrade, so let's get a look at that now. There's a few things that it currently doesn't allow such as:

  • Switching colors
  • Pressure values only available in webkit
  • Single line based brush tool

Color Swatch

From our previous article our color generation was simply a random function on load. I looked at a number of color generations and libraries and they all were quite a bit too much. Specifically many of these color picker libraries add code for converting between different color schemes like CMYK, RGB, HSL, HSV. Instead, an easier (one function) approach will be to simply create a swatch (similar to Google Docs).

All we need really to accomplish this is have an array of defined colors.

const swatch = [
    ["#000000", "#434343", "#666666", "#999999", "#b7b7b7", "#cccccc", "#d9d9d9", "#efefef", "#f3f3f3", "#ffffff"],
    ["#980000", "#ff0000", "#ff9900", "#ffff00", "#00ff00", "#00ffff", "#4a86e8", "#0000ff", "#9900ff", "#ff00ff"],
    ["#e6b8af", "#f4cccc", "#fce5cd", "#fff2cc", "#d9ead3", "#d0e0e3", "#c9daf8", "#cfe2f3", "#d9d2e9", "#ead1dc"],
    ["#dd7e6b", "#ea9999", "#f9cb9c", "#ffe599", "#b6d7a8", "#a2c4c9", "#a4c2f4", "#9fc5e8", "#b4a7d6", "#d5a6bd"],
    ["#cc4125", "#e06666", "#f6b26b", "#ffd966", "#93c47d", "#76a5af", "#6d9eeb", "#6fa8dc", "#8e7cc3", "#c27ba0"],
    ["#a61c00", "#cc0000", "#e69138", "#f1c232", "#6aa84f", "#45818e", "#3c78d8", "#3d85c6", "#674ea7", "#a64d79"],
    ["#85200c", "#990000", "#b45f06", "#bf9000", "#38761d", "#134f5c", "#1155cc", "#0b5394", "#351c75", "#741b47"],
    ["#5b0f00", "#660000", "#783f04", "#7f6000", "#274e13", "#0c343d", "#1c4587", "#073763", "#20124d", "#4c1130"]
];

The above color swatch should relatively correspond to the same colors you might see in Google Docs. Next, we need to generate a list of divs that will correspond to the given colors in this swatch.

const colorMap = swatch.flat();

let swatchContainer = document.querySelector('#color-picker');
let colorElements = {};
swatch.forEach(row => {
    let rowElem = document.createElement('div');
    rowElem.classList.add('hstack');
    row.forEach(c => {
        let elem = document.createElement('div');
        elem.classList.add('box');
        elem.style.backgroundColor = c;
        colorElements[c] = elem;
        rowElem.appendChild(elem);
    });

    swatchContainer.appendChild(rowElem);
});

Simple! Now let's go ahead and add a click behavior to this code such that the box will become active/inactive according to the current color.

elem.onclick = function (e) {
    colorPicker.dataset.color = c;
    colorPicker.style.color = c;
    if (colorElements[color]) {
        colorElements[color].classList.remove('active');
    }
    color = c;
    elem.classList.toggle('active');
    e.preventDefault();
};

Recall that our random color was generating a color based on the rgb and used Math.random() to do this. We will replace that code with the following in order to generate a random color within the existing swatch.

function randomColor() {
    return parseInt(Math.random() * colorMap.length);
}

var colorIndex = randomColor();
var color = colorMap[colorIndex];
var colorPicker = document.querySelector('[data-color]');
colorPicker.dataset.color = color;
colorPicker.style.color = color;
colorElements[color].classList.add('active');

Great! Let's get ahead and add the html to correspond to this setup. Ideally, our color picker should behave as a dropdown. Let's add the initial html there.

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>Let's Draw Together</title>
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/remixicon@2.5.0/fonts/remixicon.css">
    <link rel="stylesheet" href="/static/index.css">
    <link rel="alternate icon" type="image/png" href="/static/logo.png">
    <link rel="icon" type="image/svg+xml" href="/static/logo.png">
</head>
<body>
    <div class="flush vstack">
        <div class="menubar hstack">
            <a class="icon-link center">
                <i class="ri-lg ri-landscape-line"></i>
            </a>
            <div class="spacer"></div>
            <a class="icon-link active center" data-tool="pencil">
                <i class="ri-lg ri-pencil-fill"></i>
            </a>
            <a class="icon-link center" data-tool="rect">
                <i class="ri-lg ri-shape-line"></i>
            </a>
            <a class="icon-link center" data-tool="circle">
                <i class="ri-lg ri-checkbox-blank-circle-line"></i>
            </a>
            <a class="icon-link center" data-tool="text">
                <i class="ri-lg ri-font-size-2"></i>
            </a>
            <div class="spacer"></div>
            <div class="relative">
                <a class="icon-link center" data-color="#33ffff">
                    <i class="ri-lg ri-palette-line"></i>
                    <i class="ri-lg ri-checkbox-blank-fill center"></i>
                </a>
                <div id="color-picker" class="dropdown vstack">
                </div>
            </div>
            <div class="spacer"></div>
        </div>
        <div class="spacer app">
            <canvas></canvas>
        </div>
    </div>

    <script type="text/javascript" src="/static/load.js"></script>
    <script type="text/javascript" src="/static/draw.js"></script>
</body>
</html>

Notice that we are wrapping the data-color link in a relative class. Let's make sure we have the corresponding classes to handle this.

.relative {
    position: relative;
}

The behavior that I would like to replicate is that the dropdown shows up on hover (either hover in the swatch or hover on this data-color link).

:root {
    /** .... */
    --dropdown-background: #fff;
    --dropdown-shadow:  0px 0px 1px 0px rgba(0,0,0,0.5), 0px 2px 6px -5px rgba(0,0,0,0.75);
}

.dropdown {
    position: absolute;
    background-color: var(--dropdown-background);
    padding: 4px;
    box-shadow: var(--dropdown-shadow);
    border-radius: 4px;
    z-index: 1;
    margin-left: -80px;
    transition: all 0.25s ease-in-out;
}
.icon-link + .dropdown {
    opacity: 0;
    top: 8px;
    visibility: hidden;
    transition: all 0.25s ease-in-out;
}
.icon-link:hover + .dropdown, .dropdown:hover {
    opacity: 1;
    top: 16px;
    visibility: visible;
}
.icon-link:hover + .dropdown *, .dropdown:hover * {
    opacity: 1;
}

Finally, we need our box classes to handle hover/layout.

/** Color Picker */
.box {
    width: 24px;
    height: 24px;
    cursor: pointer;
}
.box:hover, .box.active {
    box-shadow: inset 0px 0px 2px 2px #fff;
    opacity: 0.75;
}

Great! Now we have ourselves a color swatch tool!

Color Swatch

Simulating Webkit Force

Recall that if you are in Safari you can use the webkitForce property on the onwebkitmouseforcechanged event to get the current trackpad pressure value. Since this is proprietary we have no real way of accessing this value without either working in Safari (or in Swift for desktop apps). We can however, simulate this kind of value by using a keypress value to increase or decrease a force value. This will somewhat mimic a change as we are moving around our cursor.

var force = 1;
var mouseDown = false;

function move(e) {
    mouseDown = e.buttons;
    /** ... */
}

function key(e) {
    if (e.key === 'Backspace') {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
    }
    if (mouseDown && e.key === 'ArrowUp') {
        force += 0.025;
    }
    if (mouseDown && e.key === 'ArrowDown') {
        force -= 0.025;
    }
}

window.onkeydown = key;

Now whenever we press the key up or down we can change the value while we are pressing down on the mouse!

Force Brush

Changing Colors with Left/Right Arrow Keys

One additional thing I'd like to add is the ability to change the colors on keypress as well. We can simply update the current color index to do this.

function key(e) {
    if (e.key === 'Backspace') {
        ctx.clearRect(0, 0, canvas.width, canvas.height);
    }
    if (e.key === 'ArrowRight') {
        colorIndex++;
    }
    if (e.key === 'ArrowLeft') {
        colorIndex--;
    }
    if (e.key === 'ArrowRight' || e.key === 'ArrowLeft') {
        if (colorIndex >= colorMap.length) {
            colorIndex = 0;
        }
        if (colorIndex < 0) {
            colorIndex = colorMap.length - 1;
        }
        if (colorElements[color]) {
            colorElements[color].classList.remove('active');
        }
        color = colorMap[colorIndex];
        colorPicker.dataset.color = color;
        colorPicker.style.color = color;
        colorElements[color].classList.toggle('active');
    }
    if (mouseDown && e.key === 'ArrowUp') {
        force += 0.025;
    }
    if (mouseDown && e.key === 'ArrowDown') {
        force -= 0.025;
    }
}

Switching Colors

What about combining them at the same time? To do this, we'll need to change our keys for force changes to something like SHIFT and ALT. We still want to be able to control with the up/down arrows and we want to limit shift/alt to only when left or right is pressed.

    if (mouseDown && (e.key === 'ArrowUp' || (e.shiftKey && ['ArrowLeft', 'ArrowRight'].includes(e.key)))) {
        force += 0.025;
    }
    if (mouseDown && (e.key === 'ArrowDown' || (e.altKey && ['ArrowLeft', 'ArrowRight'].includes(e.key)))) {
        force -= 0.025;
    }

Pressure and Color

Awesome! If you liked this article, give me a follow and a like if you are so inclined. Also, feel free to check out my twitter where I post similar and other interesting stuff on physics, code, cs theory and more!

Cheers! 🍻

CODE

If you're interested in the code for this series, check out my repository on GitHub below:

https://github.com/nyxtom/drawing-webrtc

Posted on by:

nyxtom profile

Tom Holloway πŸ•

@nyxtom

Software Developer for as long as I can remember, building startups and projects. I've worked mostly in analytics, devops, and data science research. Running has proved useful so I enjoy doing it.

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